emkornfield commented on code in PR #588:
URL: https://github.com/apache/parquet-format/pull/588#discussion_r3453829691
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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)) |
+| `crc32` | u32 LE | 8 | CRC32 checksum (see [Integrity
Check](#integrity-check)) |
+| `magic` | [u8; 4] | 12 | Always the bytes `P A R X` (0x50 0x41
0x52 0x58) |
+
+## Feature Flags
+
+The `flags` field is a 32-bit bitfield. A reader **must** reject any file
whose `flags` field
+contains bits that are not recognized or not supported, because unknown flags
may imply structural changes
+to the metadata or semantic changes to the file layout that the reader cannot
properly interpret.
+
+The PARX format is independent of the `version` field in `FileMetaData`; a
file may use the PARX
+magic number regardless of which specification version its metadata declares.
+
+| Bit Index| Name | Description
|
+|----------|-------------------------|-------------------------------------------------------------------------------------------------------------|
+| 0 | `ENCRYPTED_FOOTER` | The `FileMetaData` block is encrypted
(equivalent to the `PARE` format). |
Review Comment:
Seems reasonable, I'm not clear on the support for constants in thrift, I'll
do some research if we can align with this approach.
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