[Live Migration] Skipped Events (No Table in Session) are lost silently

[aasthabharill]

https://b.corp.google.com/issues/479380497

[gemini-code-assist]

Summary of Changes

Hello @aasthabharill, 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 addresses a critical issue where certain events, specifically those related to tables being dropped during a live migration, were silently lost within the Datastream-to-Spanner pipeline. The changes introduce a robust mechanism to capture, categorize, and persist these 'skipped' events to a dedicated Dead Letter Queue in GCS. This significantly improves the observability and reliability of the pipeline by preventing silent data loss and providing a clear audit trail for such occurrences.

Highlights

• Introduced SKIPPED_EVENT_TAG: A new TupleTag, SKIPPED_EVENT_TAG, has been added to explicitly categorize and track events that are skipped, primarily due to DroppedTableException.
• Dedicated Dead Letter Queue for Skipped Events: Events tagged with SKIPPED_EVENT_TAG are now routed to a dedicated Dead Letter Queue (DLQ) directory in Google Cloud Storage (GCS) under /skipped/, ensuring they are no longer silently lost.
• Enhanced Error Handling in SpannerTransactionWriter: The SpannerTransactionWriter now explicitly handles and outputs SKIPPED_EVENT_TAG events, integrating them into its result structure alongside permanent and retryable errors.
• Improved DroppedTableException Handling: The SpannerTransactionWriterDoFn and ChangeEventTransformerDoFn now output DroppedTableException events using the new SKIPPED_EVENT_TAG to the DLQ, instead of silently logging them as errors or warnings.
• Updated Test Coverage: An existing test case in ChangeEventTransformerDoFnTest has been updated to verify that DroppedTableException correctly triggers an output with the SKIPPED_EVENT_TAG.

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[codecov]

Codecov Report

❌ Patch coverage is 8.00000% with 23 lines in your changes missing coverage. Please review.
✅ Project coverage is 54.87%. Comparing base (f2c600c) to head (2503f4e).

Files with missing lines	Patch %	Lines
...oud/teleport/v2/templates/DataStreamToSpanner.java	0.00%	17 Missing ⚠️
...eleport/v2/templates/SpannerTransactionWriter.java	0.00%	5 Missing ⚠️
...ort/v2/templates/SpannerTransactionWriterDoFn.java	0.00%	1 Missing ⚠️

❌ Your patch check has failed because the patch coverage (8.00%) is below the target coverage (80.00%). You can increase the patch coverage or adjust the target coverage.

Additional details and impacted files

@@             Coverage Diff              @@
##               main    #3276      +/-   ##
============================================
+ Coverage     50.92%   54.87%   +3.94%     
+ Complexity     5166     2277    -2889     
============================================
  Files           979      493     -486     
  Lines         60503    28882   -31621     
  Branches       6617     3036    -3581     
============================================
- Hits          30813    15848   -14965     
+ Misses        27533    12070   -15463     
+ Partials       2157      964    -1193     

Components	Coverage Δ
spanner-templates	71.83% <8.00%> (+0.93%)	⬆️
spanner-import-export	∅ <ø> (∅)
spanner-live-forward-migration	79.74% <8.00%> (-0.30%)	⬇️
spanner-live-reverse-replication	77.57% <ø> (-0.07%)	⬇️
spanner-bulk-migration	88.07% <ø> (-0.02%)	⬇️

Files with missing lines	Coverage Δ
...emplates/transform/ChangeEventTransformerDoFn.java	77.41% <100.00%> (+0.18%)	⬆️
...ort/v2/templates/SpannerTransactionWriterDoFn.java	51.56% <0.00%> (-0.24%)	⬇️
...eleport/v2/templates/SpannerTransactionWriter.java	0.00% <0.00%> (ø)
...oud/teleport/v2/templates/DataStreamToSpanner.java	17.97% <0.00%> (-1.15%)	⬇️

... and 504 files with indirect coverage changes

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