emkornfield commented on code in PR #588:
URL: https://github.com/apache/parquet-format/pull/588#discussion_r3453737867
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ParxMagicNumber.md:
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@@ -0,0 +1,89 @@
+# PARX Parquet Format Specification
+
+This specification details a new magic number and associated fixed length
footer metadata changes
+that accompany the footer.
+
+## Motivation
+
+Most parts of the parquet specification lend themselves naturally to
comptability checks
+when a new feature is added (e.g. encodings and compression values have an
enum value added)
+and fail appropriately.
+However, some semantic changes or footer changes are impossible to communicate
appropriately
+within existing structures (e.g. changing the serialization of the footer).
The motivation
+for the new magic number and layout is to accomodate the latter set of changes
by introducing
+a new extensible mechanism for readers to detect these changes and fail
accordingly.
+
+## Design Motivations
+
+* Provide a mechanism to only introduce a single new magic number for parquet
that can
+ last at least a decade.
+* Provide integrity checks for the footer.
+* Provide the ability for readers to have a granular understanding of
structural and semantic
+ backward incompatible features that are required to read a particular file.
+
+## File Layout
+
+A PARX file has the same overall structure as a standard Parquet file, with
two differences:
+the leading and trailing magic bytes are `PARX` instead of `PAR1/PARE`, and
the trailing footer is
+16 bytes instead of 8.
+
+The file layout is as follows:
+
+```
++-----------+-------------------+--------------------+-------------+
+| 'PARX' | File Data | Footer Metadata | Footer tail |
+| (4 bytes) | (variable length) | (variable length) | (16 bytes) |
++-----------+----------+--------+--------------------+-------------+
+```
+
+All multi-byte integer fields are **little-endian**.
+
+
+### PARX Footer Tail — 16 bytes
+
+```
++------------------+-----------+----------+--------+
+| metadata_len | flags | crc32 | 'PARX' |
++------------------+-----------+----------+--------+
+ offset 0 offset 4 offset 8 offset 12
+```
+
+| Field | Type | Offset | Description
|
+|----------------|---------|--------|-------------------------------------------------------------------|
+| `metadata_len` | i32 LE | 0 | Byte length of the Thrift-encoded
`FileMetaData` block |
+| `flags` | u32 LE | 4 | Feature flags (see [Feature
Flags](#feature-flags)) |
Review Comment:
I don't feel too strongly. I think even the ones listed only gets us less
then 10 bits. And if we end up going with a new footer hopefully that would
take 1 bit and hopefully keep this pretty low.
I do think it is a good idea to update the wording that this might not be
fixed size (could grow in the future, but that would be detectable via
inspecting the bitmap). Another option, given we expect this field to not
evolve too quickly past an initial set of changes is to have bit 31,
effectively be a version bit, which indicates all structural changes as of a
certain date are always true.
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