Improve bq cdc scalability

[claudevdm]

• Further split initial read streams
• Fan out decompression of arrow batches (by far the most expensive step and limited by parallization of number of read streams otherwise)
• Use more efficient compression

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[claudevdm]

R: @ahmedabu98

[claudevdm]

R: @damccorm

[claudevdm]

/gemini review

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[gemini-code-assist]

Summary of Changes

Hello, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request introduces significant scalability improvements to the BigQuery Change Data Capture (CDC) reader. By enabling recursive stream splitting and offloading the CPU-intensive task of decompressing and converting Arrow batches to a distributed fan-out step, the reader can now handle much larger workloads more efficiently. Additionally, the switch to ZSTD compression provides better performance for data transfer.

Highlights

• Enhanced Parallelism: Implemented recursive stream splitting in the BigQuery Read SDF to increase parallelism for large tables.
• Distributed Decompression: Added an option to emit raw Arrow batches, allowing decompression and conversion to be distributed across multiple workers via a fan-out pattern.
• Performance Optimization: Switched to ZSTD compression for Arrow serialization to improve efficiency.

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[gemini-code-assist]

Code Review

This pull request enhances the BigQuery change history reader by introducing recursive stream splitting for better parallelism and a fan-out mechanism to distribute Arrow decompression across workers. Key changes include the addition of _split_all_streams to subdivide read streams during initial restriction and the _DecompressArrowBatchesFn to handle deferred row decoding. The default compression codec has also been updated to ZSTD. Review feedback highlights potential performance issues regarding synchronous RPC calls during splitting, high gRPC connection churn from frequent client resets, and redundant schema parsing during batch decompression.

[claudevdm]

Lint workflow is failing on master too

[ahmedabu98]

Overall looks good, just left a few comments.

Can we add a test for the max_split_rounds case too?

P.S. let's update this documentation to say to_pylist: https://github.com/claudevdm/beam/blob/e08e9d56e5ee8ece43cc15967d0edff107651554/sdks/python/apache_beam/io/gcp/bigquery_change_history.py#L882

[damccorm]

I'll defer to Ahmed's review here since he has started it and he knows more than me here :)

[ahmedabu98]

LGTM