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Contents

Notices . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . vii
Trademarks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . viii

Preface . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ix
Authors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ix
Now you can become a published author, too! . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . xi
Comments welcome. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . xi
Stay connected to IBM Redbooks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . xi

Chapter 1. Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
1.1 Cloud native applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2
1.2 Twelve factors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2
1.3 Microservices . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.3.1 The meaning of “small” . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.3.2 Independence and autonomy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.3.3 Resilience and Fault tolerance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
1.3.4 Automated environment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
1.4 Philosophy and team structure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
1.5 Examples . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
1.5.1 Online retail store . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
1.5.2 Game On! . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

Chapter 2. Creating Microservices in Java . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
2.1 Java platforms and programming models . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
2.1.1 Spring Boot . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
2.1.2 Dropwizard . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
2.1.3 Java Platform, Enterprise Edition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
2.2 Versioned dependencies. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
2.3 Identifying services . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
2.3.1 Applying domain-driven design principles. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
2.3.2 Translating domain elements into services . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
2.3.3 Application and service structure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
2.3.4 Shared library or new service? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
2.4 Creating REST APIs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
2.4.1 Top down or bottom up? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
2.4.2 Documenting APIs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
2.4.3 Use the correct HTTP verb . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
2.4.4 Create machine-friendly, descriptive results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
2.4.5 Resource URIs and versioning . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

Chapter 3. Locating services . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
3.1 Service registry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26
3.1.1 Third-party registration versus self-registration. . . . . . . . . . . . . . . . . . . . . . . . . . . 26
3.1.2 Availability versus consistency . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27
3.2 Service invocation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27
3.2.1 Server side . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27
3.2.2 Client side . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29
3.3 API Gateway . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32

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Chapter 4. Microservice communication . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33
4.1 Synchronous and asynchronous . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34
4.1.1 Synchronous messaging (REST) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34
4.1.2 Asynchronous messaging (events) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35
4.1.3 Examples . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36
4.2 Fault tolerance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37
4.2.1 Resilient against change . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37
4.2.2 Timeouts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39
4.2.3 Circuit breakers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39
4.2.4 Bulkheads . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39

Chapter 5. Handling data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41
5.1 Data-specific characteristics of a microservice . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42
5.1.1 Domain-driven design leads to entities . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42
5.1.2 Separate data store per microservice . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43
5.1.3 Polyglot persistence . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45
5.1.4 Data sharing across microservices . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45
5.1.5 Event Sourcing and Command Query Responsibility Segregation. . . . . . . . . . . . 47
5.1.6 Messaging systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48
5.1.7 Distributed transactions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49
5.2 Support in Java . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50
5.2.1 Java Persistence API . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50
5.2.2 Enterprise JavaBeans . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 57
5.2.3 BeanValidation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59
5.2.4 Contexts and Dependency Injection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61
5.2.5 Java Message Service API . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62
5.2.6 Java and other messaging protocols . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 65

Chapter 6. Application Security . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67
6.1 Securing microservice architectures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 68
6.1.1 Network segmentation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 69
6.1.2 Ensuring data privacy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 69
6.1.3 Automation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 70
6.2 Identity and Trust . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 70
6.2.1 Authentication and authorization. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 71
6.2.2 Delegated Authorization with OAuth 2.0 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 71
6.2.3 JSON Web Tokens . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 73
6.2.4 Hash-Based Messaging Authentication Code. . . . . . . . . . . . . . . . . . . . . . . . . . . . 75
6.2.5 API keys and shared secrets . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 76

Chapter 7. Testing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 77
7.1 Types of tests . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 78
7.2 Application architecture . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 78
7.3 Single service testing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 79
7.3.1 Testing domain or business function. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 79
7.3.2 Testing resources . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 81
7.3.3 Testing external service requests . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 82
7.3.4 Testing data requests . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 84
7.3.5 Component testing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 84
7.3.6 Security verification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 85
7.4 Staging environment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 85
7.4.1 Test data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 85
7.4.2 Integration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 86
7.4.3 Contract . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 86

iv Microservices Best Practices for Java

7.4.4 End-to-end . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 86
7.4.5 Fault tolerance and resilience . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 86
7.5 Production environment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 87
7.5.1 Synthetic monitoring . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 88
7.5.2 Canary testing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 88

Chapter 8. From development to production . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 89
8.1 Deployment patterns . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 90
8.2 Deployment pipelines and tools . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 91
8.3 Packaging options . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 92
8.3.1 JAR, WAR, and EAR file deployment on preinstalled middleware . . . . . . . . . . . . 93
8.3.2 Executable JAR file . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 94
8.3.3 Containerization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 96
8.3.4 Every microservice on its own server . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 97
8.3.5 Aspects of running multiple microservices on one server . . . . . . . . . . . . . . . . . . . 98
8.3.6 Preferred practices on packaging . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 98
8.4 Configuration of the applications across stages . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 100
8.4.1 Environment variables . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 100
8.4.2 Configuration files . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 101
8.4.3 Configuration systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 103
8.4.4 Programming considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 104
8.4.5 Preferred practices on configuration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 105

Chapter 9. Management and Operations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 107
9.1 Metrics and health checks. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 108
9.1.1 Dynamic provisioning . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 108
9.1.2 Health check . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 111
9.2 Logging . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 111
9.3 Templating . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 113

Related publications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 115
IBM Redbooks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 115
Other publications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 115
Online resources . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 115
Help from IBM . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 116

Contents

vi Microservices Best Practices for Java

Notices

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vii

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viii

Microservices Best Practices for Java

Preface

Microservices is an architectural style in which large, complex software applications are
composed of one or more smaller services. Each of these microservices focuses on
completing one task that represents a small business capability. These microservices can be
developed in any programming language.

This IBM® Redbooks® publication covers Microservices best practices for Java. It focuses on
creating cloud native applications using the latest version of IBM WebSphere® Application
Server Liberty, IBM Bluemix® and other Open Source Frameworks in the Microservices
ecosystem to highlight Microservices best practices for Java.

Authors

This book was produced by a team of specialists from around the world working at the
International Technical Support Organization, Raleigh Center.

Michael Hofmann has a Master’s degree in Compute Science
(Diploma from the Technical University of Munich) and a
certificate in Economics (Verwaltungs- und
Wirtschaftsakademie Munich). He has been working with Java
and Java EE since 1999 in different positions. He is currently
the manager of a software development department, but
always with a hands-on mindset. He still works as senior
software architect (technical project lead) in several large
projects. As trainer and coach in Java EE, software architect,
DevOps consultant, and as a speaker at conferences, he
shares his experience with others.

Erin Schnabel is a Senior Software Engineer at IBM. She
specializes in composable runtimes and microservice
architectures, including the application of object-oriented and
service-oriented technologies and design patterns to
decompose existing software systems. Erin has over 15 years
of experience in the WebSphere Application Server
development organization, with 7 years spent as development
lead and architect for WebSphere Liberty. Erin is passionate
about the Java developers’ experience, particularly concerning
the role of community-driven, standards-compliant software in
the cloud environment.

Katherine Stanley is a Software Engineer at IBM in the
WebSphere Application Server Liberty development
organization. She specializes in microservices architectures
built using Java. As a part of the Liberty team, Katherine has
created samples to demonstrate industry best practices for
microservices. She has run workshops and presentations at
European conferences, including DevoxxUK in London and
JFokus in Sweden. Katherine has been at IBM since 2014 and
is based in the Hursley UK lab.

This project was led by:

(cid:2) Margaret Ticknor is an IBM Technical Content Services Project Leader in the Raleigh

Center. She primarily leads projects about WebSphere products and IBM
PureApplication® System. Before joining the IBM Technical Content Services, Margaret
worked as an IT specialist in Endicott, NY. Margaret attended the Computer Science
program at the State University of New York at Binghamton.

Thanks to the following people for their contributions to this project:

Roland Barcia
Distinguished Engineer, CTO: Cloud Programming Models / NYC Bluemix Garage, IBM
Cloud

Kyle Brown
Distinguished Engineer, CTO: Cloud Architecture, IBM Cloud

Andy Gibbs
Offering Management, IBM Cloud

Alan Little
Distinguished Engineer, Architect for WebSphere Foundation, IBM Cloud

Ozzy Osborne
Software Developer - WebSphere Liberty Profile, IBM Cloud

Ilan Pillemer
Senior Solutions Architect (Global) at CHEP Edgware, Greater London, United Kingdom

Shriram Rajagopalan
Research Staff Member, Cloud Platforms & Services, IBM Research

Kevin Sutter
STSM, Java EE, and Liberty SLE Architect, IBM Cloud

Thanks to the following people for their support of this project:

(cid:2) Diane Sherman, IBM Redbooks Content Specialist
(cid:2) Ann Lund, IBM Redbooks Residency Administrator

Microservices Best Practices for Java

Now you can become a published author, too!

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Find out more about the residency program, browse the residency index, and apply online at:

ibm.com/redbooks/residencies.html

Comments welcome

Your comments are important to us!

We want our books to be as helpful as possible. Send us your comments about this book or
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(cid:2) Use the online Contact us review Redbooks form found at:

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Preface

xii

Microservices Best Practices for Java

Chapter 1. Overview

Application architecture patterns are changing in the era of cloud computing. A convergence
of factors has led to the concept of “cloud native” applications:

(cid:2) The general availability of cloud computing platforms

(cid:2) Advancements in virtualization technologies

(cid:2) The emergence of agile and DevOps practices as organizations looked to streamline and

shorten their release cycles

To best take advantage of the flexibility of cloud platforms, cloud native applications are
composed of smaller, independent, self-contained pieces that are called microservices.

This chapter provides a brief overview of the concepts and motivations that surround cloud
native applications and microservice architectures:

(cid:2) Cloud native applications
(cid:2) Twelve factors
(cid:2) Microservices
(cid:2) Philosophy and team structure
(cid:2) Examples

1.1 Cloud native applications

Cloud computing environments are dynamic, with on-demand allocation and release of
resources from a virtualized, shared pool. This elastic environment enables more flexible
scaling options, especially compared to the up-front resource allocation typically used by
traditional on premises data centers.

According to the Cloud Native Computing Foundation, cloud native systems have the
following properties:

(cid:2) Applications or processes are run in software containers as isolated units.

(cid:2) Processes are managed by using central orchestration processes to improve resource

utilization and reduce maintenance costs.

(cid:2) Applications or services (microservices) are loosely coupled with explicitly described

dependencies.

Taken together, these attributes describe a highly dynamic system composed of many
independent processes working together to provide business value: a distributed system.

For more information, see the Cloud Native Computing Foundation (“CNCF”) Charter at:

https://cncf.io/about/charter

Distributed computing is a concept with roots that stretch back decades. The eight fallacies of
distributed computing were drafted in 1994, and deserve a mention:

1. The network is reliable.
2. Latency is zero.
3. Bandwidth is infinite.
4. The network is secure.
5. Topology doesn’t change.
6. There is one administrator.
7. Transport cost is zero.
8. The network is homogeneous.

Taking the fallacies into account, something becomes clear: Cloud native applications run in
an environment of constant, unpredictable change, and must expect failures to occur at any
time. For more information about the fallacies, see Fallacies of Distributed Computing
Explained, available at:

http://www.rgoarchitects.com/Files/fallacies.pdf

1.2 Twelve factors

The 12-factor application methodology was drafted by developers at Heroku. The
characteristics mentioned in the 12 factors are not specific to cloud provider, platform, or
language. The factors represent a set of guidelines or best practices for portable, resilient
applications that will thrive in cloud environments (specifically software as a service
applications). The following are the 12 factors in brief:

1. There should be a one-to-one association between a versioned codebase (for example,

an IT repository) and a deployed service. The same codebase is used for many
deployments.

2. Services should explicitly declare all dependencies, and should not rely on the presence

of system-level tools or libraries.

Microservices Best Practices for Java

3. Configuration that varies between deployment environments should be stored in the

environment (specifically in environment variables).

4. All backing services are treated as attached resources, which are managed (attached and

detached) by the execution environment.

5. The delivery pipeline should have strictly separate stages: Build, release, and run.

6. Applications should be deployed as one or more stateless processes. Specifically,

transient processes must be stateless and share nothing. Persisted data should be stored
in an appropriate backing service.

7. Self-contained services should make themselves available to other services by listening

on a specified port.

8. Concurrency is achieved by scaling individual processes (horizontal scaling).

9. Processes must be disposable: Fast startup and graceful shutdown behaviors lead to a

more robust and resilient system.

10.All environments, from local development to production, should be as similar as possible.

11.Applications should produce logs as event streams (for example, writing to stdout and

stderr), and trust the execution environment to aggregate streams.

12.If admin tasks are needed, they should be kept in source control and packaged alongside

the application to ensure that it is run with the same environment as the application.

These factors do not have to be strictly followed to achieve a good microservice environment.
However, keeping them in mind allows you to build portable applications or services that can
be built and maintained in continuous delivery environments.

For more information about the 12 factors, see The Twelve-Factor App by Adam Wiggins,
available at:

http://12factor.net

1.3 Microservices

Although there is no standard definition for microservices, most reference Martin Fowler’s
seminal paper. The paper explains that microservices are used to compose complex
applications by using small, independent (autonomous), replaceable processes that
communicate by using lightweight APIs that do not depend on language.

For more information, see “Microservices: A new architectural term” by Martin Fowler, which
is available at:

http://martinfowler.com/articles/microservices.html

Chapter 1. Overview

Each microservice in the simple example that is shown in Figure 1-1 is a 12-factor
application, with replaceable backing services providing a message broker, service registry,
and independent data stores. For more information, see 1.2, “Twelve factors” on page 2.

Figure 1-1 A microservices application

1.3.1 The meaning of “small”

Many descriptions make parallels between the roles of individual microservices and chained
commands on the UNIX command line:

$ ls | grep ‘service’ | sort -r

These UNIX commands each do different things. You can use the commands without
awareness of what language the commands are written in, or how large their codebases are.

The size of a microservice is not related to the number of lines of code. Instead, it describes
its scope. The use of the word “small”, as applied to a microservice, essentially means that it
is focused in purpose. The microservice should do one thing, and do that one thing well.

Strategies for defining the scope of a service are discussed in detail in Chapter 2, “Creating
Microservices in Java” on page 9.

1.3.2 Independence and autonomy

The emotion and passion that accompany a grass roots push for creating microservices can
be viewed as a mutiny against monolithic environments. This trend is due to a broad range of
reasons, including these:

(cid:2) Infrequent, expensive, all-or-nothing maintenance windows.

(cid:2) Constrained technology choice due to well intentioned, but heavy handed, centralized

governance.

In these environments, change cannot be made without significant risk because everything is
interrelated with everything else. It is the opposite of an agile ecosystem.

Microservices Best Practices for Java

A dominant factor in creating and preserving agility is ensuring each service is independent
and autonomous. Each service must be capable of being started, stopped, or replaced at any
time without tight coupling to other services. Many other characteristics of microservice
architectures follow from this single factor.

Microservices should act as encapsulations (or bounded contexts, as discussed in Chapter 2,
“Creating Microservices in Java” on page 9) guarding the implementation details of the
application. This configuration allows the service’s implementation to be revised and
improved (or even rewritten) over time. Data sources, especially, must be private to the
service, as described in depth in Chapter 5, “Handling data” on page 41.

If deploying changes requires simultaneous or time sensitive updates to multiple services,
you are doing it wrong. There might be good, pragmatic, reasons behind requiring these
updates, but it should prompt a review of the relationships between your services. There are a
few possible outcomes:

(cid:2) You need to revise your versioning strategy to preserve the independent lifecycle of

services.

(cid:2) The current service boundaries are not drawn properly, and services should be refactored

into properly independent pieces.

Refactoring services will happen, especially when creating an application from scratch.
Identifying services is described more in Chapter 2, “Creating Microservices in Java” on
page 9, but take it as a given that you will get some things wrong. The ability to react and
refactor services as problems are discovered is a benefit of the microservices approach.

A quick note about polyglot and language-agnostic protocols
Polyglot is a frequently cited benefit of microservice-based architectures. Being able to
choose the appropriate language or data store to meet the needs of each service can be
powerful, and can bring significant efficiency. However, the use of obscure technologies can
complicate long-term maintenance and inhibit the movement of developers between teams.
Create a balance between the two by creating a list of supported technologies to choose from
at the outset, with a defined policy for extending the list with new technologies over time.
Ensure that non-functional or regulatory requirements like maintainability, auditability, and
data security can be satisfied, while preserving agility and supporting innovation through
experimentation.

Polyglot applications are only possible with language-agnostic protocols. Representational
State Transfer (REST) architecture patterns define guidelines for creating uniform interfaces
that separate the on-the-wire data representation from the implementation of the service.
RESTful architectures require the request state to be maintained by the client, allowing the
server to be stateless. The shared concepts between REST and microservices should be
apparent.

For more information about REST, see “Chapter 5: Representational State Transfer (REST)”,
Fielding, Roy Thomas (2000), which is available at:

http://www.ics.uci.edu/~fielding/pubs/dissertation/rest_arch_style.htm

Hypertext Transfer Protocol (HTTP) is a common communication language for all applications
on the internet. It has an expressive set of verbs and return codes that are only just realizing
their potential with the adoption of REST.

REST/HTTP interactions are inherently synchronous (request/response) in nature.
Asynchronous communications also play a significant role in maintaining loose coupling
between microservices. Again, language-agnostic protocols come to the forefront, with
standards like Advanced Message Queuing Protocol (AMQP) and Message Queuing

Chapter 1. Overview

Telemetry Transport (MQTT) edging out language-specific mechanisms like Java Message
Service (JMS). Chapter 3, “Locating services” on page 25 explores the differences between
synchronous and asynchronous interaction patterns in more detail.

JavaScript Object Notation (JSON) has emerged in microservices architectures as the wire
format of choice for text-based data, displacing Extensible Markup Language (XML) with its
comparative simplicity and conciseness. Binary serialization frameworks do exist, such as
Apache Avro1, Apache Thrift2, and Google Protocol Buffers3. However, use of these protocols
should be limited to internal services and deferred until the interactions between services are
understood.

1.3.3 Resilience and Fault tolerance

Creating a resilient system places requirements on all of the services in it. As mentioned
earlier, the dynamic nature of cloud environments demands that services be written to expect
and respond gracefully to the unexpected. This condition could mean receiving bad data,
being unable to reach a required backing service, or dealing with conflicts due to concurrent
updates in a distributed system.

Concerning independently evolving but inter-dependent services, the robustness principle
provides the best guidance: “Be liberal in what you accept, and conservative in what you
send”4. Assume that APIs will evolve over time, and be tolerant of data you do not
understand. To quote the RFC:

As a simple example, consider a protocol specification that contains an enumeration of
values … this enumeration must be assumed to be incomplete. Thus, if a protocol
specification defines four possible error codes, the software must not break when a fifth
code shows up. An undefined code might be logged…, but it must not cause a failure.

Microservices must also prevent failures from cascading through the system. Chapter 4.2,
“Fault tolerance” on page 37 describes strategies for dealing with API changes. Isolation
patterns like circuit breakers and bulkheads are described in Chapter 3, “Locating services”
on page 25.

1.3.4 Automated environment

Microservice architectures in cloud environments create a sprawling landscape of
independently moving parts. Creating visualizations for interaction patterns between
microservices eventually results in a confusing mess of lines. A high degree of automation is
required for applications that use microservices.

Strategies for building and deploying Java based services using continuous integration or
continuous deployment practices and tools are described in Chapter 8, “From development to
production” on page 89.

Automated monitoring and alerting is also important. Although individual services should be
less concerned about managing their log data in cloud environments, they should ensure that
appropriate log data and metrics are produced to facilitate automated problem detection and
alerting. This topic is explored further in Chapter 9, “Management and Operations” on
page 107.

1 https://avro.apache.org/
2 https://thrift.apache.org/
3 https://developers.google.com/protocol-buffers/
4 “1.2.2 Robustness Principle”, RFC 1122, https://tools.ietf.org/html/rfc1122#page-12

Microservices Best Practices for Java

1.4 Philosophy and team structure

There is a philosophical aspect to the relationship between microservices and team
organization. This aspect comes from the application of Conway’s law5, which states that “any
organization that designs a system will inevitably produce a design whose structure is a copy
of the organization’s communication structure.” In “Building Microservices”, Sam Newman
puts Conway’s law into context, and devotes an entire chapter to the interaction between an
organization’s structure and system design.

Conway's law specifically focuses on communication and coordination between disparate
groups of people. If the organization sequesters database administrators off into a group
separate from application developers, the resulting architecture has a distinct data tier
managed in a way that is optimal for database administrators. Application developers would
then need to coordinate changes that they need with database administrators. Conversely, if
the development team handles data management itself, this kind of coordination is
unnecessary.

A move to building microservices applications should include a discussion about
organizational structure and what changes might be necessary to streamline communication
patterns to complement the creation and growth of independent microservices. This is a
common thread in the convergence of DevOps and microservices. Creating an agile
ecosystem that is capable of fast and frequent change requires streamlined communications,
and lots of automation.

1.5 Examples

Throughout this book, we use two example applications to help explain concepts and provide
perspective:

(cid:2) Online retail store
(cid:2) Game On!

1.5.1 Online retail store

The online retail store is an example that is often used when discussing application
architectures. This book considers a set of microservices that together provide an application
where users can browse for products and place orders.

The architecture includes the following services:

(cid:2) Catalog service

Contains information about what can be sold in the store.

(cid:2) Account service

Manages the data for each account with the store.

(cid:2) Payment service

Provides payment processing.

(cid:2) Order service

Manages the list of current orders and performing tasks that are required in the lifecycle of
an order.

5 “How Do Committees Invent”, Melvin Conway, Datamation magazine, April 1968

Chapter 1. Overview

There are other ways to structure services for an online retail store. This set of services was
chosen to illustrate specific concepts that are described in this book. We leave the
construction of alternate services as an exercise for the reader.

1.5.2 Game On!

Game On!6 is an extensible, text-based adventure game built by using microservices. Unlike
the online store, Game On! is a real application, with code available on GitHub7, and a
GitBook8 describing the architecture and tools that are used to create the game in depth. This
deliberately non-standard example is used as a contrast to traditional enterprise applications.

The game is essentially a boundless two-dimensional network of interconnected rooms.
Rooms are provided by third parties (for example, developers at conferences or in
workshops). A set of core services provide a consistent user experience. The user logs in and
uses simple text commands to interact with and travel between rooms. Both synchronous and
asynchronous communication patterns are used. Interactions between players, core services,
and third-party services are also secured.

The architecture of Game On! is described in Chapter 2, “Creating Microservices in Java” on
page 9.

6 https://game-on.org/#/
7 https://github.com/gameontext/
8 https://gameontext.gitbooks.io/gameon-gitbook/content/

Microservices Best Practices for Java

Chapter 2. Creating Microservices in Java

The prospect of creating an application composed of microservices raises some questions in
any language:

(cid:2) How large should a microservice be?

(cid:2) What does the notion of focused services that do one thing mean for traditional centralized

governance?

(cid:2) What does it do to traditional ways of modeling data?

Some of these topics are covered in more depth in later chapters. This chapter focuses on
how you identify and create the microservices that compose your app, with specific emphasis
on how identified candidates are converted into RESTful APIs, and then implemented in Java.

This chapter covers the following topics:

(cid:2) Java platforms and programming models
(cid:2) Versioned dependencies
(cid:2) Identifying services
(cid:2) Creating REST APIs

2.1 Java platforms and programming models

New Java applications should minimally use Java 8, and should embrace new language
features like lambdas, parallel operations, and streams. Using annotations can simplify the
codebase by eliminating boilerplate, but can reach a subjective point where there is “too
much magic”. Try to establish a balance between code cleanliness and maintainability.

You can create perfectly functional microservices using nothing but low-level Java libraries.
However, it is generally recommended to have something provide a reasonable base level of
support for common, cross-cutting concerns. It allows the codebase for a service to focus on
satisfying functional requirements.

2.1.1 Spring Boot

Spring Boot provides mechanisms for creating microservices based on an opinionated view
of what technologies should be used. Spring Boot applications are Spring applications, and
use a programming model unique to that ecosystem.

Spring Boot applications can be packaged to run as an application hosted by an application
server, or run as a runnable .jar file containing the composition of dependencies and some
type of embedded server (usually Tomcat). Spring Boot emphasizes convention over
configuration, and uses a combination of annotations and classpath discovery to enable
additional functions.

Spring Cloud is a collection of integrations between third-party cloud technologies and the
Spring programming model. Some are integrations between the spring programming model
and third-party technologies. In fewer cases, Spring Cloud technologies are usable outside of
the Spring programming model, like Spring Cloud Connectors Core.

For more information about Spring Cloud Connectors, see the following website:

http://cloud.spring.io/spring-cloud-connectors/spring-cloud-connectors.html

2.1.2 Dropwizard

Dropwizard is another technology for creating microservices in Java. It also has an
opinionated approach, with a preselected stack that enhances an embedded Jetty servlet
container with additional capabilities. The set of technologies that Dropwizard supports is
comparatively narrow, although third party or community provided plug-ins are available to fill
the gaps. Some of their capabilities, specifically their support for application metrics, are quite
good.

2.1.3 Java Platform, Enterprise Edition

Java Platform, Enterprise Edition (Java EE) is an open, community driven standard for
building enterprise applications.

Java EE technologies are suited for creating microservices, especially Java API for RESTful
Web Services (JAX-RS), and Contexts and Dependency Injection (CDI) for Java EE.
Annotations are used for both specifications, with support for dependency injection,
lightweight protocols, and asynchronous interaction patterns. Concurrency utilities in Java
EE7 further provide a straightforward mechanism for using reactive libraries like RxJava in a
container friendly way.

Microservices Best Practices for Java

For more information about RxJava, see:

https://github.com/ReactiveX/RxJava

Modern Java EE application servers (WebSphere Liberty, Wildfly, TomEE, Payara) undercut
claims of painful dealings with application servers. All of these servers can produce
immutable, self-contained, lightweight artifacts in an automated build without requiring extra
deployment steps. Further, composable servers like WebSphere Liberty allow for explicit
declaration of spec-level dependencies, allowing the run time to be upgraded without creating
compatibility issues with the application.

Applications use a reduced set of Java EE technologies and are packaged with application
servers like WebSphere Liberty that tailor the run time to the needs of the application. This
process results in clean applications that can move between environments without requiring
code changes, mock services, or custom test harnesses.

The examples in this book primarily use Java EE technologies like JAX-RS, CDI, and Java
Persistence API (JPA). These specifications are easy to use, and are broadly supported
without relying on vendor-specific approaches.

2.2 Versioned dependencies

The twelve factor application methodology was introduced in Chapter 1, “Overview” on
page 1. In keeping with the second factor, use explicit versions for external dependencies.
The build artifact that is deployed to an environment must not make assumptions about what
is provided.

Build tools like Apache Maven1 or Gradle2 provide easy mechanisms for defining and
managing dependency versions. Explicitly declaring the version ensures that the artifact runs
the same in production as it did in the testing environment.

Tools are available to make creating Java applications that use Maven and Gradle easier.
These tools accept as inputs a selection of technologies, and output project structures
complete with the Maven or Gradle build artifacts that are required to build and either publish
or deploy the final artifact:

(cid:2) WebSphere Liberty App Accelerator

http://wasdev.net/accelerate

(cid:2) Spring Initializr

http://start.spring.io/

(cid:2) Wildfly Swarm Project Generator

http://wildfly-swarm.io/generator/

2.3 Identifying services

As mentioned in Chapter 1, “Overview” on page 1, microservices are described as being
“small.” The size of a service should be reflected in its scope, rather than in the size of its
codebase. A microservice should have a small, focused scope: It should do one thing, and do
that thing well. Eric Evans’ book, Domain-Driven Design, describes many concepts that are

1 https://maven.apache.org/
2 https://gradle.org/

Chapter 2. Creating Microservices in Java

helpful when deciding how to build an application as a composition of independent pieces.
Both the online retail store and Game On! are referenced in the next few sections to explain
why these concepts are useful for developing and maintaining microservice architectures in
the long term.

2.3.1 Applying domain-driven design principles

In domain-driven design, a domain is a particular area of knowledge or activity. A model is an
abstraction of important aspects of the domain that emerges over time, as the understanding
of the domain changes. This model is then used to build the solution, for cross-team
communications.

With monolithic systems, a single unified domain model exists for the entire application. This
concept works if the application remains fairly simple. Using a single unified model falls apart
when different parts of your application use common elements in different ways.

For example, the Catalog service in the Online Retail Store sample is focused on the items for
sale, including descriptions, part numbers, product information, pictures, and so on. The
Order service, by contrast, is focused on the invoice, with items represented minimally,
perhaps only the part number, summary, and calculated price.

Bounded contexts allow a domain to be subdivided into independent subsystems. Each
bounded context can then have its own models for domain concepts. Continuing the above
example, both the Catalog and Order services can each have independent representations of
items that best fit the needs of their service. This system usually works great, but how do the
two services share information with other services, given they have different representations
for the same concept?

Context mapping, including the development and use of ubiquitous language, can help
ensure that disparate teams working on individual services understand how the big picture
should fit together. In the case of modeling a domain for a microservices application, it is
important not to get down to the class or interface level when identifying the pieces that
comprise the system. For the first pass, keep things at a high, coarse level. Allow individual
teams working on the individual services work within their bounded contexts to expand on the
details.

Ubiquitous language in Game On!
To give a concrete example of the importance of language, we take a small detour into how
Game On! was developed. The game was built to make it easier for developers to experiment
with microservices concepts first hand, combining the instant gratification of a quick hello
world example with different ways to think about the more complex aspects of microservices
architectures. It is a text-based adventure game in which developers add their own individual
services (rooms) to extend the game.

The original metaphor for the game was akin to a hotel, with three basic elements:

(cid:2) A Player service to deal with all things relating to game players

(cid:2) An initial Room service as a placeholder for what developers will build

(cid:2) An additional service called the Concierge, the original purpose of which was to help

facilitate the transition of a player from room to room

After letting these three pieces of the system evolve over time, it was discovered that different
people working on the game had different interpretations of what the Concierge was
supposed to do. The Concierge did not work as a metaphor because it was too ambiguous.

Microservices Best Practices for Java

The Concierge was replaced with a new Map service. The Map is the ultimate source of truth
for where a room is located in the game, which is clearly understood from the name alone.

2.3.2 Translating domain elements into services

Domain models contain a few well-defined types:

(cid:2) An Entity is an object with a fixed identity and a well-defined “thread of continuity” or

lifecycle. An frequently cited example is a Person (an Entity). Most systems need to track a
Person uniquely, regardless of name, address, or other attribute changes.

(cid:2) Value Objects do not have a well-defined identity, but are instead defined only by their

attributes. They are typically immutable so that two equal value objects remain equal over
time. An address could be a value object that is associated with a Person.

(cid:2) An Aggregate is a cluster of related objects that are treated as a unit. It has a specific
entity as its root, and defines a clear boundary for encapsulation. It is not just a list.

(cid:2) Services are used to represent operations or activities that are not natural parts of entities

or value objects.

Domain elements in Game On!
As mentioned in 1.3.1, “The meaning of “small”” on page 4, the game was built around the
notion of a Player, an obvious Entity. Players have a fixed ID, and exist for as long as the
player’s account does. The player entity has a simple set of value objects, including the
player’s user name, favorite color, and location.

The other significant element of the game, rooms, do not map so obviously to an Entity. For
one thing, room attributes can change in various ways over time. These changeable attributes
are represented by a Value Object containing attributes like the room’s name, descriptions of
its entrances, and its public endpoint. Further, the placement of rooms in the map can change
over time. If you consider each possible room location as a Site, you end up with the notion of
a Map as an Aggregate of unique Site entities, where each Site is associated with changeable
room attributes.

Mapping domain elements into services
Converting domain elements into microservices can be done by following a few general
guidelines:

(cid:2) Convert Aggregates and Entities into independent microservices, by using

representations of Value Objects as parameters and return values.

(cid:2) Align Domain services (those not attached to an Aggregate or Entity) with independent

microservices.

(cid:2) Each microservice should handle a single complete business function.

Chapter 2. Creating Microservices in Java

This section explores this process by applying these guidelines to the domain elements listed
above for Game On!, as shown in Figure 2-1.

The Player service provides the resource APIs3 to work with Player entities. In addition to
standard create, read, update, and delete operations, the Player API provides additional
operations for generating user names and favorite colors, and for updating the player’s
location.

The Map service provides the resource APIs4 to work with Site entities, allowing developers
to manage the registration of room services with the game. The Map provides create, retrieve,
update, and delete operations for individual Sites within a bounded context. Data
representations inside the Map service differ from those shared outside the service, as some
values are calculated based on the site’s position in the map.

Figure 2-1 The Player and Map services

Some additional services from the game have not been discussed:

(cid:2) Mediator: The Mediator is a service that splintered out of the first implementation of the

Player service. Its sole responsibility is mediating between these WebSocket connections:

– One long running connection between the client device and the Mediator.

– Another connection between the Mediator and the target independent room service.

From a domain modeling point of view, the Mediator is a domain service.

(cid:2) Rooms: What about individual room services? Developers stand up their own instances of

rooms. For modeling purposes, treat rooms, even your own, as outside actors.

3 https://game-on.org/swagger/#!/players Game On! Player API
4 https://game-on.org/swagger/#!/map Game On! Map API

Microservices Best Practices for Java

2.3.3 Application and service structure

Monolithic Java EE applications are typically structured and deployed in tiers (client, web,
business, and data), with long lived application servers hosting web archives (WAR files),
enterprise archives (EAR files), or both for disparate parts of an application, as shown in the
left side of Figure 2-2. Naming and packaging conventions become important as JSPs, JSFs,
servlets, and EJBs for different parts of the application are deployed to the same application
server. Common data stores provide consistency for data representation, but also introduce
constraints in how data can be represented and stored.

Figure 2-2 Order and Catalog Services as part of monolithic or microservices application

A microservices architecture will likely still have these application tiers, but the elements of
each tier will likely be provided by independent services that run in independent processes,
as shown in the right side of Figure 2-2. Note these details:

(cid:2) The backend-for-frontend pattern is commonly used to support specialized behavior for

specific front ends, like optimizations for specific devices or providing additional
capabilities in a web application.

(cid:2) Although each service owns and maintains its own data store, remember that the data

store is itself an independent service.

Not all microservice applications will have all of these elements. For example, web
applications that use JSPs or JSF, or interpreted scripts like PHP run in the backend. They
could be considered as “backend-for-frontend”, especially if they have a close relationship
with JavaScript running in the browser. Game On! however, does not use the
backend-for-frontend pattern (at the time of this writing). The JavaScript single page web
application is served as a set of static resources from an independent service. The
application then runs in the browser, and calls backend APIs directly.

Chapter 2. Creating Microservices in Java

Internal structure
Structure your code inside a service so that you have a clear separation of concerns to
facilitate testing. Calls to external services should be separate from the domain logic, which
should also be separate from logic specific to marshalling data to/from a backing data service.

Figure 2-3 shows a simplified version of a microservice architecture. This architecture
separates the domain logic from any code that interacts with or understands external
services. The architecture is similar to the Ports and Adapters (Hexagonal) architecture5, and
to Toby Clemson’s approach in “Testing Strategies in a Microservice Architecture”6.

Figure 2-3 The internal structure of a microservice

The example internal structure considers the following elements:

(cid:2) Resources expose JAX-RS resources to external clients. This layer handles basic
validation of requests and then passes the information into the domain logic layer.

(cid:2) Domain logic, as a general concept, takes many forms. In Boundary-Entity-Control

patterns, domain logic represents the entity itself, with parameter validation, state change
logic, and so on.

(cid:2) Repositories are optional, but can provide a useful abstraction between the core

application domain logic and the data store when present. This configuration allows the
backing data store to be changed or replaced without extensive changes to the domain
logic.

(cid:2) Service connectors are similar to the Repository abstraction, encapsulating

communications with other services. This layer functions as either a façade or an
“anti-corruption” layer to protect domain logic from changes to external resource APIs, or
to convert between API wire formats and internal domain model constructs.

5 http://alistair.cockburn.us/Hexagonal+architecture
6 http://martinfowler.com/articles/microservice-testing/#anatomy-modules

Microservices Best Practices for Java

This architecture does require you to follow a few rules when creating classes. For example,
each class that you use to implement your service should perform one of these tasks:

(cid:2) Perform domain logic
(cid:2) Expose resources
(cid:2) Make external calls to other services
(cid:2) Make external calls to a data store

These are general recommendations for code structure that do not have to be followed
strictly. The important characteristic is to reduce the risk of making changes. For example, if
you want to change the data store, you only need to update the repository layer. You do not
have to search through every class looking for the methods that call the data store. If there
are API changes to external services, these changes can similarly be contained within the
service connector. This separation further simplifies testing, as described in Chapter 7,
“Testing” on page 77.

2.3.4 Shared library or new service?

A common principle in both Agile and Object Oriented methodologies is “don’t repeat
yourself” (DRY). Repeating the same code multiple times generally indicates that code should
be turned into a reusable “thing.” In traditional OO, this thing is an object, or perhaps a utility
class. However, especially in the case of ubiquitous utility classes, strict application of DRY
principles is the end of neatly decoupled modules.

Microservices development makes this backsliding into code spaghetti more difficult. Hard
process boundaries make oversharing of implementation details much harder, but sadly not
impossible. So, what do you do with common code?

(cid:2) Accept that there might be some redundancy
(cid:2) Wrap common code in a shared, versioned library
(cid:2) Create an independent service

Depending on the nature of the code, it might be best to accept redundant code and move on.
Given that each service can and should evolve independently, the needs of the service over
time might change the shape of that initially common code.

It is especially tempting to create shared libraries for Data Transfer Objects, which are thin
classes that serve as vehicles to change to and from the wire transfer format (usually JSON).
This technique can eliminate a lot of boiler plate code, but it can leak implementation details
and introduce coupling.

Client libraries are often recommended to reduce duplication and make it easier to consume
APIs. This technique has long been the norm for data stores, including new NoSQL data
stores that provide REST-based APIs. Client libraries can be useful for other services as well,
especially those using binary protocols. However, client libraries do introduce coupling
between the service consumer and provider. The service may also be harder to consume for
platforms or languages that do not have well-maintained libraries.

Shared libraries can also be used to ensure consistent processing for potentially complex
algorithms. For Game On!, for example, a shared library is used to encapsulate request
signing and signature verification, which is described in more detail in Chapter 6, “Application
Security” on page 67. This configuration allows you to simplify services within the system and
ensure that values are computed consistently. However, we do not provide a library for all
languages, so some services still must do it the hard way.

Chapter 2. Creating Microservices in Java

When should a shared library really be an independent service? An independent service is
independent of the language used, can scale separately, and can allow updates to take effect
immediately without requiring coordinated updates to consumers. An independent service
also adds extra processing, and is subject to all of the concerns with inter-service
communication that are mentioned in Chapter 4, “Microservice communication” on page 33. It
is a judgment call when to make the trade off, although if a library starts requiring backing
services, that is generally a clue that it is a service in its own right.

2.4 Creating REST APIs

So, what does a good microservices API look like? Designing and documenting your APIs is
important to ensure that it is useful and usable. Following conventions and using good
versioning policies can make the API easier to understand and evolve over time.

2.4.1 Top down or bottom up?

When creating a service from scratch, you can either define the APIs that you need first and
write your code to fulfill the APIs, or write the code for the service first and derive your APIs
from the implementation.

In principle, the first option is always better. A microservice does not operate in isolation.
Either it is calling something, or something is calling it. The interactions between services in a
microservices architecture should be well defined and well documented. Concerning domain
driven design, APIs are the known intersection points between bounded contexts.

As Alan Perlis says, “Everything should be built from the top down, except the first time7.”
When doing a proof of concept, you might be unclear about what APIs you are providing. In
those early days, write something that works to help refine ideas about what it is that you are
trying to build. However, after you have a better understanding of what your service needs to
do, you should go back and refine the domain model and define the external APIs to match.

2.4.2 Documenting APIs

Using tools to document your APIs can make it easier to ensure accuracy and correctness of
the published documentation, which can then be used in discussions with API consumers.
This documentation can also be used for your consumer driven contract tests (see Chapter 7,
“Testing” on page 77).

The Open API Initiative (OAI)8 is a consortium focused on standardizing how RESTful APIs
are described. The OpenAPI specification9 is based on Swagger10, which defines the
structure and format of metadata used to create a Swagger representation of a RESTful API.
This definition is usually expressed in a single, portable file (swagger.json by convention,
though YAML is supported by using swagger.yaml). The swagger definition can be created by
using visual editors or generated based on scanning annotations in the application. It can
further be used to generate client or server stubs.

7 http://www.cs.yale.edu/homes/perlis-alan/quotes.html
8 https://openapis.org/
9 https://github.com/OAI/OpenAPI-Specification
10 http://swagger.io/

Microservices Best Practices for Java

The apiDiscovery feature11 in WebSphere Application Server Liberty integrates support for
Swagger definitions into the run time. It automatically detects and scans JAX-RS applications
for annotations that it uses to generate a Swagger definition dynamically. If Swagger
annotations are present, information from those annotations is included to create informative
documentation that can be maintained inline with the code. This generated documentation is
made available by using a system provided endpoint, as shown in Figure 2-4.

Figure 2-4 The user interface provided by the apiDisovery-1.0 feature to document APIs

The Swagger API definition, especially as rendered in a web interface, is useful for visualizing
the external, consumer view of the API. Developing or designing the API from this external
view helps ensure consistency. This external view aligns with Consumer Driven Contracts12,
a pattern that advocates designing APIs with a focus on consumer expectations.

2.4.3 Use the correct HTTP verb

REST APIs should use standard HTTP verbs for create, retrieve, update, and delete
operations, with special attention paid to whether the operation is idempotent (safe to repeat
multiple times).

POST operations may be used to Create(C) resources. The distinguishing characteristic of a
POST operation is that it is not idempotent. For example, if a POST request is used to create
resources, and it is started multiple times, a new, unique resource should be created as a
result of each invocation.

11 https://developer.ibm.com/wasdev/blog/2016/02/17/exposing-liberty-rest-apis-swagger/
12 http://martinfowler.com/articles/consumerDrivenContracts.html

Chapter 2. Creating Microservices in Java

GET operations must be both idempotent and nullipotent. They should cause no side effects,
and should only be used to Retrieve(R) information. To be specific, GET requests with query
parameters should not be used to change or update information (use POST, PUT, or PATCH
instead).

PUT operations can be used to Update(U) resources. PUT operations usually include a
complete copy of the resource to be updated, making the operation idempotent.

PATCH operations allow partial Update(U) of resources. They might or might not be
idempotent depending on how the delta is specified and then applied to the resource. For
example, if a PATCH operation indicates that a value should be changed from A to B, it
becomes idempotent. It has no effect if it is started multiple times and the value is already B.
Support for PATCH operations is still inconsistent. For example, there is no @PATCH annotation
in JAX-RS in Java EE7.

DELETE operations are not surprisingly used to Delete(D) resources. Delete operations are
idempotent, as a resource can only be deleted once. However, the return code varies, as the
first operation succeeds (200), while subsequent invocations do not find the resource (204).

2.4.4 Create machine-friendly, descriptive results

An expressive REST API should include careful consideration of what is returned from started
operations. Given that APIs are started by software instead of by users, care should be taken
to communicate information to the caller in the most effective and efficient way possible.

As an example, it used to be common practice to return a 200 (OK) status code along with
HTML explaining the error message. Although this technique might work for users viewing
web pages (for whom the HTTP status code is hidden anyway), it is terrible for machines
determining whether their request succeeded.

The HTTP status code should be relevant and useful. Use a 200 (OK) when everything is fine.
When there is no response data, use a 204 (NO CONTENT) instead. Beyond that technique,
a 201 (CREATED) should be used for POST requests that result in the creation of a resource,
whether there is a response body or not. Use a 409 (CONFLICT) when concurrent changes
conflict, or a 400 (BAD REQUEST) when parameters are malformed. The set of HTTP status
codes is robust, and should be used to express the result of a request as specifically as
possible. For more information, see the following website:

https://www.w3.org/Protocols/rfc2616/rfc2616-sec10.html

You should also consider what data is being returned in your responses to make
communication efficient. For example, when a resource is created with a POST request, the
response should include the location of the newly created resource in a Location header. Yo
do not need to return additional information about the resource in the response body.
However, in practice the created resource is usually included in the response because it
eliminates the need for the caller to make an extra GET request to fetch the created resource.
The same applies for PUT and PATCH requests.

2.4.5 Resource URIs and versioning

There are varying opinions about some aspects of RESTful resource URIs. In general,
resources should be nouns, not verbs, and endpoints should be plural. This technique results
in a clear structure for create, retrieve, update, and delete operations:

POST /accounts
GET /accounts

Create a new item
Retrieve a list of items

Microservices Best Practices for Java

GET /accounts/16
Retrieve a specific item
PUT /accounts/16
Update a specific item
PATCH /accounts/16 Update a specific item
DELETE /accounts/16 Delete a specific item

The general consensus is to keep things simple with URIs. If you create an application that
tracks a specific kind of rodent, the URI would be /mouses/ and not /mice/, even if it makes
you wince.

Relationships are modeled by nesting URIs, for example, something like
/accounts/16/credentials for managing credentials associated with an account.

Where there is less agreement is with what should happen with operations that are
associated with the resource, but that do not fit within this usual structure. There is no single
correct way to manage these operations. Do what works best for the consumer of the API.

For example, with Game On!, the Player service provides operations that assist with
generating user names and favorite colors. It is also possible to retrieve, in bulk, the location
of every player within the game. The structure that was chosen reflects operations that span
all players, while still allowing you to work with specific players. This techniques has its
limitations, but illustrates the point:

GET /players/accounts
POST /players/accounts
GET /players/accounts/{id}
GET /players/accounts/{id}/location Retrieve a specific player’s location
PUT /players/accounts/{id}/location Update a specific player’s location
GET /players/color
GET /players/name
GET /players/locations

Return 10 generated colors
Return 10 generated names
Retrieve the location of all players

Retrieve a list of all players
Create a new player
Retrieve a specific player

Versioning
One of the major benefits of microservices is the ability to allow services to evolve
independently. Given that microservices call other services, that independence comes with a
giant caveat. You cannot cause breaking changes in your API.

The easiest approach to accommodating change is to never break the API. If the robustness
principle is followed, and both sides are conservative in what they send and liberal in what
they receive, it can take a long time before a breaking change is required. When that breaking
change finally comes, you can opt to build a different service entirely and retire the original
over time, perhaps because the domain model has evolved and a better abstraction makes
more sense.

If you do need to make breaking API changes for an existing service, decide how to manage
those changes:

(cid:2) Will the service handle all versions of the API?

(cid:2) Will you maintain independent versions of the service to support each version of the API?

(cid:2) Will your service support only the newest version of the API and rely on other adaptive

layers to convert to and from the older API?

After you decide on the hard part, the much easier problem to solve is how to reflect the
version in your API. There are generally three ways to handled versioning a REST resource:

(cid:2) Put the version in the URI
(cid:2) Use a custom request header
(cid:2) Put the version in the HTTP Accept header and rely on content negotiation

Chapter 2. Creating Microservices in Java

Put the version in the URI
Adding the version into the URI is the easiest method for specifying a version. This approach
has these advantages:

(cid:2) It is easy to understand.
(cid:2) It is easy to achieve when building the services in your application.
(cid:2) It is compatible with API browsing tools like Swagger and command-line tools like curl.

If you’re going to put the version in the URI, the version should apply to your application as a
whole, so use, for example, /api/v1/accounts instead of /api/accounts/v1. Hypermedia as
the Engine of Application State (HATEOAS) is one way of providing URIs to API consumers
so they are not responsible for constructing URIs themselves. GitHub, for example, provides
hypermedia URLs in their responses for this reason.13 HATEOAS becomes difficult if not
impossible to achieve if different backend services can have independently varying versions
in their URIs.

If you decide to place the version in your URI, you can manage it in a few different ways,
partly determined by how requests are routed in your system. However, assume for a moment
that your gateway proxy coverts the external /api/v1/accounts URI to /accounts/v1, which
maps to the JAX-RS endpoint provided by your service. In this case, /accounts is a natural
context root. After that you can either include the version in the @ApplicationPath annotation,
as shown in Example 2-1, which works well when the application is supporting only one
version of the API.

Example 2-1 Using the @ApplicationPath annotation to add the version to the URI

package retail_store.accounts.api;
@ApplicationPath("/v1")
public class ApplicationClass extends Application {}

Alternately, if the service should support more than one version of the application, you can
push the version to @Path annotations, as shown in Example 2-2.

Example 2-2 The @ApplicationPath defines a rest endpoint without changing the URI

package retail_store.accounts.api.v1;
@ApplicationPath("/")
public class ApplicationClass extends Application {}

Example 2-3 shows the version added to the annotation.

Example 2-3 The version is added to the API using the @Path annotation

package retail_store.accounts.api.v1;
@Path("v1/accounts")
public class AccountsAPI {
@GET
@Path("{id}")
@Produces(MediaType.APPLICATION_JSON)
public Response getAccounts(@PathParam("id") String id) {
getId(id);
…
}
}

13 https://developer.github.com/v3/#hypermedia

Microservices Best Practices for Java

One of the arguments against including the version in the URL is a strict interpretation of the
HTML standard that a URL should represent the entity and if the entity being represented
didn’t change, the URL should not change.

Another concern is that putting versions in the URI requires consumers to update their URI
references. This concern can be partly resolved by allowing requests to be made without a
version, and then mapping those requests to the latest version. However, this approach is
prone to unexpected behavior when the latest version changes.

Add a custom request header
You can add a custom request header to indicate the API version. Custom headers can be
taken into consideration by routers and other infrastructure to route traffic to specific backend
instances. However, this mechanism is not as easy to use for all of the same reasons that
Accept headers are not easy to use. In addition, it is a custom header that only applies to your
application, which means consumers need to learn how to use it.

Modify the Accept header to include the version
The Accept header is an obvious place to define a version, but is one of the most difficult to
test. URIs are easy to specify and replace, but specifying HTTP headers requires more
detailed API and command-line invocations.

Plenty of articles describe the advantages and disadvantages of all three methods. This blog
post by Troy Hunt is a good place to start:

https://www.troyhunt.com/your-api-versioning-is-wrong-which-is/

The following are the most important things to take away from this chapter:

(cid:2) Design your APIs from the consumer point of view.
(cid:2) Have a strategy for dealing with API changes.
(cid:2) Use a consistent versioning technique across all of the services in your application.

Chapter 2. Creating Microservices in Java

Microservices Best Practices for Java

Chapter 3. Locating services

Microservices are designed to be easily scaled. This scaling is done horizontally by scaling
individual services. With multiple instances of microservices, you need a method for locating
services and load balancing over the different instances of the service you are calling. This
chapter describes the options available for locating and making requests to microservices in a
system.

The following topics are covered:

(cid:2) Service registry
(cid:2) Service invocation
(cid:2) API Gateway

3.1 Service registry

A service registry is a persistent store that holds a list of all the available microservices at any
time and the routes that they can be reached on. There are four reasons why a microservice
might communicate with a service registry:

(cid:2) Registration

After a service has been successfully deployed, the microservice must be registered with
the service registry.

(cid:2) Heartbeats

The microservice should send regular heartbeats to the registry to show that it is ready to
receive requests.

(cid:2) Service discovery

To communicate with another service, a microservice must call the service registry to get
a list of available instances (see 3.2, “Service invocation” on page 27 for more details).

(cid:2) De-registration

When a service is down, it must be removed from the list of available services in the
service registry.

3.1.1 Third-party registration versus self-registration

The registration of the microservice with the service registry can be done by the microservice
or by a third party. Using a third party requires said party to inspect the microservice to
determine the current state and relay that information to the service registry. The third party
also handles de-registration. If the microservice itself performs registration and heartbeats,
the service registry can unregister the service if it misses a heartbeat.

The advantage of using a third party is that the registration and heartbeat logic is kept
separate from the business logic. The disadvantage is that there is an extra piece of software
to deploy and the microservice must be exposed to a health endpoint for the third party to
poll.

Using self-registration pulls the registration and heartbeat logic into the microservice itself.
Using this method requires careful testing considerations. See Chapter 7, “Testing” on
page 77 for more information about separation of code. However, many service registry
solutions provide a convenience library form registration, reducing the complexity of the code
required.

Generally, use a service registry that provides a Java based library for registration and
heartbeats. This configuration makes your services quicker to code and provide consistency
across every microservice in the system (that is in Java). The following service registry
solutions are available:

(cid:2) Consul

https://www.consul.io/

(cid:2) Eureka

https://github.com/Netflix/eureka

(cid:2) Amalgam8

https://github.com/amalgam8/

Microservices Best Practices for Java

All of these registries are open source and provide integration for Java. As a part of the Netflix
stack, Eureka is well integrated with the other Netflix solutions for load balancing and fault
tolerance. Using Eureka as the service registry allows you to easily add the other solutions.
The Amalgam8 open source project provides both service registration and load balancing. It
also provides routing configuration that can be used during testing. For more information, see
Chapter 7, “Testing” on page 77.

3.1.2 Availability versus consistency

Most service registries provide partition tolerance and either consistency or availability. They
cannot provide all three due to the CAP theorem1. For example, Eureka provides availability
and both Consul and Apache Zookeeper2 provide consistency. Some articles argue that you
should always choose one or the other (this blog post3 on the Knewton blog is worth a read),
but the decision comes down to the precise needs of your application. If you require all of your
microservices to have the same view of the world at a time, then choose consistency. If it is
essential that requests are answered quickly rather than waiting for a consistent answer, then
choose availability. Solutions are available for both that integrate well with Java.

3.2 Service invocation

When a service needs to communicate with another service, it uses the information stored in
the service registry. The call to the actual microservice is then either made on the server side
or the client side.

3.2.1 Server side

Server-side communication with the microservice is performed by using a service proxy. The
service proxy is either provided as part of the service registry, or as a separate service. When
a microservice needs to make a call, it calls the service proxy's well-known endpoint. The
service proxy is responsible for getting the location of the microservice from the service
registry and forwarding the request. The proxy takes the response and routes it back to the
original microservice that made the request. In the example scenario, load balancing is
handled completely on the server side.

1 https://en.wikipedia.org/wiki/CAP_theorem
2 https://zookeeper.apache.org/
3 https://tech.knewton.com/blog/2014/12/eureka-shouldnt-use-zookeeper-service-discovery/

Chapter 3. Locating services

Figure 3-1 shows the flow of requests when using a service proxy.

Figure 3-1 Calls to other microservices are made by using the service proxy

Using server-side invocation provides these key advantages:

(cid:2) Simple requests

The request made by the microservice can be simple because it is calling a well-known
endpoint.

(cid:2) Easier testing

Using a server proxy takes any load balancing or routing logic out of the microservice. This
configuration allows you to test the full function of the service using a mocked proxy.

In cloud platforms such as Cloud Foundry and Bluemix, the proxy is provided by the platform.
In Bluemix, every deployed application has a well-known endpoint, called a route. When
another service or application calls this route, Bluemix does load balancing in the
background.

This approach has the following key disadvantage:

(cid:2) Greater number of hops

Adding in a call to the service proxy increases the number of network hops per request. As
a result, the number of hops per request is generally higher than for client-side solutions. It
is worth noting that the service proxy can hold cached information about the other
microservices, so in many cases the request would go through four hops: 1 → 4 → 5 → 6.

Microservices Best Practices for Java

Figure 3-2 shows the network hops required.

Figure 3-2 Network hops are required to complete a request service when using a service proxy

3.2.2 Client side

The request from one microservice to another can be made directly from the client side. First,
the location of one or several instances of a service is requested from the service registry.
Second, the request to the microservice is made on the client side. The location of the
microservice is usually cached so that, in the future, a request can be made without returning
to the service registry. If in the future the request fails, then the client can repeat the call to the
service registry. It is a best practice to have the cached microservice location on a timeout.
This configuration means that if a new version of a service is deployed, the other
microservices do not have to wait for an error from their cached instance before being
informed about the new instance.

Chapter 3. Locating services

In comparison to the service proxy approach, this approach requires fewer network hops.
Figure 3-3 shows the network hops required. Again, this number of hops is for when the client
does not have cached information about the services. In many cases, the request to the
service registry is not required, making it just two hops: 3 → 4.

Figure 3-3 Network hops when making a client-side request to the service registry

The request and any load balancing that is required can be handled by one of two
mechanisms:

(cid:2) Client library
(cid:2) Sidecar

Microservices Best Practices for Java

Both mechanisms make requests on the client side. However, the client library runs within the
microservice and the sidecar is deployed with the microservice, but runs on a separate
process. Figure 3-4 shows an example architecture diagram.

Figure 3-4 Requests are either made by the client library or by the sidecar process

Client library
Client libraries can be useful for many reasons, one of which is isolating the specifics of
communicating with a remote resource. For service registries, client libraries like those
provided by Consul or Netflix Eureka handle service registration and heart beating. Other
libraries, such as Netflix Ribbon, provide client-side load balancing.

Generally start with an existing library, rather than writing your own. If all of the microservices
in your system are written in Java, then you should standardize your library. If you have a
polyglot system of microservices, then standardize on one for each language. By using the
same library everywhere, you maintain the flexibility to move developers from microservice to
microservice and avoid over-complicating your infrastructure and build process.

The disadvantage of using a client library is that you have now moved the complex service
calls back into your application. This configuration makes it more complicated to test your
service. Make sure to keep a good separation between the code that uses the client library
and the business logic in your microservice. See Chapter 7, “Testing” on page 77 for more
details about code separation to enable easier testing.

Sidecar
A good middle ground between the service proxy and a client library is a sidecar. A sidecar is
a separate process that is deployed with your microservice, either as a separate process
within a container, or as a separate, but closely related container.

Sidecars maintain the service registration with the service registry, and usually also perform
client-side load balancing for outbound calls to other microservices.

Chapter 3. Locating services

Because sidecars run in their own process, they are independent of language. You can use
the same sidecar implementation for all of the microservices in a polyglot application,
regardless of the language that the service is written in.

Netflix Prana is an open source sidecar that provides the capabilities of the Java based
Ribbon and Eureka client libraries to non-Java applications. For more information, see the
following website:

http://github.com/Netflix/Prana

Kubernetes is an orchestration engine for containers that provides sidecar like service
discovery and load balancing capabilities. For more information, see the following website:

http://kubernetes.io/docs/user-guide/services

Amalgam8 is a solution that can be used with and without containers. It manages service
registration and provides client-side service discovery, routing, and load balancing. For more
information, see the following website:

http://amalgam8.io

Choosing a solution that handles several of these elements can reduce the complexity of your
services and your infrastructure.

3.3 API Gateway

An API Gateway can be used to perform service invocation for both internal and external
clients. The API Gateway performs a similar role to a service proxy (see 3.2.1, “Server side”
on page 27). Services make requests to the API Gateway that look up the service in the
registry, makes the request, and then returns the response. There is a difference between the
two, however. Requests to a service proxy use the exact API that the end service provides.
Requests to an API Gateway use the API that the gateway provides. This configuration
means that the API Gateway can provide different APIs to the ones that the microservice
provides.

The internal microservices can use the exact APIs, while API Gateways provide most benefit
to external clients. Typically microservices provide fine grained APIs. By contrast, an external
client that uses the application probably does not need an API that is as fine grained. It might
also need an API that uses information from several microservices. The API Gateway
exposes APIs to the external client that are helpful to them and hides the actual
implementation of the microservices behind the Gateway. All external clients access the
application through this API Gateway.

Microservices Best Practices for Java

Chapter 4. Microservice communication

In a distributed system, inter-service communication is vital. The microservices that make up
your application must work together seamlessly to provide value to your clients.

Chapter 3, “Locating services” on page 25 discussed how you can locate a particular service
and the options for load balancing across different instances.

This chapter covers the communication between different services in a microservice
architecture after the location of the required service is determined. This description includes
synchronous and asynchronous communication, and how to achieve resilience.

The following topics are covered:

(cid:2) Synchronous and asynchronous
(cid:2) Fault tolerance

4.1 Synchronous and asynchronous

Synchronous communication is a request that requires a response, whether that be
immediately or after some amount of time. Asynchronous communication is a message
where no response is required.

A strong case can be made for using asynchronous events or messaging in highly distributed
systems that are built from independent parts. There are situations where using synchronous
calls is more appropriate, which are described in 4.1.1, “Synchronous messaging (REST)”.

For either invocation style, services should document any published APIs using tools like
Swagger. Event or message payloads should also be documented to ensure that the system
is comprehensible, and enable future consumers. Event subscribers and API consumers
should tolerate unrecognized fields, as they might be new. Services should also expect and
handle bad data. Assume that everything will fail at some point.

4.1.1 Synchronous messaging (REST)

As stated previously, in distributed systems, asynchronous forms of messaging are highly
valuable. Where clear request/response semantics apply, or where one service needs to
trigger a specific behavior in another service, synchronous APIs should be used instead.

These are usually RESTful operations that pass JSON formatted data, but other protocols
and data formats are possible. In a Java based microservice, having the applications pass
JSON data is the best choice. There are libraries to parse JSON to Java objects, and JSON is
becoming widely adopted, making it a good choice to make your microservice easily
consumable.

Async support for JAX-RS
Although JAX-RS requests will always be synchronous (you always require a response), you
can take advantage of asynchronous programming models. The asynchronous support in
JAX-RS 2.0 allows threads to perform other requests while waiting for the HTTP response. In
a stateless EJB bean, an AsyncResponse1 object instance is injected and bound to the
processing of the active request by using the @Suspended2 annotation. The @Asynchronous3
annotation allows work to be offloaded from the active thread. Example 4-1 shows the code
that is used to return the response.

Example 4-1 JAX-RS 2.0 includes asynchronous support

@Stateless
@Path("/")
public class AccountEJBResource {
@GET
@Asynchronous
@Produces(MediaType.APPLICATION_JSON)
public void getAccounts(@Suspended final AsyncResponse ar) {
Collection<Account> accounts = accountService.getAccounts();
Response response = Response.ok(accounts).build();
ar.resume(response);
}

1 http://docs.oracle.com/javaee/7/api/javax/ws/rs/container/AsyncResponse.html
2 http://docs.oracle.com/javaee/7/api/javax/ws/rs/container/Suspended.html
3 http://docs.oracle.com/javaee/6/api/javax/ejb/Asynchronous.html

Microservices Best Practices for Java

To see more examples of using the asynchronous support in JAX-RS 2.0, see the following
GitHub project:

https://github.com/WASdev/sample.async.jaxrs

Another option is using reactive libraries like RxJava1. RxJava is a Java implementation of
ReactiveX2, a library that is designed to aggregate and filter responses from multiple parallel
outbound requests. It extends the observer pattern3 and also integrates well with libraries that
are designed to handle load balancing and add resilience to your endpoints such as FailSafe4
and Netflix Hystrix5.

4.1.2 Asynchronous messaging (events)

Asynchronous messaging is used for decoupled coordination. An asynchronous event can
only be used if the creator of the event does not need a response.

A request from an external client generally must go through several microservices before
returning a response. If each of these calls is made synchronously, then the total time for the
call holds up other requests. The more complex the microservice system and the more
interactions between microservices required per external request, the bigger the resultant
latency in your system. If the requests made in the process can be replaced with
asynchronous events, then you should implement the event instead.

Use a reactive style for events. A service should only publish events about its own state or
activity. Other services can then subscribe to those events and react accordingly.

Events are an excellent way to compose new interaction patterns without introducing
dependencies. They enable the extension of the architecture in ways you had not considered
when it was first created.

The microservices pattern requires each microservice to own its own data. This requirement
means when a request comes in, there might be several services that need to update their
databases. Services should advertise data changes by using events that other interested
parties can subscribe to. To learn more about using events for data transactions and
consistency, see Chapter 5, “Handling data” on page 41.

To coordinate the messaging of your application, you can use any one of the available
message brokers and matching client libraries. Some examples that integrate with Java are
the AMQP broker6 with the RabibitMQ Java client7, Apache Kafka8, and MQTT9, which is
aimed at the Internet of Things space.

Internal messaging
Events can be used within a single microservice. For example, an order request could be
processed by creating a set of events that the service connector classes subscribe to which
triggers them to publish the external events. Using internal events can increase the speed of
your response to synchronous requests because you do not have to wait for other
synchronous requests to complete before returning a response.

1 https://github.com/ReactiveX/RxJava
2 http://reactivex.io/
3 https://en.wikipedia.org/wiki/Observer_pattern
4 https://github.com/jhalterman/failsafe
5 https://github.com/Netflix/Hystrix
6 https://www.amqp.org/
7 https://www.rabbitmq.com/
8 http://kafka.apache.org/
9 http://mqtt.org/

Chapter 4. Microservice communication

Take advantage of the Java EE specification support for events. Internal asynchronous events
can be done by using Contexts and Dependency Injection (CDI)1. In this model, an event
consists of a Java event object and a set of qualifier types. Any methods wanting to react to
events fired use the @Observes annotation. Events are fired by using the fire() method.
Example 4-2 shows the methods and annotations in use.

Example 4-2 Events using Context and Dependency Injection

public class PaymentEvent {
public String paymentType;
public long value;
public String dateTime;
public PaymentEvent() {}
}
public class PaymentEventHandler {
public void credit(@Observes @Credit PaymentEvent event) {
…
}
}
public class Payment {
@Inject
@Credit
Event<PaymentEvent> creditEvent;
Public String pay() {
PaymentEvent paymentPayload = new PaymentEvent();
// populate payload…
crediteEvent.fire(paymentPayload);
…
}
}

4.1.3 Examples

To illustrate clearly the scenarios where you should use synchronous and asynchronous
communication this section covers two examples: Game On! and an online retail store. See
1.5, “Examples” on page 7 for an introduction to the examples.

Game On!
Consider a user who has authenticated with Game On! and wants to update a room.
Figure 4-1 on page 37 shows the communication flow with the requests numbered for
reference. When the user sends an update room request to the map service, they expect a
confirmation response, which means the request must be synchronous. This synchronous
request (1) returns a confirmation that either the update was successful or that the update
has been successfully added to a queue (depending on the implementation of the update
function).

After the update has occurred, that is the room has been updated in the database using a
synchronous request (2), the map service needs to notify the other services about the
change. This update does not require a response, so it is sent by using an asynchronous
event (3) that other services are subscribed to (4).

1 http://docs.oracle.com/javaee/6/tutorial/doc/gkhic.html

Microservices Best Practices for Java

Figure 4-1 Communication pattern used to update a room

Online retail store
Consider a user wanting to place an order on an online retail store. After the user has entered
all the details of the order, they click the Submit button. The user expects a response to
confirm that the order has gone through, so the Submit button triggers a synchronous
request that returns a response. After the order has been submitted, the application starts
processing the order. The service that received the order request sends out asynchronous
events that various other services are subscribed to. These events include inventory updates,
shipping events, and payment events. Some services trigger other synchronous or
asynchronous requests in response to the events. For example, the inventory service uses a
synchronous request to update the inventory database.

4.2 Fault tolerance

A strong motivator for moving to a microservice architecture is to get a more fault tolerant and
resilient application. Modern applications are required to have near zero downtime and
respond to requests within seconds rather than minutes. Each individual service in a
microservice must be able to continue functioning even if the other services have gone down.
This section focuses on the techniques and tools you should use when communicating with
other microservices to make your microservice fault tolerant and resilient.

4.2.1 Resilient against change

When a microservice makes a synchronous request to another microservice, it uses a
specific API. The API has defined input attributes that must be included in the request and
output attributes that are included in the response. In a microservice environment, this is

Chapter 4. Microservice communication

usually in the form of JSON data that is passed from service to service. It is unrealistic to
assume that these input and output attributes will never change. Even in the best designed
application, requirements are constantly changing, causing attributes to be added, removed,
and changed. To be resilient against these forms of changes. the producer of a microservice
must carefully consider the APIs they produce, and the APIs they consume. For more best
practices concerning defining APIs including versioning, see Chapter 2, “Creating
Microservices in Java” on page 9.

Consuming APIs
As the consumer of an API, it is necessary to do validation on the response you receive to
check that it contains the information needed for the function being performed. If you are
receiving JSON, you also need to parse the JSON data before performing any Java
transformations. When doing validation or JSON parsing, you must do these two things:

(cid:2) Only validate the request against the variables or attributes that you need

Do not validate against variables just because they are provided. If you are not using them
as part of your request, do not rely on them being there.

(cid:2) Accept unknown attributes

Do not issue an exception if you receive an unexpected variable. If the response contains
the information you need, it does not matter what else is provided alongside.

Select a JSON parsing tool that allows you to configure the way it parses the incoming data.
The Jackson Project1 provides annotations to configure the parser as shown in Example 4-3.
The @JsonInclude and @JsonIgnoreProperties annotations provided by the Jackson library
are used to indicate which values should be used during serialization and whether to ignore
unknown properties.

Example 4-3 Jackson annotations for JSON parsing

@JsonInclude(Include.NON_EMPTY)
@JsonIgnoreProperties(ignoreUnknown = true)
Public class Account {
// Account ID
@JsonProperty("id")
protected String id;
}

By following both these rules, your microservice can be resilient against any changes that do
not directly affect it. Even if the producer of the microservice removes or adds attributes, if
they are not the ones you are using, your microservice continues functioning normally.

Producing APIs
When providing an API to external clients, do these two things when accepting requests and
returning responses:

(cid:2) Accept unknown attributes as part of the request

If a service has called your API with unnecessary attributes, discard those values.
Returning an error in this scenario just causes unnecessary failures.

(cid:2) Only return attributes that are relevant for the API being invoked

Leave as much room as possible for the implementation of a service to change over time.
Avoid leaking implementation details by oversharing.

1 https://github.com/FasterXML/jackson

Microservices Best Practices for Java

As mentioned earlier, these rules make up the Robustness Principle. By following these two
rules you put yourself in the best possible position for changing your API in the future and
adapting to changing requirements from consumers. For more information about the
Robustness Principle, see the following website:

https://en.wikipedia.org/wiki/Robustness_principle

4.2.2 Timeouts

When making a request to another microservice, whether asynchronously or not, the request
must have a timeout. Because we expect services to come and go, we should not be waiting
indefinitely for a response.

The asynchronous support in JAX-RS 2.0 has a setTimeout function on the AsynResponse
object (see “Synchronous messaging (REST)” on page 34). Tools like Failsafe and Netflix
Hystrix have built-in support for timeouts.

Setting a timeout improves the resilience of your microservice, but can still result in a poor
user experience because the end-to-end path will be slow. It is also possible for timeouts to
be badly aligned so that the request from service A to service B times out while service B is
waiting for a response from service C.

4.2.3 Circuit breakers

Using a timeout prevents a request from waiting indefinitely for a response. However, if a
particular request always produces a timeout, you waste a lot of time waiting for the timeout to
occur.

A circuit breaker is designed to avoid repeating timeouts. It works in a similar way to the
electronic version of the same name. The circuit breaker makes a note every time a specific
request fails or produces a timeout. If this count reaches a certain level, it prevents further
calls from occurring, instead of returning an error instantly. It also incorporates a mechanism
for retrying the call, either after a certain amount of time or in reaction to an event.

Circuit breakers are vital when calling external services. Generally, use one of the existing
libraries for circuit breakers in Java.

4.2.4 Bulkheads

In shipping, a bulkhead is a partition that prevents a leak in one compartment from sinking the
entire ship. Bulkheads in microservices are a similar concept. You need to make sure that a
failure in one part of your application doesn’t take down the whole thing. The bulkhead pattern
is about how you create microservices rather than the use of any specific tool or library. When
creating microservices, always ask yourself how you can isolate the different parts and
prevent cascading failures.

The simplest implementation of the bulkhead pattern is to provide fallbacks. Adding a fallback
allows the application to continue functioning when non-vital services are down. For example,
in an online retail store there might be a service that provides recommendations for the user.
If the recommendation service goes down, the user should still be able to search for items
and place orders. An interesting example is chained fallbacks, where a failed request for
personalized content falls back to a request for more general content, which would in turn fall
back to returning cached (and possibly stale) content in preference to an error.

Chapter 4. Microservice communication

Another strategy for preventing a slow or constrained remote resource from bringing down the
whole ship is to limit the resources available for those outbound requests. The most common
ways of doing this use queues and Semaphore. For information about Semaphore, see the
following website:

https://docs.oracle.com/javase/7/docs/api/java/util/concurrent/Semaphore.html

A queue has a depth that indicates a maximum amount of pending work. Any requests that
are made after the queue is full get a fast failure. Queues are also assigned a finite number of
workers, which defines the maximum number of server threads that can be blocked by the
remote resource.

A semaphore mechanism works by having a set number of permits. A permit is required to
make the remote request. After a request has completed successfully, the permit is released.

The significant difference between the two is in what happens when the allocated resources
are all used up. With a semaphore approach, the outbound request is skipped entirely if the
permit cannot be obtained, although the queue approach does offer some padding (at least
until queue is full).

These approaches can also be used to deal with remote resources that impose rate limits,
which are yet another form of unexpected failure to deal with.

Microservices Best Practices for Java

Chapter 5. Handling data

In an environment with polyglot persistence, which can exist in a system of microservices, it is
necessary to keep the data handling manageable. To explain how this goal can be achieved,
this chapter provides a short description of characteristics of microservices concerning data
handling and then looks how it can be done with Java based microservices.

The following topics are covered:

(cid:2) Data-specific characteristics of a microservice
(cid:2) Support in Java

5.1 Data-specific characteristics of a microservice

One way to identify the data, which must be stored in the data store of your microservice, is a
top down approach. Start at the business level to model your data. The following sections
show how to identify this data, how to handle the data, and how it can be shared with other
data stores of other microservices. For a description of the top down approach, see the
following website:

http://en.wikipedia.org/wiki/Top-down_and_bottom-up_design

5.1.1 Domain-driven design leads to entities

From the approach in domain-driven design, you get the following objects, among others:

(cid:2) Entity

“An object that is not defined by its attributes, but rather by a thread of continuity and its
identity.”

(cid:2) Value Objects

“An object that contains attributes but has no conceptual identity. They should be treated
as immutable.”

(cid:2) Aggregate

“A collection of objects that are bound together by a root entity, otherwise known as an
aggregate root. The aggregate root guarantees the consistency of changes being made
within the aggregate by forbidding external objects from holding references to its
members.”

(cid:2) Repository

“Methods for retrieving domain objects should delegate to a specialized Repository object
such that alternative storage implementations may be easily interchanged.”

These quotations are taken from the following source:

http://en.wikipedia.org/wiki/Domain-driven_design

These objects should be mapped to your persistence storage (you should only aggregate
entities when they have the same lifecycle). This domain model is the basis for both a logical
data model and a physical data model.

For more information about domain-drive design terms, see the following website:

http://dddcommunity.org/resources/ddd_terms/

Microservices Best Practices for Java

5.1.2 Separate data store per microservice

Every microservice should have its own data store (Figure 5-1) and is decoupled from the
other microservice and its data. The second microservice must not directly access the data
store of the first microservice.

Reasons for this characteristic:

(cid:2) If two microservices share a data store, they are tightly coupled. Changing the structure of
the data store (such as tables) for one microservice can cause problems for the other
microservice. If microservices are coupled this way, then deployment of new versions must
be coordinated, which must be avoided.

(cid:2) Every microservice should use the type of database that best fits its needs (polyglot

persistence, see 5.1.3, “Polyglot persistence” on page 45 for more details). You do not
need to make a trade off between different microservices when choosing a database
system. This configuration leads to a separate data store for each microservice.

(cid:2) From a performance aspect, it can be useful for every microservice to have its own data

store because scaling can become easier. The data store can be hosted on its own server.

Figure 5-1 Separate data store per microservice

The separation of data stores for a relational database can be achieved in one of the following
ways:

(cid:2) Schema per microservice

Each service has its own schema in the database (Figure 5-2 on page 44). Other services
can use the same database but must use a different schema. This configuration can be
enforced with the database access control mechanism (using grants for connected
database users) because developers suffering from time pressures tend to use shortcuts
and can access the other schema directly.

(cid:2) Database per microservice

Each microservice can have its own database but share database server with the other
microservices (Figure 5-2 on page 44). With a different database, users can connect to
the database server and there is a good separation of the databases.

Chapter 5. Handling data

Figure 5-2 Schema per microservice and database per microservice

(cid:2) Database server per microservice

This is the highest degree of separation. It can be useful, for example, when performance
aspects need to be addressed (Figure 5-3).

Figure 5-3 Database server per microservice

Microservices Best Practices for Java

5.1.3 Polyglot persistence

Every microservice should use its own data store, which means that it can also use a different
data storage technology. The NoSQL movement has led to many new data storage
technologies, which can be used alongside a traditional relational database. Based on the
requirements that an application must implement, it can choose between different types of
technologies to store its data. Having a range of data storage technologies in one application
is known as polyglot persistence.

For some microservices, it is best to store their data in a relational database. Other services,
with a different type of data (such as unstructured, complex, and graph oriented) can store
their data in some of the NoSQL databases.

Polyglot programming is a term that means that applications should be written in a
programming language that best fits the challenge that an application must deal with.

A more in-depth description of these two terms can be found on the Martin Fowler website at:

http://martinfowler.com/bliki/PolyglotPersistence.html

5.1.4 Data sharing across microservices

In some situations, a client of a microservice application might want to request data that is
owned by a different service. For example, the client might want to see all his Payments and
the corresponding state of his payments. Then, he would have to query the service for his
Account and afterward the service for Payment. It is against the best practices for
microservices to join the data by using the database. To handle this business case, you must
implement an adapter service (Figure 5-4) that queries the Account service and the Payment
service, and then returns the collected data to the client. The adapter service is also
responsible for doing the necessary data transformations on the data it has received.

Figure 5-4 Adapter microservice

Chapter 5. Handling data

Changing data can become more complicated. In a system of microservices, some business
transactions can span multiple microservices. In a microservice application for a retail store,
for example, there might be a service to place an Order and a service to do the Payment.
Therefore, if a customer wants to buy something in your shop and pays for it, the business
transaction spans the two microservices. Every microservice has its own data store so, with a
business transaction spanning two or more microservices, two or more data stores are
involved in this business transaction. This section describes how to implement these business
transactions.

Event-Driven architecture
You need a method to ensure the consistency of the data involved in a business transaction
that spans two or more microservices. One way would be a distributed transaction, but there
are many reasons why this should not be done in a microservice application. The main
reason is the tight coupling of the microservices that are involved in a distributed transaction.
If two microservices are involved in a distributed transaction and one service fails or has a
performance problem, the other service must wait for the time out to roll back the transaction.

The best way to span a business transaction across microservices is to use an event-driven
architecture. To change data, the first service updates its data and, in the same (inner)
transaction, it publishes an event. The second microservice, which has subscribed to this
event, receives this event and does the data change on its data. Using a publish/subscribe
communication model, the two microservices are loosely coupled. The coupling only exists on
the messages that they exchange. This technique enables a system of microservices to
maintain data consistency across all microservices without using a distributed transaction.

If there are microservices in the microservice application that send many messages to each
other, then they might be good candidates for merging into one service. But be careful
because this configuration might break the domain-driven design aspect of your
microservices. Adapter services doing complex updates, which span more than one service,
can also be implemented by using events.

The programming model of an event-driven architecture is more complex, but Java can help
to keep the complexity manageable.

Eventual Consistency
In an event-driven architecture, sending messages to other microservices creates a problem
called Eventual Consistency. It is most often a runtime problem resulting from the following
situation: microservice A changes data in its data store and sends, in the same inner
transaction, a message to microservice B. After a short amount of time, microservice B
receives the message and changes the data in its data store. In this normally short period, the
data in the two data stores is not consistent. For example: service A updates the Order data
in its data store and sends a message to service B to do the Payment. Until the Payment gets
processed, there is an Order that has not been paid. Things get worse when the receiver of
the message is not able to process the message. In this case, the messaging system or the
receiving microservice must implement strategies to cope with this problem1.

In a microservice application, every microservice has its own database. A business
transaction that spans more than one microservice introduces eventual consistency because
distributed transactions are discouraged to solve this problem. One way to cope with such
business transactions is shown in Figure 5-5 on page 47. The Order microservice saves the
Order in its data store and sends an event, for example OrderCreated, to the Payment
microservice. While the Order microservice has not received the confirmation of the Payment
from the Payment microservice, the Order is in a pending state.

1 “Enterprise Integration Patterns: Designing, Building, and Deploying Messaging Solutions”, Gregor Hohpe and

Booby Woolf, Addison-Wesley Professional

Microservices Best Practices for Java

The Payment service is subscribed to the OrderCreated event so it processes this event and
does the Payment in its data store. If the Payment succeeds, then it publishes a
PaymentApproved event that is subscribed by the Order microservice. After processing the
PaymentApproved event, the state of the Order gets changed from Pending to Approved. If
the customer queries the state of his Order he gets one of the following two responses: Order
is in a pending state or Order is approved.

Figure 5-5 Event messaging between microservices

In situations where the data is not available, the service can send out to the client something
like the following message: “Sorry, please try again later”.

Data replication
The separation of the data stores and the requirement to get data from different data stores
can lead to the idea that using the data replication mechanisms of the database system would
solve the problem. Doing this, for example, with a database trigger or a timed stored
procedure or other processes, has the same disadvantage as sharing the database with more
microservices. Changing the structure of the data on one side of the replication pipe leads to
problems with the replication process. The process must be adapted when a new version of a
service is being deployed. This is also a form of tight coupling and must be avoided.

As stated, the event-based processing can decouple the two data stores. The services
processing the events can do the relevant data transformation, if needed, and store the data
in their own data stores.

5.1.5 Event Sourcing and Command Query Responsibility Segregation

In an event-driven architecture, consider Command Query Responsibility Segregation
(CQRS) and Event Sourcing. These two architectural patterns can be combined to handle the
events flowing through your microservice application.

Chapter 5. Handling data

CQRS splits the access to the data store into two separate parts: One part for the read
operations and the other part for the write operations. Read operations do not change the
state of a system. They only return the state. Write operations (commands) change the state
of the system, but do not return values. Event Sourcing stores sequences of these events that
occur as changes happened to your data. Figure 5-6 shows a sample of using CQRS.

Figure 5-6 Sample of using CQRS

As shown in Figure 5-6, the events are stored sequentially in the event store. The data in the
query model gets synchronized with the data from the event store. To support the event store
or the query model, you can use specialized systems, for example Elastic Search to support
the queries of your microservice. For more information about Elastic Search, see:

https://www.elastic.co/

This architecture can also be used to deal with events in your microservice application.

5.1.6 Messaging systems

You can use messaging systems or a message-oriented middleware to support your
event-driven architecture.

“Message-oriented middleware (MOM) is software or hardware infrastructure supporting
sending and receiving messages between distributed systems. MOM allows application
modules to be distributed over heterogeneous platforms and reduces the complexity of
developing applications that span multiple operating systems and network protocols. The
middleware creates a distributed communications layer that insulates the application
developer from the details of the various operating systems and network interfaces. APIs that
extend across diverse platforms and networks are typically provided by MOM. MOM provides
software elements that reside in all communicating components of a client/server architecture
and typically support asynchronous calls between the client and server applications. MOM

Microservices Best Practices for Java

reduces the involvement of application developers with the complexity of the master-slave
nature of the client/server mechanism.”2

To communicate with message oriented middleware, you can use different protocols. The
following are the most common protocols:
(cid:2) Advanced Message Queuing Protocol (AMQP)3

AMQP “mandates the behavior of the messaging provider and client to the extent that
implementations from different vendors are interoperable, in the same way as SMTP,
HTTP, FTP, etc. have created interoperable systems.”4

(cid:2) MQ Telemetry Transport (MQTT)5

MQTT is “publish-subscribe-based “lightweight” messaging protocol for use on top of the
TCP/IP protocol. It is designed for connections with remote locations where a “small code
footprint” is required or the network bandwidth is limited.”6 Mostly it is used in Internet of
Things (IoT) environments.

In the Java world there exists an API to communicate with message-oriented middleware:
Java Message Service (JMS) which is part of the Java EE Specification. The specific
version is JMS 2.07.

Due to the long existence of JMS (since 2001), many JMS Message Brokers are available
that can be used as MOM systems8. But there also exist messaging systems that
implement AMQP:

– RabbitMQ
– Apache Qpid9

– Red Hat Enterprise MRG

All of these systems provide Java APIs so they can be used in your Java based microservice.

5.1.7 Distributed transactions

Most, if not all, of the messaging systems support transactions. It is also possible to use
distributed transactions when sending messages to the message system and changing data
in a transactional data store.

Distributed transactions and two-phase commit can be used between a microservice and its
backing store, but should not be used between microservices. Given the independent nature
of microservices, there must not be an affinity between specific service instances, which is
required for two-phase commit transactions.

For interactions spanning services, compensation or reconciliation logic must be added to
ensure consistency is maintained.

2 http://en.wikipedia.org/wiki/Message-oriented_middleware
3 http://www.amqp.org/
4 http://en.wikipedia.org/wiki/Advanced_Message_Queuing_Protocol
5 http://mqtt.org/
6 http://en.wikipedia.org/wiki/MQTT
7 http://jcp.org/en/jsr/detail?id=343
8 http://en.wikipedia.org/wiki/Java_Message_Service
9 http://qpid.apache.org/index.html

Chapter 5. Handling data

5.2 Support in Java

In a polyglot persistence environment, the programming language you use to implement your
microservices must deal with the different persistence technologies. Your programming
language must be able to support each of the different ways to persist your data. Java as a
programming language has many APIs and frameworks that can support the developer to
deal with different persistence technologies.

5.2.1 Java Persistence API

“The Java Persistence API (JPA) is a Java specification for accessing, persisting, and
managing data between Java objects / classes and a relational database. JPA was defined
as part of the EJB 3.0 specification as a replacement for the EJB 2 CMP Entity Beans
specification. JPA is now considered the standard industry approach for Object to
Relational Mapping (ORM) in the Java Industry.

JPA itself is just a specification, not a product; it cannot perform persistence or anything
else by itself. JPA is just a set of interfaces, and requires an implementation. There are
open-source and commercial JPA implementations to choose from and any Java EE 5
application server should provide support for its use. JPA also requires a database to
persist to.”10

Java Enterprise Edition 7 (Java EE 7) has included Java Persistence 2.1 (JSR 338)11.

The main focus for inventing JPA was to get an object-relational mapper to persist Java
objects in a relational database. The implementation behind the API, the persistence provider,
can be implemented by different open source projects or vendors. The other benefit that you
get from using JPA is your persistence logic is more portable.

JPA has defined its own query language (Java Persistence Query Language (JPQL)) that can
be used to generate queries for different database vendors. Java classes get mapped to
tables in the database and it is also possible to use relationships between the classes to
correspond to the relationships between the tables. Operations can be cascaded by using
these relationships, so an operation on one class can result in operations on the data of the
other class. JPA 2.0 has introduced the Criteria API that can help to get a correct query at
both run time and compile time. All queries that you implement in your application can get a
name and can be addressed with this name. This configuration makes it easier to know after
a few weeks of programming what the query does.

The JPA persistence providers implement the database access. The following are the most
common:

(cid:2) Hibernate

http://hibernate.org/

(cid:2) EclipseLink

http://www.eclipse.org/eclipselink/

(cid:2) Apache OpenJPA

http://openjpa.apache.org/

The default JPA provider of Liberty for Java EE 7 is EclipseLink.

10 http://en.wikibooks.org/wiki/Java_Persistence/What_is_JPA%3F
11 http://jcp.org/en/jsr/detail?id=338

Microservices Best Practices for Java

JPA from a birds eye view
The following short explanations and code snippets show some of the features of JPA. The
best way to start with JPA is to create the Entity-Classes to hold the data (Example 5-1).

Example 5-1 JPA Class to hold entity data

@Entity
public class Order {
@Id
@GeneratedValue(strategy = GenerationType.IDENTITY)
private Long id;
private String description;
@Temporal(TemporalType.DATE)
private Date orderDate;
public String getDescription() {
return description;
}
public void setDescription(String description) {
this.description = description;
}
public Date getOrderDate() {
return orderDate;
}
public void setOrderDate(Date orderDate) {
this.orderDate = orderDate;
}
…
}

Every entity class needs an @Entity annotation to be managed by the persistence provider.
The entity class gets mapped by name to the corresponding table in the database (convention
over configuration). Different mappings can also be applied. The attributes of the class get
mapped by name to the columns of the underlying table. The automatic mapping of the
attributes can also be overwritten (@Column). Every entity class must have an identity (see
Domain-Driven Design in“Mapping domain elements into services” on page 13). The identity
column (or columns), annotated with @Id are necessary for the persistence provider to map
the values of the object to the data row in the table. The value of the identity column can be
generated in different ways by the database or the persistence provider. Some of the
attributes of an entity class must be transformed in a special way to be stored in the database.
For example, a database column DATE must be mapped in the entity class with the
annotation @Temporal.

The main JPA interface used to query the database is the EntityManager12. It has methods to
create, read, update, and delete data from the database. There are different ways to get a
reference to the EntityManager, depending on the environment that the application is running.
In an unmanaged environment (no servlet, EJB, or CDI container) you must use a factory
method of the class EntityManagerFactory as shown in Example 5-2.

Example 5-2 How to get an EntityManager in a Java SE environment

EntityManagerFactory entityManagerFactory =
Persistence.createEntityManagerFactory("OrderDB");
EntityManager entityManager =
entityManagerFactory.createEntityManager();

12 http://docs.oracle.com/javaee/7/api/javax/persistence/EntityManager.html

Chapter 5. Handling data

The String OrderDB is the name of the persistence unit that the EntityManager is created for.
Persistence units are used to logically group entity classes and related properties to configure
the persistence provider (configuration in the persistence.xml file).

In a managed environment, things are easier. The EntityManager can be injected from the
container as shown in Example 5-3.

Example 5-3 How to get an EntityManager In a Java EE environment

@PersistenceContext
EntityManager em;

If the persistence context gets injected without using a unitName, which is the name of the
persistence unit configured in the configuration file (persistence.xml), then it uses a default
value. If there is only one persistence unit, then this is the default that JPA uses.

The following sections show how to implement some simple create, retrieve, update, and
delete operations with the methods from the EntityManager as shown in Example 5-4.

Example 5-4 JPA create operation

@PersistenceContext
EntityManager em;
...
public Order createOrder(Order order) {
em.persist(order);
return order;
}

The persist method of the EntityManager does two things. First, the EntityManager manages
the object, which means it holds the object in its persistence context. The persistence context
can be seen as a cache holding the objects that are related to the rows in the database. The
relation is done by using the database transactions. Second, the object gets persisted. It gets
stored in the database. If the Order entity class has an ID attributed whose value gets
generated by the database, then the attribute of the value will be set by the EntityManager
after the insertion into the database. That is the reason why the return value of the
createOrder method is the object itself (Example 5-5).

Example 5-5 JPA read operation by using the find method

@PersistenceContext
EntityManager em;
...
public Order readOrder(Long orderID) {
Order order = em.find(Order.class, orderID);
return order;
}

The EntityManager method find searches the table for a row whose primary key is given as a
parameter (orderID). The result is casted to a Java class of type Order (Example 5-6).

Example 5-6 JPA read operation by using JPQL

@PersistenceContext
EntityManager em;
...
public Order readOrder(Long orderID) {

Microservices Best Practices for Java

TypedQuery<Order> query =
em.createQuery( "Select o from Order o " +
"where o.id = :id", Order.class );
query.setParameter("id", orderID);
Order order = query.getSingleResult();
return order;
}

Example 5-6 on page 52 shows the function of the find method by using a JPQL with a
parameter. Beginning from the JPQL string Select o from Order o where o.id = :id, a
TypedQuery is generated. With a TypedQuery, you can omit the Java cast after generating the
result object (see the parameter Order.class). The parameter in the JPQL gets addressed by
a name (id) that makes it more readable for the developer. The method getSingleResult
makes sure that only one row is found in the database. If there is more than one row
corresponding to the SQL, then a RuntimeException is thrown.

The merge method does the update in the database (Example 5-7). The parameter order is a
detached object, which means it is not in the persistence context. After the update in the
database, an attached (it is now part of the persistence context) order object is returned by
the EntityManger.

Example 5-7 JPA update operation

public Order updateOrder(Order order, String newDesc) {
order.setDescription(newDesc);
return em.merge(order);
}

To delete a row in the database, you need an attached object (Example 5-8). To attach the
object to the persistence context, you can do a find. If the object is already attached, you do
not need the find method. The method remove with the parameter of the attached object
deletes the object in the database.

Example 5-8 JPA delete operation

public void removeOrder(Long orderId) {
Order order = em.find(Order.class, orderId);
em.remove(order);
}

As mentioned before, some configuration must be used to tell the persistence provider where
to find the database and how to work with the database. This is done in a configuration file
called persistence.xml. The configuration file needs to be in the class path of your service.
Depending on your environment (Java containers or not), the configuration must be done in
one of two ways (Example 5-9).

Example 5-9 Persistence.xml in a Java SE environment

<class>com.service.Order</class>

Chapter 5. Handling data

To configure the persistence provider, the first thing that must be done is to define the
persistence unit (OrderDB). One attribute of the persistence unit is the transaction-type. Two
values can be set: RESOURCE_LOCAL and JTA. The first option makes the developer
responsible for doing the transaction handling in his code. If there is no transaction manager
available in your environment, then use this option. The second option is JTA, which stands
for Java Transaction API and is part of the Java Community Process (JCP). This options tells
the persistence provider to delegate transaction handling to a transaction manager that exists
in your runtime environment.

Between the XML-tags <class></class>, you can enlist the entity classes to be used in this
persistence unit.

In the properties section of the file, you can set values to configure the way the persistence
provider will work with the database. Property names beginning with
javax.persistence.jdbc are defined by the JPA standard. Example 5-9 on page 53 shows
how to set the database URL (used for database connections) and the user and password.
The javax.persistence.jdbc.driver property tells the persistence provider which
JDBC-driver class to use.

The main difference in the configuration file for the Java EE environment is shown in
Example 5-10.

Example 5-10 Persistence.xml in a Java EE environment

<persistence>
<persistence-unit name="OrderDB">
<jta-data-source>jdbc/OrderDB</jta-data-source>
<class>com.widgets.Order</class>

...

</persistence-unit>
</persistence>

The default transaction handling in JPA is to use JTA so that you do not need to set it in the
configuration file. The jta-data-source property points to the JNDI-Name of the data source
configured in the application server of your Java EE environment.

To do the transaction management in non Java EE environments, the EntityManager has
some methods that can be used as shown in Example 5-11.

Example 5-11 Transaction management in no Java EE environments

EntityManagerFactory emf =
Persistence.createEntityManagerFactory("OrderDB");
EntityManager em = emf.createEntityManager();
EntityTransaction tx = em.getTransaction();

Microservices Best Practices for Java

tx.begin();
try {
em.persist(yourEntity);
em.merge(anotherEntity);
tx.commit();
} finally {
if (tx.isActive()) {
tx.rollback();
}
}

Example 5-11 on page 54 shows the transaction handling in a non Java EE environment.
Avoid doing transaction management on your own in a Java EE environment. There are better
ways of doing this management as described in 5.2.2, “Enterprise JavaBeans” on page 57.

To separate the data store for your microservice, you can set the default schema of your
relational database by using a configuration file as in Example 5-12.

Example 5-12 Setting default schema in my-orm.xml

<?xml version="1.0" encoding="UTF-8"?>
<entity-mappings xmlns="http://java.sun.com/xml/ns/persistence/orm"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xsi:schemaLocation="http://java.sun.com/xml/ns/persistence/orm orm_2_0.xsd"
version="2.0">
<persistence-unit-metadata>
<persistence-unit-defaults>
<schema>ORDER</schema>
</persistence-unit-defaults>
</persistence-unit-metadata>
</entity-mappings>

This file must be referenced in the JPA configuration file persistence.xml (Example 5-13).

Example 5-13 Snippet showing reference in persistence.xml to a mapping file

<persistence-unit name="OrderDB">
<mapping-file>custom-orm.xml</mapping-file>

This configuration sets the schema name to the name given in the my-orm.xml mapping file, in
this case ORDER, to all your JPA classes. It ensures that you can only use tables in this
schema.

JPA in combination with NoSQL databases
EclipseLink is one of the JPA providers that started to support NoSQL databases (from
version 2.4 forward). From this version forward, they support MongoDB and Oracle NoSQL.
Other NoSQL databases are expected to be supported in the future releases.

MongoDB is a NoSQL document-oriented database. The data structure is favorable to JSON
like documents with the ability to have dynamic schemas. MongoDB has a specialized version
of the JSON format called BSON:

http://docs.mongodb.com/manual/reference/bson-types/

Chapter 5. Handling data

The EclipseLink Solution Guide for EclipseLink Release 2.5 shows an example of how to
access a MongoDB:

http://www.eclipse.org/eclipselink/documentation/2.5/solutions/nonrelational_db002
.htm

Before deciding to use JPA (like with EclipseLink) as the provider for MongoDB, consider the
following points:

(cid:2) SQL is a specified language that has gone through a number of revisions. The database
vendors have implemented this standard, but they have also added some features to the
SQL that have not been standardized. JPA has good support for SQL, but does not
support all of the features that are exposed by MongoDB. If some of these features are
needed in your microservice, then the benefits you get from using JPA are fewer.

(cid:2) JPA has numerous features that do not make sense in a document-oriented database, but
the EntityManager has methods for these features. So you must define which methods to
use in your service.

If you are familiar with JPA and only require simple functions for storing your data in a NoSQL
database, then it is acceptable to start doing this with your JPA provider. If the data access
gets more complicated, it is better to use the Java drivers from your NoSQL database. JPA
does not really fit with NoSQL databases, but it can be a good starting point for your
implementation. For a sample of how to use the native Java driver of MongoDB, see:

http://docs.mongodb.com/getting-started/java/

To get more out of your JPA provider, it can be useful to use Spring Data JPA. In addition to
the JPA provider, Spring Data JPA adds an extra layer on top of the JPA provider:

http://projects.spring.io/spring-data-jpa/

Suitability of JPA for data handling in microservices
The following list shows some arguments why JPA is useful for data handling in your
microservices:

(cid:2) Defining your microservices from the perspective of Domain Driven Design leads to the

situation where most of the microservices need only simple queries to persist their entities
(simple create, retrieve, update, and delete operations). JPA's EntityManager has these
create, retrieve, update, and delete methods that you need: persist, find, merge, delete).
Not much programming is needed to call these methods.

(cid:2) In some cases, the queries get more complex. These queries can be done with the query
language defined in JPA: JPQL. The need for complex queries should be an exceptional
case. JSON documents with a hierarchy of entity data should each be stored separately.
This leads to simple IDs and simple queries.

(cid:2) JPA is standardized and has a great community to support you in developing your

microservice. All Java EE servers must support JPA.

(cid:2) To implement microservices not running in a Java EE container, you can also use JPA.

(cid:2) Generating entity classes from the database (reverse engineering) can reduce the number

of lines of code you must implement by yourself.

(cid:2) For polyglot persistence, JPA has support for a relational data store and a

document-oriented data store (EclipseLink).

(cid:2) JPA as an abstraction of the relational database allows you to exchange your relational
data store with another relational data store if you need to. This portability prevents your
microservice from a vendor lock in.

Microservices Best Practices for Java

(cid:2) To implement the strategy, every microservice should have its own schema in a relational
database, you can set the default schema for your service in the JPA configuration file
persistence.xml.

5.2.2 Enterprise JavaBeans

Enterprise JavaBeans 3.2 (EJB) (specified in the JSR 34513) is part of the Java EE
specification. EJBs are more than normal Java classes for these reasons:

(cid:2) They have a lifecycle.
(cid:2) They are managed by an EJB container (runtime environment for the EJB).
(cid:2) They have a lot more features that can be helpful.

EJBs are server-side software components. Since EJB 3.0, it is no longer necessary to use a
Deployment Descriptor. All declarations of an EJB can be done with annotations in the EJB
class itself. The implementation to handle the EJB inside an EJB container is as lightweight
as it is for CDI managed beans. The thread handling of the EJBs is done by the EJB container
(similar to thread handling in a servlet container). An additional feature of EJBs is that they
can be used in combination with Java EE Security.

EJBs can be divided into the following types:

(cid:2) Stateless
(cid:2) Stateful
(cid:2) Singleton
(cid:2) Message-driven beans (MDB)

Stateless EJBs cannot hold any state, but stateful EJBs can. Due to the characteristics of a
microservice, stateful EJBs should not be used in a microservice. A Singleton Bean exists
only one time in a Java EE server. MDBs are used in the processing of asynchronous
messages in combination with a JMS provider.

EJBs can implement several business views, which must be annotated accordingly:

(cid:2) Local interface (@Local)

Methods in this Interface can only be called by clients in the same Java virtual machine
(JVM).

(cid:2) Remote interface (@Remote)

Methods that are listed in this interface can be called by clients from outside the JVM.

(cid:2) No interface (@LocalBean)

Nearly the same as local interface, except all public methods of the EJB class are exposed
to the client.

In a lightweight architecture, which a microservice should have, it is useful to implement the
EJBs as no interface EJBs.

One of the main benefits that EJBs provide is automatic transaction handling. Every time the
a business method is called, the transaction manager of the EJB container is invoked
(exception: an EJB with explicitly switched off support for transactions). So it is easy to use
EJBs in combination with a transactional data store. Integrating EJBs with JPA is also easy.

13 http://jcp.org/en/jsr/detail?id=345

Chapter 5. Handling data

The code snippet in Example 5-14 shows an example of how to combine the EJBs with the
JPA framework.

Example 5-14 Stateless (no-interface) EJB with PersistenceContext

@Stateless
@LocalBean
public class OrderEJB {
@PersistenceContext
private EntityManager entityManager;
public void addOrder(Order order) {
entityManager.persist(order);
}
public void deleteOrder(Order order) {
entityManager.remove(order);
}
. . .
}

The EntityManager gets injected as described in 5.2.1, “Java Persistence API” on page 50.

An EJB can have one of the following transaction attributes to operate with a transactional
data store (must be implemented by the EJB container):

(cid:2) REQUIRED (default)
(cid:2) MANDATORY
(cid:2) NEVER
(cid:2) NOT_SUPPORTED
(cid:2) REQUIRES_NEW
(cid:2) SUPPORTS

For more information about these attributes, see the following website:

http://docs.oracle.com/javaee/7/api/javax/ejb/TransactionAttributeType.html

These so called container-managed transactions (CMTs) can be used on every business
method of an EJB. Bean-managed transactions (BMT), which can also be used in an EJB,
should be avoided. The annotation TransactionAttribute can be set on a class level so
every business method of that class has this transaction attribute (Example 5-15). If nothing is
set, then all methods have the default transaction level (REQUIRED). A transaction attribute
at the method level overwrites the class attribute.

Example 5-15 Set transaction attribute explicit in an EJB

@TransactionAttribute(REQUIRED)
@Stateless
@LocalBean
public class OrderEJB {
...
@TransactionAttribute(REQUIRES_NEW)
public void methodA() {...}
@TransactionAttribute(REQUIRED)
public void methodB() {...}
}

Microservices Best Practices for Java

REST Endpoint not implemented as EJB
Some of the database changes or verifications are done at the point where the transaction
manager has decided to commit. In some situations, check constraints in the database for
example, are verified as one of the last steps before the transaction is committed. If this
verification fails, then the outcome is a RuntimeException that the JPA provider throws,
because JPA uses runtime exceptions to report errors. If you use EJBs for transaction
management, then the place to catch the RuntimeException is in the stub code of the EJB
where the EJB container does the transaction management. The stub code is generated by
the EJB container. Therefore, you cannot react to this RuntimeException and the exception is
thrown further to the place it will be caught.

If you implement your REST endpoint as an EJB, as some people prefer, then the exception
must be caught in your REST provider. REST providers do have exception mappers to do the
conversion of exceptions to HTTP error codes. However, these exception mappers do not
interfere in the case when the RuntimeException is thrown during the database commit.
Therefore, the RuntimeException is received by the REST client, which should be avoided.

The best way to handle these problems is to implement the REST endpoint as a CDI
managed request scoped bean. In this CDI bean, you can use injection the same way as
inside an EJB. So it is easy to inject your EJB into the CDI managed bean (Example 5-16).

Example 5-16 REST endpoint implemented as CDI managed bean with EJB injected

@RequestScoped
@Path("/Order")
public class OrderREST {

@EJB
private OrderEJB orderEJB;

...
}

It is also possible to integrate EJBs with Spring (Enterprise JavaBeans (EJB) integration -
Spring) and if preferred, the transaction management can be done using Transaction
Management Spring. However, in a Java EE world it is better to delegate transaction
management to the server.

For more information about Spring, see the following website:

http://docs.spring.io/spring/docs/current/spring-framework-reference/html/ejb.html

5.2.3 BeanValidation

BeanValidation is also part of the Java EE 7 specification: Bean Validation 1.1 JSR 34914.
The intention of Bean Validation is to easily define and enforce validation constraints on your
bean data. In a Java EE environment, Bean Validation is done by the different Java EE
containers automatically. The developer only needs to set the constraints, in the form of
annotations on attributes, methods, or classes. The validation is done on the invocation of
these elements automatically (if configured). The validation can also be done explicitly in your
source code.

For more information about Bean Validation, see the following website:

http://beanvalidation.org/

14 http://jcp.org/en/jsr/detail?id=349

Chapter 5. Handling data

Examples of built-in constraints in the javax.validation.constraints package are shown in
Example 5-17.

Example 5-17 Default built-in constraints in Bean Validation

private String username; // username must not be null
@Pattern(regexp="\$\\d{3}\$\\d{3}-\\d{4}")
private String phoneNumber; // phoneNumber must match the regular expression
@Size(min=2, max=40)
String briefMessage; // briefMessage netween 2 and 40 characters

Constraints can also be combined as shown in Example 5-18.

Example 5-18 Combination of constraints

@NotNull
@Size(min=1, max=16)
private String firstname;

It is also possible to extend the constraints (custom constraints). Example 5-19 shows how to
do the validation on your own.

Example 5-19 Validating programmatically

Order order = new Order( null, "This is a description", null );
ValidatorFactory factory =
Validation.buildDefaultValidatorFactory();
Validator validator = factory.getValidator();
Set<ConstraintViolation<Order>> constraintViolations = validator.validate(order);
assertEquals( 2, constraintViolations.size() );
assertEquals( "Id may not be null",
constraintViolations.iterator().next().getMessage() );
assertEquals( "Order date may not be null",
constraintViolations.iterator().next().getMessage() );

JPA can be configured to do the bean validation automatically. The JPA specification
mandates that a persistence provider must validate so called managed classes (see
Example 5-20). Managed classes in the sense of JPA are, for example, entity classes. All
other classes used for the JPA programming must also be validated (for example, embedded
classes, super classes). This process must be done in the lifecycle events these managed
classes are involved in.

Example 5-20 Persistence.xml with bean validation turned on

<persistence>
<persistence-unit name="OrderDB">
<provider>
org.eclipse.persistence.jpa.PersistenceProvider
</provider>
<class> . . . </class>
<properties>
<property name="javax.persistence.validation.mode"
value="AUTO" />
</properties>
</persistence-unit>
</persistence>

Microservices Best Practices for Java

All the other frameworks used from the Java EE stack can be used to validate the beans
automatically (for example JAX-RS, CDI) and also EJB as shown in Example 5-21.

Example 5-21 Bean validation in an EJB

@Stateless
@LocalBean
public class OrderEJB {
public String setDescription(@Max(80) String newDescription){
. . .
}
}

Architectural layering aspects of JPA and BeanValidation
Depending on the layers that are implemented in a microservice, there are some aspects to
address.

In a service with just a few layers, it is easier to use the JPA entity classes as a Data Transfer
Object (DTO) as well. After the JPA object gets detached from its persistence context, it can
be used as a plain old Java object (POJO). This POJO can also be used as a DTO to transfer
the data to the REST endpoint. Doing it this way has some disadvantages. The Bean
Validation annotations get mixed with the JPA annotations that can lead to Java classes with
numerous annotations and your REST endpoints become more closely related to your
database.

If the microservice is a little bit larger or has to deal with a more complex data model, then it
can be better to use a separate layer to access the database. This extra layer implements all
the data access methods based on the JPA classes. The classes in this layer are data access
objects (DAOs). You can use the DAO classes to generate the DTO classes for the REST
endpoint and focus, on the one side, on the data model (DAO) and, on the other side, on the
client (DTO). The disadvantage of this pattern is that you must convert your JPA classes
handled in the DAO layer to DTOs and vice versa. To avoid creating a lot of boilerplate code to
do this, you can use some frameworks to help you do the converting. The following
frameworks are available to convert Java objects:

(cid:2) ModelMapper

http://modelmapper.org/

(cid:2) MapStruct

http://mapstruct.org/

To extend the possibilities you get from Bean Validation, it can be useful to get additional
features by using Springs. For more information, see “Validation, Data Binding, and Type
Conversion” at:

http://docs.spring.io/spring/docs/current/spring-framework-reference/html/validati
on.html

5.2.4 Contexts and Dependency Injection

If your microservice does not store its data in a transactional data store, then consider using
CDI managed beans instead of EJBs. CDI 1.1 is specified in the JSR 34615 and is part of the
Java EE 7 specification. CDI managed beans can be a good alternative in these situations.

15 http://jcp.org/en/jsr/detail?id=346

Chapter 5. Handling data

For more information about CDI managed beans, see the following website:

http://docs.oracle.com/javaee/6/tutorial/doc/giwhl.html

In comparison to EJBs, CDI has no Java EE security on its own and does not get the
persistence context injected. This process must be done by the developer themselves or by
using additional frameworks. Apache DeltaSpike, for example, has many modules that you
can use to extend the functions of CDI. For more information about Apache DeltaSpike, see:

http://deltaspike.apache.org/index.html

Additional frameworks can be used to extend the functions of CDI managed beans. EJBs
have a thread pool that can be managed in an application server. CDI has nothing that
corresponds to this function yet. To be able to configure a thread pool can be helpful in an
environment with high loads.

To implement a microservice not running in a Java EE application server, CDI and additional
modules provide you many functions that are useful in these environments.

5.2.5 Java Message Service API

To implement an event-driven architecture in a Java world, JMS API provides support, which
is also specified in Java Message Service 2.0 JSR 34316. JMS is used to communicate to a
so-called message provider that must be implemented by the message-oriented middleware
(MOM).

When JMS was updated from version 1.1 to version 2.0, which is part of the Java EE 7
specification, much rework was done to make the API easier to use. JMS 2.0 is compatible
with an earlier version, so you can use your existing code or use the new simplified API for
your new microservices. The old API will not be deprecated in the next version.

According to the smart endpoints and dump pipes methodology17, your Java based
microservice must just send your JSON message to an endpoint hosted by the JMS provider.
The sender of the messages is called producer and the receiver of the message is the
consumer. These endpoints can be of these types:

(cid:2) Queue

Messages in a queue are consumed by only one consumer. The sequence of the
messages in the queue might be consumed in a different order. Queues are used in a
point to point semantic.

(cid:2) Topic

These messages can be consumed by more than one consumer. This is the
implementation for publish/subscribe semantics.

In a REST-based microservice, where JSON-based requests are sent by the client, it is a
good idea to also use the JSON format for your messaging system. Other messaging
applications use XML. If you have only one format in your system of microservice, it is easier
to implement JSON.

Example 5-22 Producer sending messages to a JMS queue with EJB

@Stateless
@LocalBean
public class OrderEJB {

16 http://jcp.org/en/jsr/detail?id=343
17 http://martinfowler.com/articles/microservices.html#SmartEndpointsAndDumbPipes

Microservices Best Practices for Java

@Inject
@JMSConnectionFactory("jms/myConnectionFactory")
JMSContext jmsContext;
@Resource(mappedName = "jms/PaymentQueue")
Queue queue;
public void sendMessage(String message) {

jmsContext.createProducer().send(queue, message);
}

A JMSContext and a Queue are needed to send a message (Example 5-22 on page 62). These
objects are injected if the message sender is running in a Java EE application server.
Example 5-22 on page 62 uses an EJB so these resources are injected. The configuration for
the injected objects must be done in the application server. If an exception occurs, a runtime
exception JMSRuntimeException is thrown.

The scope of an injected JMSContext in a JTA transaction is transaction. So if your message
producer is an EJB, your message is delivered in the context of a transaction, which avoids
loosing messages.

To consume the message from a queue, it is easy to use a message driven EJB (MDB) as
shown in Example 5-23. Using an MDB also has the advantage that consuming messages is
done in a transaction, so no message is lost.

Example 5-23 Consumer - process messages from a JMS queue

@MessageDriven(
name="PaymentMDB",
activationConfig = {
@ActivationConfigProperty(
propertyName="messagingType",
propertyValue="javax.jms.MessageListener"),
@ActivationConfigProperty(
propertyName = "destinationType",
propertyValue = "javax.jms.Queue"),
@ActivationConfigProperty(
propertyName = "destination",
propertyValue = "PaymentQueue"),
@ActivationConfigProperty(
propertyName = "useJNDI",
propertyValue = "true"),
}
)
public class PaymentMDB implements MessageListener {
@TransactionAttribute(
value = TransactionAttributeType.REQUIRED)
public void onMessage(Message message) {

if (message instanceof TextMessage) {
TextMessage textMessage = (TextMessage) message;
String text = message.getText();
. . .
}
}
}

Chapter 5. Handling data

You must use the @MessageDriven annotation to declare the EJB type as message-driven
EJB. Within this annotation, you can set the name of the MDB and some activation
configurations. The properties of the activation configuration tie the MDB to the JMS
messaging system that handles the queues or topics. In an application server environment,
where your MDB gets hosted, it is easy to configure these elements. The MDB itself
implements a MessageListener interface that has only one method: onMessage. Every time the
JMS provider has a message to process, it calls this method. The method is annotated with a
transactional attribute to show that this method is called in a transaction.The default
transaction attribute for an MDB is TransactionAttributeType.REQUIRED. Inside the method,
the message object must be converted and the message can be extracted as a String. Other
message types are also possible.

It is good practice to use the MDB only for the message handling. Keep the MDB as a
technical class. The rest of your business code should be implemented in a Java POJO that
gets called by the MDB. This configuration makes the business code easier to test in your
JUnits.

As stated before, every MDB runs in a transaction, so no message gets lost. If an error occurs
during the processing of the message and the EJB container, which handles the MDB,
receives this runtime exception, then the message is redelivered to the MDB (error loop). The
retry count, which can be configured in your JMS provider, specifies how often this occurs.
After the maximum number of retries, the message is normally put into an error queue.
Messages in an error queue must be handled separately.

If a business transaction spans more than one microservice, use an event-driven architecture
(see section 5.1.4, “Data sharing across microservices” on page 45). This means that the
microservice sending the event must perform the following tasks:

(cid:2) Change data in its data store
(cid:2) Send the message to the second microservice

The receiving microservice must perform these tasks:

(cid:2) Receive the message from the queue
(cid:2) Change data in its data store

To be consistent, these two things must be done in one transaction if the data store is
transactional. This requirement is also true for the producer and the consumer side of the
messaging system. In these cases, you must use a distributed transaction. The transaction
partners are the data store and the JMS provider (not the two data stores of the two
microservices).

To track the messages produced and consumed, it is useful to use a correlation ID. A
correlation ID specified by the producer of the message correlates to the message consumed
by the consumer. This correlation ID can also be used in the logging of the microservice to get
the complete communication path of your microservices call (see 9.2, “Logging” on
page 111).

Java provides a class to generate a unique Id: UUID. This class can be used to generate a
correlation ID. Example 5-24 shows setting a correlation ID.

Example 5-24 Setting correlation ID in a JMS message

// JMSContext injected as before
JMSProducer producer = jmsContext.createProducer();
producer.setJMSCorrelationID(UUID.randomUUID().toString());
producer.send(queue, message);

Microservices Best Practices for Java

Example 5-25 shows retrieving the correlation ID.

Example 5-25 Retrieving the correlation ID from a JMS message

// message received as before
String correlationId = message.getJMSCorrelationID();

For more information about UUID, see the following website:

http://docs.oracle.com/javase/7/docs/api/java/util/UUID.html

If you are using a non-JMS provider as the message-oriented middleware, JMS might not be
the correct way to send messages to this system. Using RabbitMQ (which is an AMQP
Broker) can be done with JMS because Pivotal has implemented a JMS Adapter for
RabbitMQ. For more information about RabbitMQ, see the following website:

http://www.rabbitmq.com/

Apache Qpid also implements a JMS Client for the AMQP Protocol. These are just a few
examples to show that it can also be useful to use JMS to communicate with non-JMS
providers. But, depending on your requirements, it can be better to use the native Java driver
of the messaging system. For more information about Apache Qpid, see the following
website:

http://qpid.apache.org/index.html

Spring's support for JMS Message Providers is another alternative for handling JMS
messages. Fore more information about Spring, see the following website:

http://docs.spring.io/spring/docs/current/spring-framework-reference/html/jms.html

5.2.6 Java and other messaging protocols

Depending on the features that you need from your message-oriented middleware, you might
need a non-JMS message provider system. MQTT is a messaging protocol that best fits the
needs of Internet of Things (IoT) and AMQP. It was developed to be portable across vendors.
JMS cannot be used to communicate with these systems. Using their provided clients allows
you to use all special features they provide. For more information about MQTT, see the
following website:

http://mqtt.org/

Apache Kafka is a non-JMS provider, but provides a Java driver. There is an enhancement
request to implement an adapter to let the clients talk JMS with Apache Kafka18 but this issue
is still open. So it is better to use the Java driver that Kafka provides. For more information
about Kafka, see the following website:

http://kafka.apache.org/

RabbitMQ is a message broker system which implements the AMQP protocol. It provides a
Java client19. For more information about RabbitMQ, see the following website:

https://www.rabbitmq.com/

18 http://issues.apache.org/jira/browse/KAFKA-1995
19 http://www.rabbitmq.com/java-client.html

Chapter 5. Handling data

Spring has a library to talk to AMQP based messaging systems. Spring also provides support
for the MQTT protocol20. For more information about Spring, see the following website:

http://projects.spring.io/spring-amqp/

As you can see, there is support for Java with non JMS messaging systems.

20 http://docs.spring.io/spring-integration/reference/html/mqtt.html

Microservices Best Practices for Java

Chapter 6. Application Security

As always, security is an important consideration. This chapter describes security
requirements within the context of a cloud native microservices architecture. The following
topics are covered:

(cid:2) Securing microservice architectures
(cid:2) Identity and Trust

6.1 Securing microservice architectures

The dynamic nature of microservice architectures changes how security should be
approached. This consideration is specifically relevant to how application or service
boundaries are defined. Figure 6-1 shows simplified representations of how requests flow in
monolithic and microservice architectures. Connections between microservices and
persistent storage services (RDBMS, NoSQL) have been hidden for general simplicity, but
represent yet another dimension of network traffic.

Figure 6-1 Simplified topology for monolithic and microservice architectures

The left side of Figure 6-1 shows a simplification of a traditional monolithic architecture.
Requests come through the gateway, go through a load balancer, to business logic in the
middleware, and then down to the data tier.

On the right side of Figure 6-1, things are less organized. Collections of microservices are
grouped as applications, and surface external APIs as endpoints through a gateway. One
obvious difference is the number of moving parts. The other is how many more requests there
are.

Microservices Best Practices for Java

The biggest difference is that the composition of services shown on the right side are under
constant change. Individual service instances come and go based on load. Microservices are
continuously updated independently. Overall, there can be hundreds of updates a day across
services. Securing the perimeter is not going to be enough. The question is how to secure a
rapidly changing infrastructure in an organic way that still allows the individual services to
change and grow without requiring central coordination.

6.1.1 Network segmentation

Figure 6-1 on page 68 shows an isolated subnet in both systems. Using an extra firewall or
gateway to guard resources that require more levels of protection is a good idea in either
environment.

Something else to observe in microservices approach shown in Figure 6-1 on page 68 is that
the externally exposed APIs are connected to two different microservices-based applications.
Defined application boundaries provide a reasonable amount of isolation between
independently varying systems, and are a good way to maintain a reasonable security
posture in a dynamic environment.

Network segmentation happens naturally in hosted environments. For example, each
microservices application that is shown in Figure 6-1 on page 68 could be a separate tenant
in a multi-tenant environment. Where it does not happen naturally, you might want to
encourage it. However, describing the best practices for managing networks in microservice
architectures is well outside the scope of this publication.

6.1.2 Ensuring data privacy

Different kinds of data require different levels of protection, and that can influence how data is
accessed, how it is transmitted, and how it is stored.

When dealing with data, take into account these considerations:

(cid:2) Do not transmit plain text passwords.

(cid:2) Protect private keys.

(cid:2) Use known data encryption technologies rather than inventing your own.

(cid:2) Securely store passwords by using salted hashes. For more information about salted

hashing, see the following website:

https://crackstation.net/hashing-security.htm

(cid:2) Further, sensitive data should be encrypted as early as possible, and decrypted as late as
possible. If sensitive data must flow between services, only do so while it is encrypted, and
should not be decrypted until the data needs to be used. This process can help prevent
accidental exposure in logs, for example.

Backing services
As mentioned earlier, connections to backing services are additional sources of network
traffic that need to be secured. In many cloud environments, backing services are provided by
the platform, relying on multi-tenancy and API keys or access tokens to provide data isolation.
It is important to understand the characteristics of backing services, especially how they store
data at rest, to ensure regulatory requirements (HIPAA, PCI, and so on) are satisfied.

Chapter 6. Application Security

Log data
Log data is a trade-off between what is required to diagnose problems and what must be
protected for regulatory and privacy reasons. When writing to logs, take full advantage of log
levels to control how much data is written to logs. For user-provided data, consider whether it
belongs in logs at all. For example, do you really need the value of the attribute written to the
log, or do you really only care about whether it was null?

6.1.3 Automation

As mentioned way back in Chapter 1, “Overview” on page 1, as much as possible should be
automated in microservice environments, which includes general operation. Repeatable
automated processes should be used for applying security policies, credentials, and
managing SSL certificates and keys across segmented environments to help avoid human
error.

Credentials, certificates, and keys must be stored somewhere for automation to use.
Remember the following considerations:

(cid:2) Do not store your credentials alongside your applications.

(cid:2) Do not store your credentials in a public repository.

(cid:2) Only store encrypted values.

(cid:2) Spring Cloud Config, as an example, uses Git to back up their configuration. It provides
APIs to automatically encrypt configuration values on the way into the repository, and to
decrypt them on the way out. For more information about Spring Cloud Config, see this
website:

https://cloud.spring.io/spring-cloud-config/

Hashicorp’s Vault is similar, but adds extra capabilities to allow use of dynamic keys with
expiry. For more information about Vault, see the following website:

https://www.vaultproject.io/

Other key/value stores, for example etcd, zookeeper, or consul, could also be used, but
care should be taken with how credentials are stored in what are otherwise plain text
repositories.

6.2 Identity and Trust

A highly distributed, dynamic environment like microservices architectures place some strain
on the usual patterns of establishing identity. You must establish and maintain the identity of
users without introducing extra latency and contention with frequent calls to a centralized
service.

Establishing and maintaining trust throughout this environment is not the easiest either. It is
inherently unsafe to assume a secure private network. End-to-end SSL can bring some
benefit in that bytes are not flowing around in plain text, but it does not establish a trusted
environment on its own, and requires key management.

The following sections outline techniques for performing authentication and authorization and
identity propagation to establish and maintain trust for inter-service communications.

Microservices Best Practices for Java

6.2.1 Authentication and authorization

There are new considerations for managing authentication and authorization in microservice
environments. With a monolithic application, it is common to have fine grained roles, or at
least role associated groups in a central user repository. With the emphasis on independent
lifecycles for microservices, however, this dependency is an anti-pattern. Development of an
independent microservice is then constrained by and coupled with updates to the centralized
resource.

It is common to have authentication (establishing the user’s identity) performed by a
dedicated, centralized service or even an API gateway. This central service can then further
delegate user authentication to a third party.

When working with authorization (establishing a user’s authority or permission to access a
secured resource), in a microservices environment keep group or role definitions coarse
grained in common, cross cutting services. Allow individual services to maintain their own fine
grained controls. The guiding principle again is independence. A balance must be found
between what can be defined in common authorization service to meet requirements for the
application as a whole, and what authorization requirements are implementation details for a
particular service.

Java EE security
Traditional enterprise access control tokens also have less traction in a microservice
environment. The presence of other programming languages and the inability to rely on
long-lived sessions undercut the operating assumptions of traditional Java EE security
mechanisms.

For Java EE security, each Java EE server must be trusted by all of the other services. The
security token (in WebSphere Application Server, this is an LTPA Token) that the user
receives after authentication must be valid on the other servers. Establishing trust this way
means that either (a) every server needs the certificates of every other server in its keystore,
or (b) all of the services need to use the same key. A user authenticating to the system must
also be in the security realm defined for your Java EE server. Key management, specifically
keystore management for Java applications, in a dynamically provisioned environment is a
non-trivial problem to solve. For more information about Java EE Security, see this website:

https://dzone.com/refcardz/getting-started-java-ee

If you compare the effort that is required to achieve and maintain this environment to the
benefit you get from Java EE security, then it is difficult to recommend using Java EE security.

6.2.2 Delegated Authorization with OAuth 2.0

OAuth provides an open framework for delegating authorization to a third party. Although the
framework does define interaction patterns for authorization, it is not a strictly defined
programming interface, with some variations between OAuth providers. For more information
about the OAuth Authorization Framework, see the following website:

https://tools.ietf.org/html/rfc6749

OpenID Connect (OIDC) provides an identity layer on top of OAuth 2.0, making it possible to
also delegate authentication to a third party. It defines an interoperable REST-based
mechanism for verifying identity based on the authentication steps performed by the OAuth
provider, and to obtain basic profile information about the authorized user. For more
information about ODIC, see the following website:

https://openid.net/connect/

Chapter 6. Application Security

Game On! uses three-legged authentication defined by OAuth and OIDC to delegate to social
sign-on providers, specifically Facebook, GitHub, Google, and Twitter. As shown in
Figure 6-2, the game has a dedicated Auth service that acts as a proxy for the various social
login providers. This Auth service triggers a request to the target OAuth provider including
information required to identify the Game On! application.

Figure 6-2 OAuth invocation sequence diagram for Game On!

The service interaction goes through these steps:

1. The front end (web client or mobile application) makes a request to the game’s Auth

service on behalf of the user.

2. The Auth service returns a forwarding response that is automatically handled by the

browser to forward the user to the selected OAuth provider with the application identifier.

3. After the user has authenticated and granted permission, the OAuth provider returns an
authorization code in another redirect response. The browser automatically handles the
redirect to invoke a callback on the Auth service with the authorization code.

4. The Auth service then contacts the OAuth provider to exchange the authorization code for

an access token.

5. The Auth service then converts data from that token into a Signed JSON Web Tokens

(JWTs), which allows you to verify the identity of the user over subsequent inter-service
calls without going back to the OAuth provider.

Some application servers, like WebSphere Liberty, have OpenID Connect features to facilitate
communication with OAuth providers. As of this writing, those features do not provide the
access token that was returned from the OAuth provider. Also, minute protocol differences
can make using individual provider libraries easier than more generic solutions.

Microservices Best Practices for Java

A simpler approach might be to perform authentication with an OAuth provider by using a
gateway. If you take this approach, it is still a good idea to convert provider-specific tokens
into an internal facing token that contains the information that you need.

6.2.3 JSON Web Tokens

Identity propagation is another challenge in a microservices environment. After the user
(human or service) has been authenticated, that identity needs to be propagated to the next
service in a trusted way. Frequent calls back to a central authentication service to verify
identity are inefficient, especially given that direct service-to-service communication is
preferred to routing through central gateways whenever possible to minimize latency.

JWTs can be used to carry along a representation of information about the user. In essence,
you want to be able to accomplish these tasks:

(cid:2) Know that the request was initiated from a user request
(cid:2) Know the identity that the request was made on behalf of
(cid:2) Know that this request is not a malicious replay of a previous request

JWTs are compact and URL friendly. As you might guess from the name, A JWT (pronounced
“jot”) contains a JSON structure. The structure contains some standard attributes (called
claims), such as issuer, subject (the user’s identity), and expiration time, which has clear
mappings to other established security mechanisms. Room ia also available for claims to be
customized, allowing additional information to be passed along.

While building your application, you will likely be told that your JWT is not valid for reasons
that are not apparent. For security and privacy reasons, it is not a good idea to expose (to the
front end) exactly what is wrong with the login. Doing so can leak implementation or user
details that could be used maliciously. JWT.IO provides some nice web utilities for working
with JWTs, including a quick way to verify whether the encoded JWT that scrolled by in your
browser console or your trace log is valid or not. For more information about JWT.IO, see the
following website:

https://jwt.io

Dealing with time
One of the nice attributes of JWTs is that they allow identity to be propagated, but not
indefinitely. JWTs expire, which triggers revalidation of the identity that results in a new JWT.
JWTs have three fields that relate to time and expiry, all of which are optional. Generally,
include these fields and validate them when they are present as follows:

(cid:2) The time the JWT was created (iat) is before the current time and is valid.
(cid:2) The “not process before” claim time (nbf) is before the current time and is valid.
(cid:2) The expiration time (exp) is after the current time and is valid.

All of these times are expressed as UNIX epoch time stamps.

Signed JWTs
Signing a JWT helps establish trust between services, as the receiver can then verify the
identity of the signer, and that the contents of the JWT have not been modified in transit.

JWTs can be signed by using shared secrets, or a public/private key pair (SSL certificates
work well with a well known public key). Common JWT libraries all support signing. For more
information about JWT libraries, see the following website:

https://jwt.io/#libraries-io

Chapter 6. Application Security

Working with JWTs
Common JWT libraries make working with JWTs easy. For example, with JJWT1, creating a
new JWT is straightforward, as shown in Example 6-1.

Example 6-1 Creating and validating a signed JWT using JJWT

import java.time.Instant;
import java.time.temporal.ChronoUnit;
import java.util.Date;
import io.jsonwebtoken.Claims;
import io.jsonwebtoken.Jws;
import io.jsonwebtoken.Jwts;
import io.jsonwebtoken.SignatureAlgorithm;
public class JwtExample {

public String createJwt(String secret) throws Exception {

// create and sign the JWT, including a hint
// for the key used to sign the request (kid)
String newJwt = Jwts.builder()

.setHeaderParam("kid", "meaningfulName")
.setSubject("user-12345")
.setAudience("user")
.setIssuedAt(Date.from(Instant.now()))
.setExpiration(Date.from(Instant.now().plus(15, ChronoUnit.MINUTES)))
.signWith(SignatureAlgorithm.HS512, secret)
.compact();
return newJwt;

}
public void validateJwt(String jwtParameter, String secret) throws Exception {

// Validate the Signed JWT!
// Exceptions thrown if not valid
Jws<Claims> jwt = Jwts.parser()

.setSigningKey(secret)
.parseClaimsJws(jwtParameter);

// Inspect the claims, like make a new JWT
// (need a signing key for this)
Claims jwtClaims = jwt.getBody();
System.out.println(jwtClaims.getAudience());
System.out.println(jwtClaims.getIssuer());
System.out.println(jwtClaims.getSubject());
System.out.println(jwtClaims.getExpiration());
System.out.println(jwtClaims.getIssuedAt());
System.out.println(jwtClaims.getNotBefore());

}

Example 6-1 is an altered version of what the authentication service in Game On! does to
create the JWT from the returned OAuth token2. JWTs should be validated in a central place
such as a JAX-RS request filter.

A consideration is the longevity of the JWT. If your JWT is long lived, you will have significant
reduction in traffic to revalidate JWTs, but might then have a lag in the system reacting to
access changes.

1 https://github.com/jwtk/jjwt
2 https://github.com/gameontext/gameon-auth/

Microservices Best Practices for Java

In the case of Game On!, the user is connected by using long running WebSocket
connections to enable 2-way asynchronous communication between third parties. The JWT is
passed as part of the query string when the connection is established. Expiring that JWT
leads to an unpleasant user experience at the time of writing, so we chose to use a relatively
long JWT expiry period.

6.2.4 Hash-Based Messaging Authentication Code

Authentication by using Hash-Based Messaging Authentication Code (HMAC) signatures is
superior to HTTP Basic authentication. When using HMAC, request attributes are hashed and
signed with a shared secret to create a signature that is then included in the sent request.
When the request is received, the request attributes are rehashed by using the shared secret
to ensure that the values still match. This process authenticates the user, and verifies that the
content of the message has not been manipulated in transit. The hashed signature can also
be used to prevent replay attacks.

HMAC validation can be performed by an API Gateway. In this case, the Gateway
authenticates the user, and then can either serve cached responses, or produce a JWT or
other custom headers to include in the request before forwarding on to backend/internal
services. Using a JWT has the advantage of allowing downstream services to consistently
use JWTs to work with the caller’s identity, independent of the original authentication method
(such as OAuth when using a UI, versus HMAC for programmatic invocation).

HMAC is not yet a complete standard, API providers (either gateways or individual services)
often require different request attributes (http headers or a hash of the message body) in the
generated signature. Minimally, an identifying string and the date are used.

Using shared libraries is encouraged when working with HMAC signatures because HMAC
calculations must be performed in the same way by both the API consumer and the provider.
Consider providing client-side libraries to help consumers sign their requests correctly.

Game On! does not use an API gateway at the time of writing. We provide a shared library3 to
ensure that a common HMAC signing algorithm is used for both REST and WebSocket
requests. The shared library provides JAX-RS filters for inbound and outbound requests, and
a core utility that can be invoked around the WebSocket handshake between the Mediator4
and third-party Room services. See Example 6-2.

For example, the Mediator uses the shared library to sign outbound requests to the Map5.

Example 6-2 JAX-RS 2.0 client using SignedClientRequestFilter from shared library

Client client = ClientBuilder.newClient()

.register(JsonProvider.class);

// a filter is added to the JAX-RS client
// the filter will sign the request on the way out
SignedClientRequestFilter apikey = new

SignedClientRequestFilter(userid, secret);

client.register(apikey);

3 https://github.com/gameontext/signed
4 https://github.com/gameontext/gameon-mediator
5 https://github.com/gameontext/gameon-map

Chapter 6. Application Security

The shared library allows the use of simple annotations to flag server-side endpoints that
should have the inbound signature verified. JAX-RS filters and interceptors are provided to
verify both request headers and the message body as shown in Example 6-3.

Example 6-3 Map service JAX-RS endpoint: Annotation from shared library to enable signing

@POST
@SignedRequest
@Consumes(MediaType.APPLICATION_JSON)
@Produces(MediaType.APPLICATION_JSON)
public Response createRoom(RoomInfo newRoom) {

Site mappedRoom = mapRepository.connectRoom(…);
return Response.created(…).entity(mappedRoom).build();

}

Using the shared library in this way keeps the business logic clean. The configuration ensures
that the client and server side agree on included attributes and hash calculation as part of
producing the library. The shared library simplifies what both the API provider and the
consumer need to deal with.

6.2.5 API keys and shared secrets

An API key can be one of a few different things, but the usual usage is to identify the origin of
a request. Identifying the origin of a request is important for API management functions like
rate limiting and usage tracking. API keys are usually separate from an account’s credentials,
enabling them to be created or revoked at will.

An API key is sometimes a single string, but unlike a password, these strings are randomly
generated and over 40 characters long. However, sometimes the string is used directly in the
request as either a bearer token or query parameter (over https). Using an API key is more
secure when used as a shared secret for signing tokens (JWTs) or for digest methods of
request authentication (HMACs).

If your service creates API keys, ensure that API keys are securely generated with high
entropy and are stored correctly. Ensure that these keys can be revoked at any time. If your
application has distinct parts, consider using different API keys for each part. This
configuration can enable more granular usage tracking, and reduces the impact if a key is
compromised.

Asymmetric keys can also be used (such as SSL), but require a public key infrastructure (PKI)
to be maintained. Although it is more secure, this configuration is also more labor intensive,
and can be especially hard to manage when API consumers are third parties.

Consistent with the 12 factors, secrets should be injected configuration when used by
services for intra-service communication. The values should never be hardcoded because
that increases the likelihood they will be compromised (such as checked into a repository in
plain text). It either makes your code sensitive to the environment, or requires different
environments to use the same shared secret, neither of which are good practices.

Microservices Best Practices for Java

Chapter 7. Testing

It is essential that a microservice application is built with an awareness of how it can be
tested. Having good test coverage gives you more confidence in your code and results in a
better continuous delivery pipeline.

This chapter covers the tests that are required at three different stages in the build lifecycle:

(cid:2) Single service testing

Tests carried out in isolation by a team that owns a particular microservice in a system.

(cid:2) Staging environment

Tests that are run on objects in a staging environment. The microservices that form a
particular application are deployed into a staging environment for testing.

(cid:2) Production environment

Tests carried out on the live production system.

Tests should be automated as part of the build, release, run (delivery) pipeline.

The following topics are covered:

(cid:2) Types of tests
(cid:2) Application architecture
(cid:2) Single service testing
(cid:2) Staging environment
(cid:2) Production environment

7.1 Types of tests

This chapter focuses on the Java specific testing considerations, assuming some prior
knowledge of testing microservices. The following types of tests are covered:

(cid:2) Unit

Tests a single class or a set of closely coupled classes. These unit tests can either be run
using the actual objects that the unit interacts with or by employing the use of test doubles
or mocks.

(cid:2) Component

Tests the full function of a single microservice. During this type of testing, any calls to
external services are mocked in some way.

(cid:2) Integration

Integration tests are used to test the communication between services. These tests are
designed to test basic success and error paths over a network boundary.

(cid:2) Contract

Test the agreed contract for APIs and other resources that are provided by the
microservice. For more information, see “Consuming APIs” on page 38.

(cid:2) End-to-end

Tests a complete flow through the application or microservice. Usually used to test a
golden path or to verify that the application meets external requirements.

7.2 Application architecture

The internal structure of your microservice directly affects how easy it is to test. Figure 7-1
shows a simplified version of the microservice architecture described in Chapter 2, “Creating
Microservices in Java” on page 9.

Figure 7-1 The internal microservice architecture.

Microservices Best Practices for Java

The different internal sections break down into the following subsections:

(cid:2) Resources

Exposes JAX-RS resources to external clients. This layer can handle basic validation of
requests, but any business logic is passed on.

(cid:2) Domain logic

This area is where the bulk of the functional code goes. This code has no understanding of
external dependencies to the application.

(cid:2) Service connector

Handles communication with external services. This can be services within the
microservice system or third-party services. Responses are validated and then passed to
the domain logic layer.

(cid:2) Repositories

Includes data mapping and validation. The repositories layer has the logic that is required
to connect to the external data store.

The different sections of the application have different testing requirements. For example,
domain logic does not require integration testing, whereas resources do. All of the code in a
particular Java class should provide function from the same section. Separating the code in
this way results in all of the code in a particular class having the same testing requirements.
This technique makes your test suite simpler and better structured. Given the inherent
complexity of microservices, it is easy to get into a mess with your testing, so anything that
can add structure to your tests is a good idea.

7.3 Single service testing

This section defines single service testing as the tests that the owners of an individual service
must create and run. These tests exercise the code that is contained inside the logical
boundary in the diagram of the internal microservice architecture as shown in Figure 7-1 on
page 78. Three types of tests apply here: Unit tests, component tests, and integration tests.
Unit and integration tests are used to test the individual parts of the microservice, and
component tests are used to test the service as a whole.

7.3.1 Testing domain or business function

The code in your microservice that performs business function should not make calls to any
services external to the application. This code can be tested by using unit tests and a testing
framework such as JUnit1. The unit tests should test for behavior and either use the actual
objects (if no external calls are needed) or mock the objects involved in any operations.

When writing tests using the actual objects, a simple JUnit test suffices. For creating mocks of
objects, you can either use the built-in capabilities of Java EE or use a mocking framework.
The @Alternatives annotation2 in the Context and Dependency Injection (CDI) specification
enables injection of mock objects instead of the actual beans. Plenty of mocking frameworks
are available for Java. For example, JMockit3 is designed to work with JUnit to allow you to
mock objects during testing. In the most basic test using JMockit, you can create mocked
objects by using the @Mocked annotation and define behavior when this object is called by
using the Expectations() function.

1 http://junit.org/junit4/
2 http://docs.oracle.com/javaee/6/tutorial/doc/gjsdf.html
3 http://jmockit.org/

Chapter 7. Testing

Consider an online payment application that has an “Order” microservice. Suppose that a
class called OrderFormatter takes in an account ID and returns an order history in a
particular format as in Example 7-1. The DatabaseClient class handles requests to the
database to get the list of orders. The OrderFormatter is @ApplicationScoped so that it can
be injected into any REST endpoints that use it.

Example 7-1 The OrderFormatter class formats the response from the Orders class

@ApplicationScoped
public class OrderFormatter {
@Inject
DatabaseClient client;
public FormattedHistory getOrderHistory(String id) {
History orderHistory = client.getHistory(id);
FormattedHistory formatted = format(orderHistory;
return formatted;
}
private FormattedHistory format(History history) {
…
}
}

To test this class, make the assumption that the DatabaseClient class will return the correct
orders from the getHistory() function. The behavior that you want to test is the actual
formatting. However, you cannot call the format object directly because it is a private method.
You want to avoid altering the code to enable testing, such as by changing the format()
method to public. Instead, use a mocked version of the DatabaseClient class. Example 7-2
shows a simple test of the OrderFormatter class using JMockit and JUnit. The @Tested
annotation indicates that the class that is being tested. This technique allows test objects to
be injected into the class when the test is running. The @Injectable annotation is used to
inject a mocked version of the DatabaseClient into the OrderFormatter object when the test
runs. The Expectations() function defines the behavior of the mocked object.

Example 7-2 JMockit allows you to define what particular methods will return

public class TestOrderFormatter {
@Tested OrderFormatter formatter;
@Injectable DatabaseClient databaseClient;
@Test
public void testFormatter() {
String id = "Test ID";
new Expectations() {{
History history = // set history to be formatted
databaseClient.getHistory(id); returns(history);
}};
FormattedHistory expected = // set expected object
FormattedHistory actual = formatter.getOrderHistory(id);
assertEquals(expected.toString(), actual.toString());
}
}

Microservices Best Practices for Java

7.3.2 Testing resources

Classes that expose JAX-RS endpoints or receive events should be tested by using two types
of tests: Integration tests and contract tests.

Integration tests
Integration tests are used to verify the communication across network boundaries. They
should test the basic success and failure paths in an exchange. Integration tests can either be
run in the same way as unit tests, or by standing up the application on a running server. To
run integration tests without starting the server, call the methods that carry JAX-RS
annotations directly. During the tests, create mocks for the objects that the resource classes
call when a request comes in.

To run integration tests on a running server, you can use one of the methods described in
“Tests on a running server” on page 82. To drive the code under test, use the JAX-RS client
provided by JAX-RS 2.0. to send requests.

Integration tests should validate the basic success and error paths of the application.
Incorrect requests should return useful responses with the appropriate error code.

Consumer driven contract
A consumer of a particular service has a set of input and output attributes that it expects the
service to adhere to. This set can include data structures, performance, and conversations.
The contract is documented by using a tool like Swagger. See Chapter 2, “Creating
Microservices in Java” on page 9 for more information. Generally, have the consumers of a
service drive the definition of the contract, which is the origin of the term consumer driven
contract.

Consumer driven contract tests are a set of tests to determine whether the contract is being
upheld. These tests should validate that the resources expect the input attributes defined in
the contract, but also accepts unknown attributes (it should just ignore them). They should
also validate that the resources return only those attributes that are defined in the
documentation. To isolate the code under test, use mocks for the domain logic.

Maintaining consumer driven contract tests introduces some organizational complexity. If the
tests do not accurately test the contract defined, they are useless. In addition, if the contract is
out of date, then even the best tests will not result in a useful resource for the consumer.
Therefore, it is vital that the consumer driven contract is kept up to date with current consumer
needs and that the tests always accurately test the contract.

Contract tests require the actual API to be implemented. This technique requires the
application be deployed onto a server. See '“Tests on a running server” on page 82 for more
information. Use tools such as the Swagger editor4 to create these tests. The Swagger editor
can take the API documentation and produce implementations in many different languages.

Another dimension to contract testing is the tests that are run by the consumer. These tests
must be run in an environment where the consumer has access to a live version of the
service, which is the staging environment. See section 7.4.3, “Contract” on page 86 for more
details.

4 http://editor.swagger.io/

Chapter 7. Testing

Tests on a running server
A few different methods are available for starting and stopping the application server as part
of your automated tests. There are Maven and Gradle plug-ins for application servers such as
WebSphere Application Server Liberty that allow you to add server startup into your build
lifecycle. This method keeps complexity out of your application and contains it in the build
code. For more information about these plug-ins, see the following websites:

(cid:2) Maven

https://github.com/WASdev/ci.maven

(cid:2) Gradle

https://github.com/WASdev/ci.gradle

Another solution is to use Arquillian. Arquillian can be used to manage the applications server
during tests. It allows you to start and stop the server mid-test, or start multiple servers.
Arquillian is also not affected by the container, so if you write Arquillian tests, they can be
used on any application server. This feature is useful for contract testing because the
consumers do not have to understand the application server or container that is used by the
producer. For more information about Arquillian, see the following website:

http://arquillian.org

For a more detailed comparison of these different methods, see “Writing functional tests for
Liberty” at:

https://developer.ibm.com/wasdev/docs/writing-functional-tests-liberty/

7.3.3 Testing external service requests

Inevitably, your microservice must make calls to external services to complete a request, such
as calls to other microservices in the application or services external to the application. The
classes to do this construct clients that make the requests and handle any failures. The code
can be tested by using two sets of integration tests: One at the single service level and one in
the staging environment. Both sets test the basic success and error handling of the client.
More information about the tests in the staging environment is available in 7.4.2, “Integration”
on page 86.

The integration tests at the single service level do not require the service under test or the
external services to be deployed. To perform the integration tests, mock the response from
the external services. If you are using the JAX-RS 2.0 client to make the external requests,
this process can be done easily by using the JMockit framework.

Consider the online store example introduced in 1.5.1, “Online retail store” on page 7.
Suppose the Account service must make a call to the Order service to get the list of recent
orders for a particular account. The logic to make the request should be encapsulated in a
class similar to Example 7-3. The OrderServiceClient is an @ApplicationScoped bean that is
injected into the classes that use it. We have chosen to use CDI to inject the URL for the
requests, rather than using a library that gets the service list from the registry as part of the
request. In this case, we are just returning an empty History object if the response from the
Order service is bad. Take advantage of the @PostConstruct methods to initialize the JAX-RS
client because you can then call the method directly in the test if required.

Example 7-3 OrderServiceClient handles all requests to the Order service

@ApplicationScoped
public class OrderServiceClient {
@Resource(lookup = "orderServiceUrl")

Microservices Best Practices for Java

String orderServiceUrl;

private Client client;

@PostConstruct
public void initClient() {
this.client = ClientBuilder.newClient();
}
public History getHistory(String id) {
String orderUrl = orderServiceUrl + "/" + id;
WebTarget target = client.target(orderUrl);
Response response = callEndpoint(target, "GET");
History history = getHistoryFromResponse(response);
return history;
}
private History getHistoryFromResponse(Response response) {
if (response.getStatus() == Response.Status.OK.getStatusCode()) {
return response.readEntity(History.class);
} else {
return new History();
}
}
private Response callEndpoint(WebTarget target, String callType) {
Invocation.Builder invoBuild = target.request();
Response response = invoBuild.build(callType).invoke();
return response;
}
}

To test this class, mock the response from the Order service as shown in Example 7-4. By
putting the logic that interprets the response in a separate method to the one making the call,
you can test this logic by directly invoking that method. JMockit provides a Deencapsulation
class to allow invocation of private classes from tests. This technique provides test coverage
for the handling of response codes from external services. It is also possible to mock the
JAX-RS WebTarget or InvocationBuilder if your class structure requires this. Higher-level
coverage of external requests is done in the staging environment.

Example 7-4 The response from the Order service is mocked to enable isolated testing

public class OrderServiceClientTest {

@Mocked
Response response;

OrderServiceClient orderHistoryClient = new OrderServiceClient();

@Test
public void test200() {
new Expectations() {{
History user = new History();
user.addOrder("Test");
response.getStatus(); returns(200);
response.readEntity(History.class); returns(user);
}};
History history = Deencapsulation.invoke(orderHistoryClient,
"getHistoryFromResponse", response);

Chapter 7. Testing

assertTrue("Expected string Test", history.getOrders().contains("Test"));
}
@Test
public void test500() {
new Expectations() {{
response.getStatus(); returns(500);
}};
History history = Deencapsulation.invoke(orderHistoryClient,
"getHistoryFromResponse", response);
assertTrue("Expected empty order list", history.getOrders().isEmpty());
}
}

7.3.4 Testing data requests

In a microservice architecture, each microservice owns its own data. See Chapter 5,
“Handling data” on page 41 for more information. If you follow this guideline, the developers of
a microservice are also responsible for any external data stores used. As described in 7.2,
“Application architecture” on page 78, the code that makes requests to the external data store
and performs data mapping and validation is contained in the repositories layer. When testing
the domain logic, this layer should be mocked. Tests for data requests, data mapping, and
validation are done by using integration tests with the microservice and a test data store
deployed locally or on a private cloud. The tests check the basic success and error paths for
data requests. If the data mapping and validation for your application requires extensive
testing, consider separating out this code and testing it using a mocked database client class.

Test data
The local version of the data store must be populated with data for testing. Think carefully
about what data you put in the data store. The data should be structured in the same way as
production data but should not be unnecessarily complicated. It must serve the specific
purpose of enabling data request tests.

7.3.5 Component testing

Component tests are designed to test an individual microservice as one piece. The
component is everything inside the network boundary, so calls to external services are either
mocked or are replaced with a “test-service.” There are advantages and disadvantages to
both scenarios.

Using mocks
By mocking the calls to external services, you have fewer test objects to configure. You can
easily define the behavior of the mocked system by using frameworks like JMockit, and no
tests will fail due to network problems. The disadvantage of this approach is that it does not
fully exercise the component because you are intercepting some of the calls, increasing the
risk of bugs slipping through.

Test services
To fully exercise the communication boundaries of your microservice, you can create test
services to mimic the external services that are called in production. These test services can
also include a test database. The test services can also be used as a reference for
consumers of your microservice. The disadvantage of this system is that it requires you to
maintain your test services. This technique requires more processor cycles than maintaining

Microservices Best Practices for Java

a mocking system as you must fully test the test microservice and create a deployment
pipeline.

After you are using a mocking framework for other levels of testing, it makes sense to reuse
those skills. However, if you do take the mocking approach, you must make sure that the tests
in your staging environment exercise inter-service communications effectively.

7.3.6 Security verification

Security is important in a distributed system. You can no longer put your application behind a
firewall and assume that nothing will break through. For more details about securing your
microservices, see Chapter 6, “Application Security” on page 67.

Testing the security of your microservice is slightly different depending on how you implement
security. If the individual services are just doing token validation, then test at the individual
service level. If you are using another service or a library to validate the tokens, then that
service should be tested in isolation and the interactions should be tested in the staging
environment.

The final type of tests to run is security scanners such as IBM Security AppScan® to highlight
security holes. AppScan scans your code and highlights any vulnerabilities. It also provides
recommendations for fixing the vulnerabilities. For more information about Security AppScan,
see the following website:

http://www.ibm.com/software/products/en/appscan

7.4 Staging environment

This section defines a staging environment as a test environment that is identical (where
possible) to the production environment. The build pipeline deploys successfully tested
microservices to the staging environment where tests are run to verify the communication
across logical boundaries, that is, between microservices.

7.4.1 Test data

The staging environment should include any data stores that will be in your production
system. As described in 7.3.4, “Testing data requests” on page 84, the test data should not be
unnecessarily complicated. The data in this data store will be more complete than at the
individual microservice level, as these tests are testing more complicated interactions.

Use tools to inject data into tests for you. Tools allow you to have more control over the flow of
data around the system, enabling you to test what happens if bad data is introduced. The
microservice orchestration and composition solution Amalgam8 offers this feature using its
controller component. The controller passes information between the service registry and the
microservice side-car. It can inject data dynamically to simulate the introduction of bad data
into the system.

For more information about Amalgam8, see the following website:

https://www.amalgam8.io/

Chapter 7. Testing

7.4.2 Integration

Integration tests are used to test the interactions between all the services in the system. The
in-depth behavior of the individual services has already been tested at this stage. The
consumer driven contract tests should have ensured that the services interact successfully,
but these tests identify bugs that have been missed. The tests should check the basic
success and error paths of service communication with the application deployed. Use the test
data as discussed in the previous section.

Rather than testing all of the services at once, it might still be necessary to mock out some of
the services during testing. Test the interaction of two specific services, or a small set of
services, adding in mocked behavior when calls are made to outside the set. Mocking the
calls to the services outside the group under test is done in the same way as the unit tests on
the APIs. The same techniques that are used to start and stop the server or container for
contract testing also apply here (see 7.3.2, “Testing resources” on page 81).

7.4.3 Contract

Every service that consumes another service or resource should have a set of contract tests
that are run against that resource (especially in staging environments). Given that services
evolve independently over time, it is important to ensure that the consumer's contract
continues to be satisfied.

These tests are specifically written by the consumer (the client side), and are run and
managed as part of the test suite for the consuming service. By contrast, the tests that are
discussed in 7.3.2, “Testing resources” on page 81 are written by the provider to verify the
contract as a whole.

7.4.4 End-to-end

End-to-end testing is essential to find the bugs that were missed previously. End-to-end tests
should exercise certain “golden paths” through the application. It is unrealistic to test every
path through the application with an end-to-end test, so identify the key ones and test those.
A good way to identify these paths through the environment is to review the key external
requirements of an application. For example, if your application is an online retail store you
might test the following paths:

(cid:2) User logs in
(cid:2) User purchases an item
(cid:2) User views the summary of the order
(cid:2) User cancels the order

End-to-end testing should also include the user interface. Tools such as SeleniumHQ can be
used to automate interactions with a web browser for testing. For more information about
SeleniumHQ, see the following website:

http://www.seleniumhq.org/

7.4.5 Fault tolerance and resilience

A microservice should be designed in a fault tolerant way as described in 4.2, “Fault
tolerance” on page 37. Various techniques can be used to increase the resilience and fault
tolerance of individual microservices, but you should still test how fault tolerant your system is
as a whole. Tests for fault tolerance should be performed with all of the services deployed. Do
not use any mocks.

Microservices Best Practices for Java

Taking down individual microservices
In a microservice system, you must not rely on all of the microservices being available at any
one time. During the testing phase, make requests to the system while taking down and
redeploying individual services. This process should include the microservices in the system
and backend data stores. Monitor the time for requests to return and identify an acceptable
response time for your application. If the response times are too long, consider reducing the
timeout values on your services or altering the circuit breaker configurations.

There are tools that can automate this process, the best known of which within the Java
community is Netflix Chaos Monkey, which terminates instances of virtual machines to allow
testing of fault tolerance. For more information about Netflix Chaos Monkey, see the following
website:

https://github.com/Netflix/SimianArmy/wiki/Chaos-Monkey

Other tools, such as Gremlin, which is used by Amalgam8, take the approach of intercepting
and manipulating calls to microservices rather than shutting down instances. For more
information about Gremlin, see this website:

https://github.com/ResilienceTesting/gremlinsdk-python

Injecting bad data
As in the integration tests, tools such as Amalgam8 can be used to automate the injection of
bad data during testing. If you have successfully used the bulk head pattern, then bad data in
one microservice should not propagate to other microservices.

Stress testing
Microservices should be able to handle unexpected loads. Stress testing should be used to
test the bulk heads in your microservices. If a particular microservice is holding up requests,
look at the configured bulk heads.

Recoverability testing
Individual microservices should also be resilient to problems in their own deployment. If a
container or virtual machine goes down, the microservice needs to be able to recover quickly.
Orchestration tools such as Amalgam8 or Kubernetes5 (for container orchestration) will spin
up new instances of any application if one goes down. A microservice must have fast startup
time and, when a shutdown is required, it should shut down gracefully. Using an application
server that provides a fast startup time is also essential.

Couple your recoverability testing with taking down service instances. Allow your
orchestration system to create the new instances and monitor how long it takes to start the
new service.

7.5 Production environment

To really test your system, you should consider running tests on the production environment.
With so many moving parts and services constantly coming and going, you should not expect
the system to continue working perfectly indefinitely. It is important to add monitoring and
analytics to your application to warn of any bad behavior. For more information, see
Chapter 9, “Management and Operations” on page 107.

5 http://kubernetes.io/

Chapter 7. Testing

Repeat these specific tests from the staging environment in the production environment:

(cid:2) Injecting test or bad data

(cid:2) Taking down services, both to test the fault tolerance of other services and the

recoverability of the service you took down

(cid:2) Security verification

Use the same tools and techniques as in the staging environment. Be careful about when you
run these tests. Do not run them during peak hours. Instead, select a timeslot that has the
smallest impact on your clients as possible, just in case something goes down.

7.5.1 Synthetic monitoring

To determine the health of your production system, create synthetic transactions to run
through your application. Use a browser emulator to instigate the synthetic transactions and
monitor the responses. Look for invalid responses or large response times. You can also run
the automated build system periodically. If you are committing new code frequently, the builds
will also be running frequently, but it is likely that particular services might go through a time
where fewer commits are made. During this time, if you continue running builds, you will
discover any changes to the application dependencies that might cause problems.

7.5.2 Canary testing

Canary tests are named after the proverbial canary taken into the coal mine to serve as an
early indicator of unsafe operating conditions for miners. They are minimal tests that verify all
dependencies and basic operating requirements for a service are present and functioning. If
the environment is unhealthy, canary tests fail quickly, providing an early indicator for a
botched deployment.

Microservices Best Practices for Java

Chapter 8. From development to production

This chapter covers how individual microservices are built and deployed. To that end, review
the characteristics that microservices should have that facilitate a smooth transition from
development to production:

(cid:2) A microservice must be independently deployable.

(cid:2) A new version of a microservice should be deployable in minutes rather than in hours.

(cid:2) A microservice must be fault tolerant, and should prevent cascading failures.

(cid:2) A microservice should not require any code changes as it is deployed across target

environments.

As mentioned earlier in the book, microservice architectures require good automated
deployment tools to help manage the deployment, testing, and promotion of services across
target environments.

The following topics are covered in this chapter:

(cid:2) Deployment patterns
(cid:2) Deployment pipelines and tools
(cid:2) Packaging options
(cid:2) Configuration of the applications across stages

8.1 Deployment patterns

A traditional monolith is built and deployed as one unit. A deployment monolith is built of many
parts, many individual Java Platform, Enterprise Edition archives, for example, that are
combined into one deployed artifact. Microservices, by contrast, are many different pieces
that are all deployed independently. This configuration provides more flexibility in how
consumers are exposed to service changes:

(cid:2) Blue/Green Deployment

Blue/Green deployments are a common pattern for monoliths. The key aspect of this
approach is that you are maintaining two working versions of your environment such that
one can be used as a rollback to the other if a deployment failure occurs. The process
goes something like this:

a. You have a Blue system that is active and running.

b. You set up the Green system, test it, verify it is working properly, and then switch traffic
to the Green system. The Blue system is left alone as a rollback should something fail
as traffic moves to the Green system.

c. After the Green system is stable, the Blue system is prepared with the next outgoing

updates, and the process is repeated to switch from the Green system back to the Blue
system.

In the microservices context, this replicated environment still applies. Each system (Blue
and Green) contains all of your microservices running with stable versions in a
configuration that works. Periodic, coordinated updates of otherwise uncoupled services
can be managed this way, with updates gathered and stabilized with each other into a
known functioning system that is enabled (or disabled) as a set.

(cid:2) Canary Releasing

Canary releases are also known as phased or incremental rollouts. A canary release aims
to discover and contain failure when deploying a new service. Just as with Blue/Green
deployments, canary releases start with a duplicated environment. After the new function
is introduced into one of the two systems, traffic is gradually (and selectively) routed to the
new function to ensure it behaves and performs as expected.

(cid:2) Toggling

Feature toggles are flags that are used inside the service to enable or disable new
function. This process can effectively be used to allow service consumers to opt in to
testing new function (perhaps using custom HTTP headers) without having to maintain
different versions of the service or engineer coordinated rollouts. Toggling is common in
monolithic environments, too, as a way to protect behavior changes from impacting
existing consumers.

Toggles can be set in the environment such that a service either has the function enabled
or it does not. They can be dynamic configuration settings that act as a binary mode for
the service, or they can be combined with per request mechanisms like HTTP headers to
turn new functions on or off per request.

Toggles can be used to make routing decisions, which allows them to be a selector for
service-centric canary releases. They can also be used for A/B testing, where you are
trying to make a side-by-side comparison of alternative implementations.

Microservices Best Practices for Java

8.2 Deployment pipelines and tools

One of the benefits of microservice architectures is agility. Because each individual service
has a focused scope and an independent lifecycle, it takes less time to implement a new
feature as a microservice, test it, and deploy it into production. Microservices are also often
built iteratively, which means supporting the notion of continuous integration and deployment.

Deployment artifacts often run on different systems, so many systems must be configured to
make the services work. The more manual steps there are for deploying a microservice into
production, the more likely it is that unexpected problems will arise due to bad typing or
forgotten steps. Human intervention should be reduced to a minimum. For example, manual
approvals might be required before a new version of a service can be released into
production environment for regulatory reasons. However, everything leading up to, and
following from, that approval can be automated).

Deployment pipelines as described by Martin Fowler1 are used as a part of a Continuous
Delivery environment2. They describe or define a set of steps to follow to get microservices
into production, usually by breaking the process into stages that cover building, deploying,
and then testing the service in each stage to verify that all is well. It is helpful to use DevOps
technologies to simplify the creation of a Deployment pipeline and related tool chain to
support moving applications from development to production. For more information about
DevOps technologies, see this website:

http://blog.xebialabs.com/2015/07/20/xebialabs-launches-the-periodic-table-of-devo
ps-tools/

As mentioned earlier, the source for your microservices should be in a Source Code
Management (SCM) tool like Subversion3 or GIT4. It should be built with a build tool that
includes dependency management like Maven5 or Gradle6. Choose one of the repository
management tools (such as Nexus7 or Artifactory8) to store versioned binary files of your
shared libraries. Build your services using one of the CI-Tools like Jenkins9 or Bamboo10.

After the automated build of the microservice testing must be done, and for some of that
testing you need to deploy the microservice, which means you need the correct
configurations placed on your automatically provisioned system. You can use classic scripting
to configure the target stages correctly, but you must then manage platform or other system
differences in the scripts. Scripting frameworks/infrastructures like Chef11, Puppet12,
Ansible13, and Salt14 can handle this situation much better. They all can be used the same
way for initial installation and for modification of an existing system. You describe the target
content of the configuration files, and these tools deal with the difference if there are changes
on the target system.

1 http://martinfowler.com/bliki/DeploymentPipeline.html
2 http://martinfowler.com/bliki/ContinuousDelivery.html
3 http://subversion.apache.org/
4 http://git-scm.com/
5 http://maven.apache.org/
6 http://gradle.org/
7 http://www.sonatype.org/nexus/
8 http://www.jfrog.com/artifactory/
9 http://jenkins.io/
10 http://www.atlassian.com/software/bamboo
11 http://www.chef.io/
12 http://puppet.com/
13 http://www.ansible.com/
14 http://saltstack.com/

Chapter 8. From development to production

Containerization can also help you to get a correctly configured system. For dynamic
provisioning, you can use Cloud hosting services infrastructure as a service (IaaS) or
platform as a service (PaaS). Due to the high number of microservices in your system, it is
good to have a tool to help you with Release Management. All of these deployed systems
must now be monitored (see Chapter 9, “Management and Operations” on page 107). Logs
must also be gathered for auditing and diagnostic purposes (see 9.2, “Logging” on page 111).
Last, but not least, the security holes in your microservice should be found, as detailed in
Chapter 6, “Application Security” on page 67.

It is essential that the build steps to create the microservice are done with a build tool that can
also manage dependencies of JAR files. Maven and Gradle are the best choice to do the job.
In both build systems, you can list the needed JAR files to build your microservice to ensure
that every build of a service leads to an immutable software artifact. Generally, have a distinct
version or revision indicator for every build result of a microservice to make it clear which
version of the service is deployed. This goal can be achieved with Maven or Gradle. This
version of the service should not be confused with a semantic API version: it just determines
which version of the codebase is running,

A sample window of a build pipeline “The Pipeline View” used in Jenkins (CI server) is shown
at:

http://wiki.jenkins-ci.org/display/JENKINS/Build+Pipeline+Plugin

As you can see in the sample window, versions of the same application are running in parallel
through the pipeline. The different rectangles show the deployment state of this software
version on the stages.

Templates of microservices (9.3, “Templating” on page 113) can be helpful to develop
microservices that need the same implementations for the supporting systems they access.
Logging, for example, is necessary in a system of microservices, and is also helpful to track
the requests between microservices. If every development team of a service does its own
logging and also uses its own logging format, it can be more difficult to join the log files of the
different services. Security is also a requirement that must be developed to central
specifications. Templates can codify these common concerns, and turn them into conventions
that are easy to follow. However, if developers feel restricted by these templates, they will
ignore or get rid of them. Balance the effort spent building templates with the value they bring
to the developers who are writing services.

8.3 Packaging options

The next question, after building the microservice, is which packaging format should be used
to deploy the application. A recurring theme in both the twelve factors and the definition of
cloud native applications is the creation of immutable artifacts that can be run without change
across different deployment environments.

The more things change from deployment to deployment, the greater the risk that something
will break in unexpected ways, due to misconfiguration, missing dependencies, and so on.
Building immutable, self-contained artifacts introduces more consistency between
development and production environments, and reduces the likelihood of deployment time
surprises.

Microservices Best Practices for Java

The following packaging options are available, among others:

(cid:2) WAR/EAR file deployment on preinstalled middleware
(cid:2) Executable JAR file
(cid:2) Containers
(cid:2) Every microservice on its own virtualized server

In general, you want to minimize differences between the environment in which the service is
developed and tested, and the environment in which it runs in production. Some packaging
and deployment choices make it easier to achieve parity between development and
production environments than others. The following sections explore the pros and cons of
each packaging option.

8.3.1 JAR, WAR, and EAR file deployment on preinstalled middleware

This variant of packaging is the default way for deploying Java EE applications, where WAR or
EAR files are deployed to a dedicated system with a preinstalled application server. In
traditional environments, many EARs and WARs are deployed to the same long running
server, with deployments batched per week, month, or even longer time frames.

On the way from development to production, the application gets hosted in different
application servers in different staging environments (such as quality assurance, performance
test, and user acceptance test). Although every environment should have the same
configuration, this is not always the case. For example, a production system might use a more
conservative approach to upgrades than development or test systems do.

The following problems can arise from this approach:

(cid:2) Frequent deployments in a shorter time period (day, hour) can be hard to achieve. For

example, some application deployments need a restart of the application server and as a
consequence interrupt the other applications on the same application server. This
interruption often cannot be done daily or even hourly.

(cid:2) If different applications are hosted together on the same application server, the load from

one application can interfere with the performance of the other application.

(cid:2) High variations in the load of an application cannot easily be adopted in the infrastructure.

(cid:2) Different stages might not have the same configuration, which can lead to application

problems in one stage that cannot be reproduced in the other stage.

(cid:2) Separated responsibilities often lead to a discussion between developer and administrator

to find a “guilty” person when deployment problems arise.

If this pattern is used with microservices, the relationship should be one-to-one between the
service and the preinstalled middleware server. This configuration is common, for example, in
PaaS environments, where you push only the WAR or EAR to a freshly built, predefined
server. It can also apply to cases where application server instances are provisioned by using
tools like Chef or Puppet, with the WAR or EAR added afterward. The isolation of services
into their own middleware instances is important to allow services to scale independently, and
to reduce the risk of service updates interfering with each other.

As mentioned, this approach has a deployment risk even if services are isolated because the
service still relies on the preinstalled middleware to supply all of the dependencies that it
requires. However, because it mirrors much of the traditional monolith application server
experience, it can be an easy first step when starting a new microservices application.

Chapter 8. From development to production

8.3.2 Executable JAR file

One way to avoid depending on the deployment environment to provide dependencies is to
pack everything that is needed to run the service inside an executable JAR file. These fat or
über JAR files contain the service class files and supporting shared libraries, including servlet
containers or other Java EE middleware. This technique eliminates problems introduced by
dependency mismatches between development and test or production deployment
environments.

These self-contained JAR files are generally easy to handle, and help create a consistent,
reproducible environment for local development and test in addition to target deployment
environments.

As mentioned in 2.1, “Java platforms and programming models” on page 10, there are various
programming models available that produce stand-alone executable JAR files.

Spring Boot
The executable JAR files produced by Spring Boot15 contain a Spring application, its
dependent JAR files, and an underlying web application run time (Tomcat16, Jetty17, or
Undertow18). Most of the application configuration is automatically done by Spring Boot itself
and can be changed from outside the application. Additional functions for production support,
like metrics and health checks, are also included in this package.

There is a quick and easy way to get started. First, you need a build script, like the maven file
shown in Example 8-1.

Example 8-1 Spring Boot Maven Quick Start

<parent>
<groupId>org.springframework.boot</groupId>
<artifactId>spring-boot-starter-parent</artifactId>
<version>1.4.1.RELEASE</version>
</parent>
<dependencies>
<dependency>
<groupId>org.springframework.boot</groupId>
<artifactId>spring-boot-starter-web</artifactId>
</dependency>
</dependencies>

You must implement a Controller to respond to the HTTP requests that is started by
SpringApplication (Example 8-2).

Example 8-2 Spring Boot Controller

@Controller
@EnableAutoConfiguration
public class MyController {
@RequestMapping("/myservice")
@ResponseBody
public String sayHello() {
return "Greetings from Spring Boot!";

15 http://projects.spring.io/spring-boot/
16 http://tomcat.apache.org/
17 http://www.eclipse.org/jetty/
18 http://undertow.io/

Microservices Best Practices for Java

}
public static void main(String[] args) throws Exception {
SpringApplication.run(MyController.class, args);
}
}

The dependency management of the JAR files of the application can be done with Maven or
Gradle. Configuration follows the “Convention over Configuration19” method. An
auto-configuration feature scans for included JAR files in the application and provides a
default behavior to these Spring modules. This process can also be done by hand to
overwrite the default behavior. The application should be implemented with frameworks from
the Spring ecosystem to get the most from Spring Boot.

Microservices built and packaged by using Spring Boot have everything that they need to run
as 12-factor applications across different deployment environments. Spring Cloud, another
set of packages from the Spring ecosystem, provides an additional collection of integrations
between third-party technologies and the Spring programming model to support by using
Spring Boot applications in cloud environments. For example, Spring Cloud Configuration
supports injecting application configuration from external configuration sources in support of
immutable application artifacts and Spring Cloud Netflix provides language bindings and
libraries to easily build microservice applications by using the popular Netflix OpenSource
Software framework.

Dropwizard
Dropwizard also produces stand-alone executable JAR files by using a preselected stack that
enhances an embedded Jetty servlet container with extra capabilities.

Wildfly Swarm
Wildfly Swarm20 packages the application and the subset of Wildfly server21 the application
needs into the executable JAR file. Microservices produced with Wildfly swarm can use a
wider set of pre-packaged Java EE technologies, rather than starting from a servlet engine
and having to gather and assemble the rest.

WebSphere Liberty
IBM WebSphere Liberty can also produce immutable executable JAR files containing only the
technologies the application needs. The packaging tool of Liberty can combine the following
elements:

(cid:2) A customized (minified) Liberty run time
(cid:2) Java EE applications packaged as a WAR file or EAR file (usually only one)
(cid:2) Supporting configuration like shared libraries, JNDI values, or data source configurations

Liberty has sensible defaults, requiring configuration only for customized elements with ample
support for injected configuration. Liberty is able to directly interpret environment variables,
which can eliminate startup scripts that pre-process configuration files to insert
environment-specific values. For example, a data source can be defined in server.xml as
shown in Example 8-3.

Example 8-3 Example 3. Configuring a data store by using environment variables

<couchdb id="couchdb" jndiName="couchdb/connector"
libraryRef="couchdb-lib"

19 https://en.wikipedia.org/wiki/Convention_over_configuration
20 http://wildfly-swarm.io
21 http://wildfly.org/

Chapter 8. From development to production

password="${env.COUCHDB_PASSWORD}"
url="${env.COUCHDB_SERVICE_URL}"
username="${env.COUCHDB_USER}"/>

8.3.3 Containerization

The previous strategies are all running on an operating system that must be installed and
configured to work with the executable JAR file. A strategy to avoid the problems resulting
from inconsistently configured operating systems is to bring more of this operating system
with you. This strategy leads to the situation that a microservice runs on an operating system
with the same configuration on every stage.

Containers are lightweight virtualization artifacts that are hosted on an existing operating
system. Docker provides an infrastructure capability to build a complete file system that
contains all elements (tools, libraries) of the operating system that are needed to run the
application, including the application itself. Docker Engine is a lightweight run time that
provides both the abstraction and isolation between Docker containers and the host, and the
supporting tools to work with hosted containers. For more information about Docker, see this
website:

http://www.docker.com

Docker containers isolate applications from other applications (residing in peer containers)
and also separates applications from the host’s operating system. The isolation of these
containers and their lightweight attributes make it possible to run more than one Docker
container on one operating system (see Figure 8-1). This configuration means that you do not
need as many virtualized servers to host your system of microservices.

Figure 8-1 Multiple Docker containers on one server

Microservices Best Practices for Java

Docker images are immutable artifacts. When applications are built into Docker images, those
images can then be used to start containers in different deployment environments, ensuring
that the same stack is used across all environments. Using Docker containers makes the
exact packaging format (executable JAR, WAR, EAR, ZIP file) irrelevant because the image
configuration determines how a container is started. Configuration values for the microservice
hosted inside a container can be provided either by using environment variables or mounted
volumes.

Data stores and all other backends needed by the microservice should not be hosted in the
same container as the microservice itself, as is mentioned in the 12 factors. These backends
should be treated as services, which is an essential characteristic of a microservice. Data
stores, for example, can also be managed as Docker containers. For failover reasons, you
should not host multiple instances of the same microservice on the same host.

8.3.4 Every microservice on its own server

Another alternative to keep the operating system as identical as possible across all stages is
to use the same virtualized server as the basis for every stage. To achieve this goal, install the
operating system and other tools that you need, and then create a template of that server.
Use this template as the basis for all servers that host your system of microservices, which is
another form of immutable server.

To manage these servers and server templates, you can use tools provided by your
virtualization provider, cloud provider or other third-party options. Some of the third-party
options available now include Vagrant22, VMware23, and VirtualBox24. When paired with
software configuration and automation technologies like Salt or Ansible, the virtualization of
microservices onto individual servers can become flexible and agile.

This configuration leads mainly to two variants:

(cid:2) Having one microservice packaged on a single virtualized server.
(cid:2) Having multiple microservice packages on a single virtualized server (see Figure 8-2).

Figure 8-2 Microservices on virtualized servers or microservices together on one server

When deploying only one microservice per virtualized server, you get good isolation from all
the other virtualized servers that host the other microservices. The operating system can be
configured corresponding to the needs of the specific microservice it is hosting. When errors
occur, problem determination is not complicated by having to separate out unrelated
information or side effects produced by other applications in the same server. The lack of side
effects from other unrelated applications is also good on the performance front. One

22 http://www.vagrantup.com/
23 http://www.vmware.com
24 http://www.virtualbox.org/

Chapter 8. From development to production

application cannot (as easily) starve other applications of resources when they are isolated
from each other in different virtualized servers.

The disadvantage of this alternative is you end up with many virtualized servers (one per
microservice instance) that must be managed and coordinated across your infrastructure.
The overall resource consumption is also higher because every virtualized server needs its
own operating system. Some of the virtualization solutions have optimizations for this
problem, which can help to reduce the processor usage caused by multiple operating
systems.

Having multiple microservices on one virtualized server should only be done when they have
similar resource usage patterns, so that one microservice does not adversely affect, or
starve, the others.

The configuration of the operating system must also be compatible with hosting multiple
microservices on the same virtualized server. Different configurations must not disturb each
other or require conflicting operating system configurations between stages. This pattern
would generally reduce the number of virtualized systems that are needed to host the
microservice application, at the cost of sharing system configuration. In this setting, you can
also use the shared class cache of the JVM, which reduces start time of the microservice.

8.3.5 Aspects of running multiple microservices on one server

The following remarks are suitable for deployments where more than one microservice gets
hosted on a server instance, regardless of whether this configuration is done with a container
or on a virtualized system.

If you host more than one microservice on your server, then pay attention to the thread pool
optimization of your Java run time. The JVM queries the operating system to get the
information about the number of CPUs on this server, and uses this information to define a
strategy for managing thread pooling. With virtualized hardware, the JVM often gets the
wrong information about available resources because every JVM on a server sees the same
information about available CPU, even though the amount available to any specific process is
considerably less. This configuration can cause unexpected side effects in some thread pool
tuning algorithms because the wrong optimization might be triggered because it does not get
as many CPUs as expected.

Another aspect that you should consider is how microservices on the same server would
share or contend for server resources. Know the resource requirements for each
microservice upfront, or gather the data to figure it out, to avoid problems that are related to
hosting shared resources. For example, caching inside the memory of a microservice can
cause problems as the cache grows. Therefore, generally perform caching on an external
system. The external caching server should be treated as an attached service just like a data
store.

8.3.6 Preferred practices on packaging

On the first glance, running multiple Docker containers containing microservices on the same
host server seems the same as running multiple microservice packages on the same server.
But upon further review, there are notable differences between the two deployment patterns.
Running multiple microservices on one server without Docker containers can only be done
when configurations of the operating system are the same for all microservices. This
configuration must be checked in advance. However, Docker provides a much greater level of
isolation while making effective use of host resources to the common underlying operating

Microservices Best Practices for Java

system. Microservices that require different or conflicting operating system configurations can
run happily side by side in Docker containers.

The following is a summary of preferred practices for packaging:

(cid:2) Virtualization or cloud services are the preferred basis for infrastructure because dynamic

provisioning can help a system of microservices deal with changing capacity
requirements.

(cid:2) System level configuration should either be as similar as possible across all stages, or

should be isolated from the application by using technology like containers.

(cid:2) An immutable artifact should be produced that is promoted unchanged between stages.

(cid:2) Immutable artifacts for microservices should be stand alone and self-contained. This item

could be an executable JAR file or a Docker container.

(cid:2) Each immutable artifact should contain only one microservice application and its

supporting run time and libraries.

(cid:2) Resource requirements should be considered when hosting shared resources to prevent

disruption due to resource contention.

(cid:2) Shared libraries should be built as part of your application, rather than being shared or

used as a dependency provided by the hosting environment.

Figure 8-3 shows a Java EE-based microservice that is packaged together with its Java EE
run time running inside a Docker container as a virtualized infrastructure. This is one of the
preferred practices for hosting a microservice.

Figure 8-3 Java Platform, Enterprise Edition based microservice in a Docker Container hosted on
virtualized infrastructure

Chapter 8. From development to production

8.4 Configuration of the applications across stages

Every microservice should be an immutable deployment artifact. After it is built, it should not
be changed for the deployments on different stages. Therefore, environment variables, which
can change from stage to stage, need to be injected from outside of the deployment artifact.
In addition to using an immutable server for deployments, the application configuration should
be immutable as far as possible. Only change values that are necessary on different server
(such as port, IP addresses, database users, thread pool size). The remainder of the
application configuration should be the same for every stage. The more values that you
change. the higher the risk of getting failures on the deployment on the next stage. If values
need to be changed on the subsequent stage, then you should test the behavior of your
service in the presence to these changes.

Configuring the system of microservices can be done in three different ways:

(cid:2) Environment variables
(cid:2) Configuration files
(cid:2) Configuration systems

The following sections describe these three variants.

8.4.1 Environment variables

One way to get the configuration from outside to your microservice is to use environment
variables. This is a common way to pass values to a running process.

Environment variables are character-based key value pairs that are both mapped to the Java
data type java.lang.String. The way to set these values varies between operating systems.
Example 8-4 shows how to set these variables in a scripting environment to start the
microservice.

Example 8-4 Setting environment variables in a Linux Bash script

#!/bin/bash
export VARIABLE1="Value 1"
export VARIABLE2="Value 2"

The Java sample shown in Example 8-5 shows how to parse these environment variables.

Example 8-5 Parsing environment variables

Map<String, String> envMap = System.getenv();
for (String envVariable : envMap.keySet()) {
System.out.format("%s=%s%n",
envVariable, envMap.get(envVariable));
}

Using environment variables is the preferred method compared to passing system properties
(Java command line option -D) to your microservice because in a Cloud environment, you
normally have no access to the command line of your process. Cloud environments normally
use environment variables.

100 Microservices Best Practices for Java

Cloud Foundry for example uses an environment variable called VCAP_SERVICES. It is a
JSON document that contains information about service instances bound to the application.
For more information about VCAP_SERVICES, see:

http://docs.cloudfoundry.org/devguide/deploy-apps/environment-variable.html#VCAP-S
ERVICES

Example 8-6 shows VCAP_SERVICES addressing an IBM Cloudant®25 database.

Example 8-6 VCAP_SERVICES

VCAP_SERVICES=
{
"cloudantNoSQLDB": [
{
"name": "myMicroservicesCloudant",
"label": "cloudantNoSQLDB",
"plan": "Shared",
"credentials": {
"username": "xyz",
"password": "password",
"host": "yourhost.yourdomain.com ",
"port": 443,
"url": "https://mycloudant.yourhost.yourdomain.com"
}
}
]
}

Connectors available to parse the VCAP_SERVICES variable. For example, Spring Boot has
a connector to use the VCAP_SERVICES in its configuration service. For more information,
see this website:

http://spring.io/blog/2015/04/27/binding-to-data-services-with-spring-boot-in-clou
d-foundry

8.4.2 Configuration files

Another alternative to get configuration values from your environment into Java is to use Java
Naming and Directory Interface (JNDI26) which is a Java API to query the directory service.
JNDI must be provided by every Java EE compatible server. Elements that are defined in the
JNDI namespace can be injected with Java CDI, so accessing these values is easy. To get
elements into the JNDI namespace, they must be added to a configuration file.

In Liberty, this process involves the following steps:

1. Activate the JNDI-Feature in the server.xml, which is the main configuration file in Liberty,

Example 8-7. After this you can define your JNDI variable.

Example 8-7 Activate JNDI feature in Liberty and set JNDI value in server.xml

25 http://cloudant.com/
26 http://de.wikipedia.org/wiki/Java_Naming_and_Directory_Interface

Chapter 8. From development to production

101

</featureManager>
<jndiEntry jndiName="myFirstJNDIEntry" value='"myFirstValue"'/>
</server>

2. Looking up the value for this JNDI entry can be done in two ways. You can do it on your

own as shown in Example 8-8.

Example 8-8 Parsing JNDI value

InitialContext ctx = new InitialContext();
String jndiValue = (String) ctx.lookup("myFirstJNDIEntry");

Or you can use CDI by using a field-based injection (Example 8-9).

Example 8-9 Field-based injection of JNDI with CDI

Public class MyClass {
@Resource(name="myFirstJNDIEntry")
private String jndiValue;
...
}

Using this technique requires that the configuration files must exist on every stage and must
have the correct stage values. This configuration can be done with tools from the DevOps
universe to ensure that the needed file with the correct values exists on the target stage.

Loading these files into the application within a Liberty server.xml can be done with a URL
resource that points to where the configuration file is stored in the file system (see
Example 8-10).

Example 8-10 JNDI URL Entry in Liberty

In Liberty, you can also use more than one server configuration file. You just have to include
other XML files into server.xml as shown in Example 8-11.

Example 8-11 Liberty include of configuration files

The included file gets parsed and the values are set in the application server. This process
gives you the opportunity to separate the configuration files. Configuration files that have
values that must be changed on every stage should be included in server.xml. The main
configuration file (server.xml) should only have values that are constant across all stages.

102 Microservices Best Practices for Java

Setting your configuration this way keeps the main configuration file (server.xml) the same
on every stage.

In a Docker based environment, it is normal to do the configuration with files placed outside
the Docker container. The Docker container itself stays unchanged and the configuration file
must be provided by DevOps tools.

Spring Boot also offers the opportunity to configure the application from outside. It is possible
to use properties files, YAML files, environment variables, and command line arguments.
These values get injected by Spring into Spring beans and can be used inside your
application. For more information, see the following website:

http://docs.spring.io/spring-boot/docs/current/reference/html/boot-features-extern
al-config.html

8.4.3 Configuration systems

The third alternative to configure the system of microservices is to use special servers. These
configuration systems are all some kind of key value stores to persist the configuration for the
different microservices.

According to the CAP theorem27 from Eric Brewer, you must choose two of the three
guarantees between consistency, availability, and partition tolerance. As the microservice
system will not work without the configuration system, the configuration system must be
partitioned. This limitation means that your configuration system will either be always
available with inconsistent values, or have limited availability but with consistent values.

The following are examples of configuration systems used in microservice environments:

(cid:2) Apache ZooKeeper

http://zookeeper.apache.org/

(cid:2) etcd

http://coreos.com/etcd/

(cid:2) Netflix archaius

http://github.com/Netflix/archaius

(cid:2) Consul

http://www.consul.io/

ZooKeeper and etcd are CP systems with regard to the CAP theorem. Archaius and Consul
follow the AP attributes. All of these systems can be used in a Java based application. Which
one to use, with regard to the CAP theorem, depends on the requirements of your
microservice application.

The advantage of using configuration systems is that configuration data can be changed at
run time. This change cannot be easily achieved with environment variables or configuration
files. If a bound JNDI value changes, the Liberty server would have to be restarted to get the
new values into the application. You should track the changes in the configuration system to
be able to reproduce the values later to help you to find the reasons for any failures.

27 http://en.wikipedia.org/wiki/CAP_theorem

Chapter 8. From development to production

103

8.4.4 Programming considerations

Configuring your microservices should not be done inside the business logic of your source
code. It is better to do this configuration in a central place. In most cases, accessing the
configuration values is based on technical decisions and is not affected by your business
logic.

If you configure your system using CDI, then you must deal with changing values at run time.
In such situations, annotate the CDI beans with @RequestScoped (or @Dependent) to make sure
that the values are refreshed on every request. Some of the configuration systems provide
callbacks to inform the client when values change. In combination with CDI producers
(@Produces), you can easily attach your configuration system to your application.

The following examples show a CDI way to set the configuration values in your application.
First, you must define a CDI qualifier that can be used to annotate the injection point
(@Configuration) as shown in Example 8-12.

Example 8-12 CDI qualifier

@Qualifier
@Retention(RUNTIME)
@Target({ METHOD, FIELD, PARAMETER, TYPE })
public @interface Configuration {

// Values without binding
@Nonbinding
String value() default "";
}

The generic producer class (MyConfigProducer) (Example 8-13) checks the name of the
attribute inside the qualifier and returns the corresponding value from its configuration cache.
The main thing to address is the InjectionPoint object that holds the information that is
needed by the producer that is set at the injection point in the EJB as shown in Example 8-14
on page 105.

Example 8-13 CDI producer

public class MyConfigProducer {
private Properties environmentProps;
@Produces
@Configuration
public String getConfigValue(InjectionPoint ip) {
Configuration config = ip.getAnnotated().
getAnnotation(Configuration.class);
String configKey = config.value();

if (configKey.isEmpty()) {
throw new IllegalArgumentException(
"Property key missing!");
}
String myValue = environmentProps.getProperty(configKey);
if (myValue == null) {
throw new RuntimeException("Property value for key " +
configKey + " could not be found!");
}
return myValue;

104 Microservices Best Practices for Java

}
. . .
}

The default CDI scope for an EJB is @RequestScoped and the default CDI scope when it is not
explicit set is @Default. The producer injects the value on every request that calls the EJB.
Other CDI scopes can also be used depending on your requirements (Example 8-14).

Example 8-14 EJB with injected configuration value

@Stateless
public class MyEJB {
@Inject
@Configuration("myConfigValue")
private String myConfigValue;
. . .
}

8.4.5 Preferred practices on configuration

In summary, the following are the preferred practices when doing configuration:

(cid:2) Configuration should live outside of the immutable deployment artifact and can be

supplied by three variations of configuration (environment variables, configuration files,
and configuration systems). The choice depends more on the runtime environment that
your system of microservices is being hosted in.

(cid:2) Dynamic configuration values should be managed with configuration systems, but your

application must handle the changing of configuration values.

(cid:2) Keep the system as unchanged as you can. This guideline is also true for configuration

values otherwise to minimize the risk of unexpected errors.

(cid:2) Change only configuration values that must be changed and use a defined way to manage

those changes (for example, track these changes).

(cid:2) The business logic of the microservice should not deal with the configuration values. This

tracking should be done in a central helper class or package.

Chapter 8. From development to production

105

106 Microservices Best Practices for Java

Chapter 9. Management and Operations

Running a system of microservices has some aspects that differ from running a monolith
application. With microservices, a single application becomes a distributed system that is
made up of a number of microservices that communicate with each other. Therefore, you
must manage a highly distributed system and worry about the communication path between
each service.

The number of running services is normally higher than the services needed for a monolith
application. When load characteristics vary, a microservice application should react by
adjusting the number of running services. Therefore, you must be able to automatically deploy
new and remove existing service instances to ensure that overall application is running at
optimum state.

In every production system, it is necessary to do health checks to get information about the
state of the running applications. This requirement is also true for a system of microservices
where you must worry about the health of each service instance.

This chapter covers the following topics:

(cid:2) Metrics and health checks
(cid:2) Logging
(cid:2) Templating

107

9.1 Metrics and health checks

Metrics and health checks are necessary to get information about the state of a running
service. As mentioned in 8.3, “Packaging options” on page 92 each Java EE-based
microservice has its own run time packaged together with the microservice code itself. These
Java based runtimes have tools to help the administrators to check the health of the running
microservice.

9.1.1 Dynamic provisioning

With dynamic provisioning, the aim is to automatically keep the system of microservices in its
optimum state. This state is where the system is properly balanced to match the load
demands of the application users. It can be reached with enough service instances being
available to manage throughput and keep application latency at a minimum. For example, an
instance of a microservice becoming overloaded can be compensated by scaling up and
starting another instance of a service. This change should be done automatically without the
need for administrator intervention. Similarly, unused microservice instances with low load
can be stopped to return the unused resources to your virtualization system.

However, in some situations it is necessary to inform the administration when some threshold
is reached so they can take action to prevent failures in the system. However, this should be
an exceptional case.

Service discovery as described in Chapter 3, “Locating services” on page 25 is a key service
that is used by microservices. The service discovery service tracks available microservice
instances and the current health state. A new instance of a service should start to take
requests to reduce load from the other instances. With this information, load balancing can be
dynamically performed to distribute load across available microservice instances for optimal
performance. A microservice that is shutting down should complete running requests before it
stops, and should not receive any further requests to process.

A system of microservices needs a high degree of automation to achieve this behavior. You
can use every tool of your administration infrastructure to help you. But a packaged
microservice as described in 8.3, “Packaging options” on page 92 gives you an alternative for
this task. The runtime environment in the microservice package has some features to support
your automation. Using Liberty as a runtime environment, for example, has some features
that offer you support in the areas of auto scaling and dynamic routing.

Auto scaling provides an autonomic scaling capability of Liberty servers. The auto scaling
function is enabled by two Liberty features: Scaling controller and scaling member. Auto
scaling dynamically adjusts the number of Java virtual machines (JVMs) used to service your
workload. This feature provides operational agility and decreases administrative requirements
to enhance the resiliency of your middleware environment. The conditions for auto scaling are
defined by scaling policies. These conditions include the minimum or maximum number of
server instances and the threshold values for each of the server resources.

A common pattern with microservices operation is to keep a specific number of microservice
instances running at all times, or more often a minimum number and some level of
permissible scaling up. A microservice application with these settings is running within its
designed bounds and can scale up an acceptable amount for unexpected loads. With the
auto scaling feature of Liberty, you can define the minimum number of service instances that
should be running for a healthy system. You can then have system management keep that
number alive. Another Liberty feature to check for the health of a service is described in 9.1.2,
“Health check” on page 111.

108 Microservices Best Practices for Java

The following metrics that can be used to do auto scaling.

The scaling member monitors the use of the following resources within the server process:

(cid:2) CPU
(cid:2) Heap
(cid:2) Memory

The scaling member monitors the use of the following resources at the host level:

(cid:2) CPU
(cid:2) Memory

For more information, see “Setting up auto scaling for Liberty collectives” at:

http://www.ibm.com/support/knowledgecenter/SSAW57_9.0.0/com.ibm.websphere.wlp.nd.d
oc/ae/twlp_wve_autoscaling.html

When you start and stop instances of microservices, your service discovery must also be
informed. One easy way to do this is to inform the HTTP server in front of your system of
microservices about started or stopped microservices. Liberty has a feature that can be
combined with the auto scaling feature. The Dynamic Routing feature enables routing of
HTTP requests to members of Liberty collectives without having to regenerate the
WebSphere plug-in configuration file when the environment changes. When servers, cluster
members, applications, or virtual hosts are added, removed, started, stopped, or modified,
the new information is dynamically delivered to the WebSphere plug-in. Requests are routed
based on up-to-date information. The feature provides the Dynamic Routing service, which
dynamically retrieves routing information from the collective repository and delivers this
information to the WebSphere plug-in.

Liberty servers that take part in this functionality must be joined in a collective. The set of
Liberty servers in a single management domain is called a collective. A collective consists of
at least one server with the collectiveController-1.0 feature enabled that is called a
collective controller. Optionally, a collective can have many servers with the
collectiveMember-1.0 feature enabled that are called collective members and a collective
can be configured to have many collective controllers.

Chapter 9. Management and Operations

109

Figure 9-1 shows an example of a Liberty collective.

Figure 9-1 Liberty collective architecture1

A sample for a topology showing the dynamic routing feature is shown in Figure 9-2.

Figure 9-2 Auto scaling and dynamic routing topology

1 http://www.ibm.com/support/knowledgecenter/en/SSAW57_9.0.0/com.ibm.websphere.wlp.nd.doc/ae/cwlp_coll

ective_arch.html

110 Microservices Best Practices for Java

9.1.2 Health check

The instances of the microservice application experiencing problems must be recognized.
This function helps to keep the whole system in its optimum state. To get more information out
of your system of microservices, Liberty has another feature to support this function. With the
health management feature in Liberty, you can take a policy driven approach to monitoring
the application server environment and take action when unhealthy criteria is discovered. You
can define the health policies, which include the health conditions to be monitored in your
environment and the health actions to take if these conditions are met.

Health management in a Liberty collective can prevent the disruption of service by detecting
common problems and generating diagnostic actions based on configured health policies.
The health management functions are enabled by two Liberty features: Health manager and
health analyzer.

The following predefined health conditions can be used:

(cid:2) Excessive request time out condition
(cid:2) Excessive response time condition
(cid:2) Memory condition: Excessive memory usage
(cid:2) Memory condition: Memory leak

When one of these health conditions is raised, a predefined action is executed. The following
actions are available:

(cid:2) Restart server
(cid:2) Take thread memory dumps
(cid:2) Take JVM heap memory dumps
(cid:2) Enter server into maintenance mode
(cid:2) Exit server out of maintenance mode

Some of these actions produce more information (thread and heap memory dumps) to get a
better understanding of the problem. A server in maintenance mode is still running, but does
not accept new requests.

To get more information about internals of the running server, the monitor feature of Liberty
can help you. This feature enables the Performance Monitoring Infrastructure (PMI) for other
features that the server is running. The values collected in the MXBeans can be seen in the
AdminCenter of the Liberty server (feature adminCenter-1.0 must be activated).

Other tools already in the infrastructure of your production system (for example Nagios2) can
also be used to help administrators keep the system of microservices running.

9.2 Logging

Logging is essential for applications. The same is true for a system of microservices. But
there are some special points of interest.

In a system of microservices, a business transaction can span multiple microservices. If your
microservices use a message-oriented middleware, this can also be part of your business
transaction. Therefore, you need something to track the request from entering your system
until the response leaves the system. A good way to track the request is to use a correlation
ID (see 5.2.5, “Java Message Service API” on page 62). The same correlation ID you use for
your message-oriented middleware can be used for your microservice requests.

2 http://www.nagios.org/

Chapter 9. Management and Operations

111

You must create a correlation ID on the first request that enters your system of microservices,
then send the same correlation ID to other microservices when you call them. The other
services only have to check whether there is a correlation ID and propagate it in their calls.
The correlation ID can be placed into the HTTP header using a special field (X-header fields,
for example) to hold the value. The handling of the header fields can be done with a REST
filter to keep this logic in a central place.

Using a correlation ID provides these benefits:

(cid:2) It is easier to find cascading errors.

(cid:2) You can track user requests to get information about typical behavior of the customer.

(cid:2) You can find unused or seldom used microservices that can be eliminated, or find heavily

used microservices where you can get more business value if you extend them.

There are some benefits with your logging framework to logging the value of the correlation ID
in your log files. Some of these frameworks use logging context. SLF4J3 or Log4j 24
implements a Mapped Diagnostic Context (MDC) or a Nested Diagnostic Context (NDC) that
you can use to put the correlation ID into it. Therefore, in every log statement written to the log
files, the NDC or MDC is included. Another benefit that you get from the NDC/MDC is that if
you are using multiple threads to get the request done, then each of these threads logs the
same NDC/MDC.

The layout of the log format should be defined in a central manner, so it is easier to aggregate
the logs produced in the different microservices. The log levels (for example, FATAL, ERROR,
WARN, INFO, DEBUG, TRACE, ALL) should be clearly defined and used corresponding in
every microservice. Otherwise, you might get misunderstandings between the microservice
teams.

In a system of microservice with dynamic provisioning, you must ensure that the server that
hosted the microservices and the log files get deleted after the shutdown of the system. In
such cases, it is better to use a logging infrastructure that is always running and gathering the
log output from your microservices. This logging system can also help you in situations where
log files are distributed over different systems. It can be time consuming to search in log files
on different servers. This limitation can also be complicated in a production environment
where you normally have no access rights or minimal rights on the systems.

Therefore, the preferred approach for a logging infrastructure in a microservice application is
the ELK stack or Elastic Stack5, which is a combination of three tools:

(cid:2) Elasticsearch (search and analytics engine)
(cid:2) Logstash (data collection and transportation pipeline)
(cid:2) Kibana (visualizing collected data)

If your system of microservices uses Liberty as the Java EE provider, then there is a feature
to use the Elastic Stack: logstashCollector-1.0. Use the Logstash collector feature in
Liberty to collect log and other events from your Liberty servers and send them to a remote
Logstash server.

3 http://www.slf4j.org/
4 http://logging.apache.org/log4j/2.x/
5 http://www.elastic.co/products

112 Microservices Best Practices for Java

Example 9-1 shows the usage of logstashCollector in Liberty.

Example 9-1 Configuring logstashCollector in Liberty6

<featureManager>
<feature>logstashCollector-1.0</feature>
</featureManager>
<keyStore id="defaultKeyStore" password="Liberty" />
<keyStore id="defaultTrustStore" location=”trust.jks” password="Liberty" />
<ssl id="mySSLConfig" trustStoreRef="defaultTrustStore"
keyStoreRef="defaultKeyStore" />
<logstashCollector

source="message,trace,garbageCollection,ffdc,accessLog"
hostName="localhost"
port="5043"
sslRef="mySSLConfig"

Splunk7 is a commercial product that can also be used to gather and analyze your log data.

9.3 Templating

From a management point of view, it can be valuable to do templating for your development
team, which is implementing the system of microservices. Templating in this sense is defining
a skeleton of code fragments that can be used to generate the needed parts of a
microservice.

A microservice template typically includes these common capabilities:

(cid:2) Communication with Service Discovery to register a service instance
(cid:2) Communication with other microservices
(cid:2) Communication with a messaging system
(cid:2) Logging
(cid:2) Security

You get these benefits from a template having all these capabilities:

(cid:2) Quicker start for the development teams
(cid:2) All teams use the same cross functional services

The following are available generators:
(cid:2) WildFly Swarm Project Generator8
(cid:2) Spring Initializr9
(cid:2) Liberty app accelerator10

If all teams use Java to implement the microservices, and do not have a polyglot
programming environment, then you must only define one template that can be evolved
occasionally to best fit your requirements.

6 http://www.ibm.com/support/knowledgecenter/SSAW57_9.0.0/com.ibm.websphere.wlp.nd.doc/ae/twlp_analyti

cs_logstash.html

7 http://www.splunk.com/
8 http://wildfly-swarm.io/generator/
9 http://start.spring.io/
10 http://liberty-app-accelerator.wasdev.developer.ibm.com/start/#1

Chapter 9. Management and Operations

113

114 Microservices Best Practices for Java

Related publications

The publications listed in this section are considered particularly suitable for a more detailed
discussion of the topics covered in this book.

IBM Redbooks

The following IBM Redbooks publications provide additional information about the topic in this
document. Note that some publications referenced in this list might be available in softcopy
only.

(cid:2) IBM WebSphere Application Server V8.5 Administration and Configuration Guide for

Liberty Profile, SG24-8170-00

You can search for, view, download or order these documents and other Redbooks,
Redpapers, Web Docs, draft and additional materials, at the following website:

ibm.com/redbooks

Other publications

These publications are also relevant as further information sources:

(cid:2) “Understanding the Differences between AMQP & JMS”, Marc Richards

(cid:2) “Building Microservices”, Sam Newman, February 2015

(cid:2) ”Release It!: Design and Deploy Production-Ready Software”, Michael T. Nygard, 2007

(cid:2) “Continuous Delivery: Reliable Software Releases through Build, Test, and Deployment

Automation”, Jez Humble, David Farley, 2011

(cid:2) “Enterprise Integration Patterns: Designing, Building, and Deploying Messaging

Solutions”, Gregor Hohpe and Booby Woolf, Addison-Wesley Professional

Online resources

These websites are also relevant as further information sources:

(cid:2) “1.2.2 Robustness Principle”, RFC 1122

https://tools.ietf.org/html/rfc1122#page-12

(cid:2) Apache Avro

https://avro.apache.org/

(cid:2) Amalgam8

https://github.com/amalgam8/

(cid:2) Apache Thrift

https://thrift.apache.org/

115

(cid:2) Blog post by Troy Hunt

https://www.troyhunt.com/your-api-versioning-is-wrong-which-is/

(cid:2) “Chapter 5: Representational State Transfer (REST)”, Fielding, Roy Thomas (2000)

http://www.ics.uci.edu/~fielding/pubs/dissertation/rest_arch_style.htm

(cid:2) Cloud Native Computing Foundation (“CNCF”) Charter

https://cncf.io/about/charter

(cid:2) Consul

https://www.consul.io/

(cid:2) Eureka

https://github.com/Netflix/eureka

(cid:2) Fallacies of Distributed Computing Explained

http://www.rgoarchitects.com/Files/fallacies.pdf

(cid:2) Game On! Player API

https://game-on.org/swagger/#!/players

(cid:2) “Microservices: A definition of this new architectural term”, Martin Fowler

http://martinfowler.com/articles/microservices.html

(cid:2) Oracle Java Platform, Enterprise Edition (Java EE) 7

http://docs.oracle.com/javaee/7/

(cid:2) Protocol Buffers:

https://developers.google.com/protocol-buffers/

(cid:2) “The Twelve-Factor App”, Adam Wiggins, 2012

http://12factor.net

Help from IBM

IBM Support and downloads

ibm.com/support

IBM Global Services

ibm.com/services

116 Microservices Best Practices for Java

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