[DataStreamToSpanner] Improvement: Adding group by key step before Spanner write step

[darshan-sj]

Improvement: Adding group by key step before Spanner write step

This would serialize events happening on same key in Dataflow job and hence avoid lock contention in Spanner when there are too many events corresponding to a small set of keys.

[gemini-code-assist]

Summary of Changes

Hello @darshan-sj, 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 a crucial optimization to the DataStreamToSpanner pipeline by implementing a primary key-based grouping mechanism for change events. This enhancement aims to serialize operations on the same key, thereby significantly reducing lock contention within Spanner during high-volume write operations. The change improves the robustness and efficiency of the data replication process, particularly for scenarios with concentrated event activity on a small set of keys.

Highlights

• New Grouping Mechanism: Introduced a new GroupChangeEventsByPrimaryKeyDoFn to group change events by their primary key, converting them into KV<Long, FailsafeElement<String, String>> pairs where the key is a hash of the table name and primary key string.
• Pipeline Integration: The SpannerTransactionWriter now incorporates this grouping step, applying the GroupChangeEventsByPrimaryKeyDoFn followed by a Reshuffle operation before writing mutations to Spanner.
• Enhanced Error Handling: Errors occurring during the new grouping step are now captured and merged with permanent errors from the subsequent Spanner write operations, ensuring comprehensive error reporting.
• Input Type Update: The SpannerTransactionWriterDoFn has been updated to accept KV<Long, FailsafeElement<String, String>> as input, aligning with the output of the new grouping function.
• New Constant for Grouped Events: A new TupleTag, SUCCESSFUL_GROUPED_EVENT_TAG, has been added to DatastreamToSpannerConstants to identify successfully grouped events within the pipeline.

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[codecov]

Codecov Report

❌ Patch coverage is 56.36364% with 24 lines in your changes missing coverage. Please review.
✅ Project coverage is 54.91%. Comparing base (63091f1) to head (826933f).
⚠️ Report is 20 commits behind head on main.

Files with missing lines	Patch %	Lines
...eleport/v2/templates/SpannerTransactionWriter.java	0.00%	24 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##               main    #3249      +/-   ##
============================================
+ Coverage     50.93%   54.91%   +3.98%     
+ Complexity     5157     2277    -2880     
============================================
  Files           977      493     -484     
  Lines         60347    28861   -31486     
  Branches       6594     3036    -3558     
============================================
- Hits          30735    15848   -14887     
+ Misses        27457    12050   -15407     
+ Partials       2155      963    -1192     

Components	Coverage Δ
spanner-templates	71.94% <56.36%> (+0.89%)	⬆️
spanner-import-export	∅ <ø> (∅)
spanner-live-forward-migration	80.04% <56.36%> (-0.08%)	⬇️
spanner-live-reverse-replication	77.58% <ø> (-0.19%)	⬇️
spanner-bulk-migration	88.08% <ø> (-0.38%)	⬇️

Files with missing lines	Coverage Δ
...tes/CreateKeyValuePairsWithPrimaryKeyHashDoFn.java	100.00% <100.00%> (ø)
...ort/v2/templates/SpannerTransactionWriterDoFn.java	51.80% <100.00%> (ø)
.../v2/templates/datastream/ChangeEventConvertor.java	87.50% <ø> (+5.55%)	⬆️
...eleport/v2/templates/SpannerTransactionWriter.java	0.00% <0.00%> (ø)

... and 518 files with indirect coverage changes

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[bharadwaj-aditya]

Couple of minor changes. otherwise LGTM

[bharadwaj-aditya]

LGTM