emkornfield commented on code in PR #186: URL: https://github.com/apache/parquet-site/pull/186#discussion_r3365993183
########## content/en/docs/File Format/versions.md: ########## @@ -0,0 +1,267 @@ +--- +title: "Parquet format versions" +linkTitle: "Format versions" +weight: 9 +--- + +This page describes how features are added to the [Parquet format +specification](https://github.com/apache/parquet-format) and how they affect +reader and writer compatibility. See the +[Implementation status](../implementationstatus/) page for which implementations +(arrow, parquet-java, arrow-rs, etc.) support each feature. + +*Note*: If you find out-of-date information, please open an issue or pull request. + +## Feature compatibility + +The Parquet format spec [classifies changes] by their effect on reader and +writer compatibility. Changes differ in their *forward* compatibility — whether +an older reader can read files that use a newer feature. + +**Forward compatible** features remain **readable by older readers**, with a +possibly degraded experience: some metadata may be missing or performance may +suffer, but the reader does not fail. Examples: + +* **Bloom filters**: a reader that ignores them skips the pruning metadata but + still reads the data correctly. +* **Logical type annotations** such as `VARIANT`: an older reader reads the + underlying physical column (e.g. `BYTE_ARRAY`) as raw bytes without applying + the logical type. + +**Forward incompatible** features make the data **unreadable** to older software. +Examples: + +* **New encodings** (e.g. the `DELTA_*` encodings, `BYTE_STREAM_SPLIT`, + `RLE_DICTIONARY`): a reader that does not implement them cannot decode the + column values. +* **Data Page V2 headers**: a reader that only understands `DataPageHeader` + cannot parse `DataPageHeaderV2` pages. + +[classifies changes]: https://github.com/apache/parquet-format/blob/master/CONTRIBUTING.md#compatibility-and-feature-enablement + +## `FileMetadata` version field + +Each Parquet file has a `version` field in the [`thrift FileMetadata`] that +declares which features the file may use, and thus what a reader **must** support +to read it. + +**Note**: Many writers set the version field to `1` even for files that use +format 2.0 features, which has caused [confusion and interoperability +issues][closing-out-2.0]. + +## `parquet-format` release versions + +The Thrift definition is released independently of implementations such as +parquet-java or arrow-rs, following the Apache release process and +[semantic versioning]: + +1. The major version corresponds to the [`thrift FileMetadata`] `version` field. +2. Minor releases (e.g. `2.10.0` to `2.11.0`) may add compatible Review Comment: Again, I'm not sure SemVer is really the right model here. Partially for reasons as outlined in [the comment above](https://github.com/apache/parquet-site/pull/186/changes#r3364542047). But also, because its philosophy is maor version bumps should be rare. If we really do add a handful of new encodings and a new footer it would get bumped pretty quickly. This might naturally lead to delays similar to what we saw with v2. From my perspective, as long as we agree we are OK bumping for every new feature I'm generally OK with this. A second alternative is to specify a very short maximum cadence that features will collect for (e.g. 1 Month). -- 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. To unsubscribe, e-mail: [email protected] For queries about this service, please contact Infrastructure at: [email protected]
