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---
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".
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