---
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,
  stored as non-negative integer value represented using decimal digits
  with no leading or trailing spaces.

#### `deletion-vector-v1` blob type

A serialized delete vector (bitmap) that represents the positions of rows in a
file that are deleted. A set bit at position P indicates that the row at
position P is deleted.

The vector supports positive 64-bit positions (the most significant bit must be
0), but is optimized for cases where most positions fit in 32 bits by using a
collection of 32-bit Roaring bitmaps. 64-bit positions are divided into a
32-bit "key" using the most significant 4 bytes and a 32-bit sub-position using
the least significant 4 bytes. For each key in the set of positions, a 32-bit
Roaring bitmap is maintained to store a set of 32-bit sub-positions for that
key.

To test whether a certain position is set, its most significant 4 bytes (the
key) are used to find a 32-bit bitmap and the least significant 4 bytes (the
sub-position) are tested for inclusion in the bitmap. If a bitmap is not found
for the key, then it is not set.

The serialized blob contains:

- Combined length of the vector and magic bytes stored as 4 bytes, big-endian
- A 4-byte magic sequence, `D1 D3 39 64`
- The vector, serialized as described below
- A CRC-32 checksum of the magic bytes and serialized vector as 4 bytes, big-endian

The position vector is serialized using the Roaring bitmap
["portable" format][roaring-bitmap-portable-serialization]. This representation
consists of:

- The number of 32-bit Roaring bitmaps, serialized as 8 bytes, little-endian
- For each 32-bit Roaring bitmap, ordered by unsigned comparison of the 32-bit keys:
    - The key stored as 4 bytes, little-endian
    - A [32-bit Roaring bitmap][roaring-bitmap-general-layout]

Note that the length and CRC fields are stored using big-endian, but the
Roaring bitmap format uses little-endian values. Big endian values were chosen
for compatibility with existing deletion vectors in Delta tables.

The blob's `properties` must:

- Include `referenced-data-file`, the location of the data file the delete
  vector applies to; must be equal to the data file's `location` in table
  metadata
- Include `cardinality`, the number of deleted rows (set positions) in the
  delete vector
- Omit `compression-codec`; `deletion-vector-v1` is not compressed

Snapshot ID and sequence number are not known at the time the Puffin file is
created. `snapshot-id` and `sequence-number` must be set to -1 in blob metadata
for Puffin v1.

[roaring-bitmap-portable-serialization]: https://github.com/RoaringBitmap/RoaringFormatSpec?tab=readme-ov-file#extension-for-64-bit-implementations
[roaring-bitmap-general-layout]: https://github.com/RoaringBitmap/RoaringFormatSpec?tab=readme-ov-file#general-layout

### 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".
