Drill recently accepted a PR for the following bug:
DRILL-3867: Store relative paths in metadata file
This PR turned out to have a flaw which affects version compatibility.
DRILL-3867 PR changed the format of Parquet metadata files to store relative paths. All Drill servers after that PR create files with relative paths. But, the version number of the file is unchanged, so that older Drillbits don't know that they can't understand the file.
Instead, if an older Drill tries to read the file, queries fail left and right. Drill will resolve the paths, but does so relative to the user's HDFS home directory, which is wrong.
What should have happened is that we should have bumped the parquet metadata file version number so that older Drillbits can’t read the file. This ticket requests that we do that.
Now, one could argue that the lack of version number change is fine. Once a user upgrades Drill, they won't use an old Drillbit. But, things are not that simple:
- A developer tests a branch based on a pre-
DRILL-3867build on a cluster in which metadata files have been created by a post- DRILL-3867build. (This has already occurred multiple times in our shop.)
- A user tries to upgrade to Drill 1.11, finds an issue, and needs to roll back to Drill 1.10. Doing so will cause queries to fail due to seemingly-corrupt metadata files.
- A user tries to do a rolling upgrade: running 1.11 on some servers, 1.10 on others. Once a 1.11 server is installed, the metadata is updated ("corrupted" from the perspective of 1.10) and queries fail.
Standard practice in this scenario is to:
- Bump the file version number when the file format changes, and
- Software refuses to read files with a version newer than the software was designed for.
Of course, it is highly desirable that newer servers read old files, but that is not the issue here.
Main technical points of working of parquet metadata caching for now.
Only process of reading the parquet metadata is changed (the process of writing isn't changed):
Metadata objects are created by deserialization of parquet metadata files in the process of creating ParquetGroupScan physical operator.
All supported versions are stored in the "MetadataVersion.Constants" class and in the Jackson annotations for Metadata.ParquetTableMetadataBase class.
JsonMappingException is obtained and swallowed without creating metadata object. Error message is logged. The state is stored in MetadataContext, therefore further there will be no attempt to deserialize metadata file again in context of performing current query. The physical plan will be created without using parquet metadata caching. Warning message is logged for every further check "is metadata corrupted".
JsonParseException is obtained. Then the same behaviour as for the unsupported version files.
FileNotFound is obtained. Then the same behaviour as for the unsupported version files.
The new versions of metadata should be added in such manner:
1. Increasing of the metadata major version if metadata structure is changed.
2. Increasing of the metadata minor version if only metadata content is changed, but metadata structure is the same.
For the first case a new metadata structure (class) should be created (possible an improvement of deserializing metadata files of any version into one strucure by using special converting)
For the second case only annotation for the last metadata structure can be updated.
1. Drill will read and use metadata files if files are valid, all present and supported. Under supported we mean that files were created before and under current Drill version.
2. Drill will ignore reading metadata files if at least one file is missing, empty, corrupted or unsupported. Under unsupported we mean files created after current Drill version. When metadata files are not used, warning message will be written to the logs.