| --- |
| title: "Puffin Spec" |
| --- |
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|
|
| # Puffin file format |
|
|
| This is a specification for Puffin, a file format designed to store |
| information such as indexes and statistics about data managed in an |
| Iceberg table that cannot be stored directly within the Iceberg manifest. A |
| Puffin file contains arbitrary pieces of information (here called "blobs"), |
| along with metadata necessary to interpret them. The blobs supported by Iceberg |
| are documented at [Blob types](#blob-types). |
|
|
| ## Format specification |
|
|
| A file conforming to the Puffin file format specification should have the structure |
| as described below. |
|
|
| ### Versions |
|
|
| Currently, there is a single version of the Puffin file format, described below. |
|
|
| ### File structure |
|
|
| The Puffin file has the following structure |
|
|
| ``` |
| Magic Blob₁ Blob₂ ... Blobₙ Footer |
| ``` |
|
|
| where |
|
|
| - `Magic` is four bytes 0x50, 0x46, 0x41, 0x31 (short for: Puffin _Fratercula |
| arctica_, version 1), |
| - `Blobᵢ` is i-th blob contained in the file, to be interpreted by application |
| according to the footer, |
| - `Footer` is defined below. |
|
|
| ### Footer structure |
|
|
| Footer has the following structure |
|
|
| ``` |
| Magic FooterPayload FooterPayloadSize Flags Magic |
| ``` |
|
|
| where |
|
|
| - `Magic`: four bytes, same as at the beginning of the file |
| - `FooterPayload`: optionally compressed, UTF-8 encoded JSON payload describing the |
| blobs in the file, with the structure described below |
| - `FooterPayloadSize`: a length in bytes of the `FooterPayload` (after compression, |
| if compressed), stored as 4 byte integer |
| - `Flags`: 4 bytes for boolean flags |
| - byte 0 (first) |
| - bit 0 (lowest bit): whether `FooterPayload` is compressed |
| - all other bits are reserved for future use and should be set to 0 on write |
| - all other bytes are reserved for future use and should be set to 0 on write |
|
|
| A 4 byte integer is always signed, in a two's complement representation, stored |
| little-endian. |
|
|
| ### Footer Payload |
|
|
| Footer payload bytes is either uncompressed or LZ4-compressed (as a single |
| [LZ4 compression frame](https://github.com/lz4/lz4/blob/77d1b93f72628af7bbde0243b4bba9205c3138d9/doc/lz4_Frame_format.md) |
| with content size present), UTF-8 encoded JSON payload representing a single |
| `FileMetadata` object. |
|
|
| #### FileMetadata |
|
|
| `FileMetadata` has the following fields |
|
|
|
|
| | Field Name | Field Type | Required | Description | |
| | ---------- | --------------------------------------- | -------- | ----------- | |
| | blobs | list of BlobMetadata objects | yes | |
| | properties | JSON object with string property values | no | storage for arbitrary meta-information, like writer identification/version. See [Common properties](#common-properties) for properties that are recommended to be set by a writer. |
|
|
| #### BlobMetadata |
|
|
| `BlobMetadata` has the following fields |
|
|
| | Field Name | Field Type | Required | Description | |
| |-------------------|-----------------------------------------|----------| ----------- | |
| | type | JSON string | yes | See [Blob types](#blob-types) |
| | fields | JSON list of ints | yes | List of field IDs the blob was computed for; the order of items is used to compute sketches stored in the blob. |
| | snapshot-id | JSON long | yes | ID of the Iceberg table's snapshot the blob was computed from. |
| | sequence-number | JSON long | yes | Sequence number of the Iceberg table's snapshot the blob was computed from. |
| | offset | JSON long | yes | The offset in the file where the blob contents start |
| | length | JSON long | yes | The length of the blob stored in the file (after compression, if compressed) |
| | compression-codec | JSON string | no | See [Compression codecs](#compression-codecs). If omitted, the data is assumed to be uncompressed. |
| | properties | JSON object with string property values | no | storage for arbitrary meta-information about the blob |
|
|
| ### Blob types |
|
|
| The blobs can be of a type listed below |
|
|
| #### `apache-datasketches-theta-v1` blob type |
|
|
| A serialized form of a "compact" Theta sketch produced by the [Apache |
| DataSketches](https://datasketches.apache.org/) library. The sketch is obtained by |
| constructing Alpha family sketch with default seed, and feeding it with individual |
| distinct values converted to bytes using Iceberg's single-value serialization. |
|
|
| The blob metadata for this blob may include following properties: |
|
|
| - `ndv`: estimate of number of distinct values, derived from the sketch. |
|
|
| ### Compression codecs |
|
|
| The data can also be uncompressed. If it is compressed the codec should be one of |
| codecs listed below. For maximal interoperability, other codecs are not supported. |
|
|
| | Codec name | Description | |
| |------------|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------| |
| | lz4 | Single [LZ4 compression frame](https://github.com/lz4/lz4/blob/77d1b93f72628af7bbde0243b4bba9205c3138d9/doc/lz4_Frame_format.md), with content size present | |
| | zstd | Single [Zstandard compression frame](https://github.com/facebook/zstd/blob/8af64f41161f6c2e0ba842006fe238c664a6a437/doc/zstd_compression_format.md#zstandard-frames), with content size present | |
| __ |
|
|
| ### Common properties |
|
|
| When writing a Puffin file it is recommended to set the following fields in the |
| [FileMetadata](#filemetadata)'s `properties` field. |
|
|
| - `created-by` - human-readable identification of the application writing the file, |
| along with its version. Example "Trino version 381". |
|
|
