alamb commented on code in PR #186: URL: https://github.com/apache/parquet-site/pull/186#discussion_r3374414298
########## 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 + +*Note*: If you find out-of-date information, please open an issue or pull request.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 Review Comment: This is a really nice description of the core tradeoffs with using Versions (and I think one of the major points we need to resolve to arrive at agreement about how to move versioning forward) I am not sure there are simple ways to resolve this tradeoff and I believe there are a variety of different opinions Thus, What I plan to do is to update this PR to *REMOVE* the a statement on semantic versioning, and keep it focused on what the current state of the spec /features is. We can then continue to have the fun (FUN!) discussion about how to version / signal new features in the corresponding mailing list thread. -- 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]
