Client { # Client
  n1: circle label:"Start Transaction"
  n2: rectangle label:"Attempt Payment"
  n5: circle label:"Payment Success"
  n1.handle(right) -> n2.handle(left)
  n2.handle(bottom) -> Server.n3.handle(top) [label="Submit"]
  n2.handle(right) -> n5.handle(left) [label="Authorized"]
}

Server { # Server
  n3: rectangle label:"Process Payment"
  n4: diamond label:"Authorized?"
  n3.handle(right) -> n4.handle(left)
  n4.handle(right) -> Client.n5.handle(bottom) [label="Approved"]
  n4.handle(top) -> Client.n2.handle(bottom) [label="Decline/Retry"]
  loop [Retry until approved] n2 n3 n4
}