[GitHub] [parquet-format] pitrou commented on a diff in pull request #164: PARQUET-1950: Define core features

["pitrou (via GitHub)" <gi...@apache.org>]

pitrou commented on code in PR #164:
URL: https://github.com/apache/parquet-format/pull/164#discussion_r1211793576


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CoreFeatures.md:
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+
+# Parquet Core Features
+
+This document lists the core features for each parquet-format release. This
+list is a subset of the features which parquet-format makes available.
+
+## Purpose
+
+The list of core features for a certian release makes a compliance level that
+the different implementations can tied to. If an implementation claims that it
+provides the functionality of a parquet-format release core features it must
+implement all of the listed features according the specification (both read and
+write path). This way it is easier to ensure compatibility between the
+different parquet implementations.
+We cannot and don't want to stop our clients to use any features that are not
+on this list but it shall be highlighted that using these features might make
+the written parquet files unreadable by other implementations. We can say that
+the features available in a parquet-format release (and one of the
+implementations of it) and not on this list are experimental.
+
+## Versioning
+
+This document is versioned by the parquet-format releases which follows the
+scheme of semantic versioning. It means that no feature will be deleted from
+this document under the same major version. (We might deprecate some, though.)
+Because of the semantic versioning if one implementation supports the core
+features of the parquet-format release `a.b.x` it must be able to read any
+parquet files written by implementations supporting the release `a.d.y` where
+`b >= d`.
+
+If a parquet file is written according to a released version of this document
+it might be a good idea to write this version into the field `compliance_level`
+in the thrift object `FileMetaData`.
+
+## Adding new features
+
+The idea is to only include features which are specified correctly and proven
+to be useful for everyone. Because of that we require to have at least two
+different implementations that are released and widely tested.
+
+## The "list"
+
+This list is based on the [parquet thrift file](src/main/thrift/parquet.thrift)
+where all the data structures we might use in a parquet file are defined.
+
+### File structure
+
+All of the required fields in the structure (and sub-structures) of
+`FileMetaData` must be set according to the specification.
+The following page types are supported:
+* Data page V1 (see `DataPageHeader`)
+* Dictionary page (see `DictionaryPageHeader`)

Review Comment:
   "Stable" is unfortunately underdefined here. It certainly seems stable as far as the _format_ is concerned. It's not necessarily widely implemented, though both parquet-mr and Parquet C++ support it.



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