rdblue commented on code in PR #4944: URL: https://github.com/apache/iceberg/pull/4944#discussion_r889751336
########## format/puffin-spec.md: ########## @@ -0,0 +1,139 @@ +--- +url: puffin-spec +toc: false +--- +<!-- + - Licensed to the Apache Software Foundation (ASF) under one or more + - contributor license agreements. See the NOTICE file distributed with + - this work for additional information regarding copyright ownership. + - The ASF licenses this file to You under the Apache License, Version 2.0 + - (the "License"); you may not use this file except in compliance with + - the License. You may obtain a copy of the License at + - + - http://www.apache.org/licenses/LICENSE-2.0 + - + - Unless required by applicable law or agreed to in writing, software + - distributed under the License is distributed on an "AS IS" BASIS, + - WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. + - See the License for the specific language governing permissions and + - limitations under the License. + --> + +# Puffin file format + +This is a specification for the 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 | list of JSON long | yes | List of field IDs the blob was computed for; the order of items is used to compute sketches stored in the blob. +| 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 +| compression-codec | JSON string | no | See [Compression codecs](#compression-codecs). If omitted, the data is assumed to be uncompressed. + +### 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. Review Comment: What about documenting the extra blob metadata that this will produce? It would be nice to have that documented for the blob type. -- This is an automated message from the Apache Git Service. To respond to the message, please log on to GitHub and use the URL above to go to the specific comment. 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