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