POJOs for MultiShard TableReader.

[VardhanThigle]

Foundations for Multi-Shard Routing (POJOs)

This is the first child of #3684

Design Decision

This PR introduces the foundational data models and services required to support multi-shard JDBC migrations.

Key Components:

• JdbcIoWrapperConfigGroup: A container that aggregates multiple JdbcIOWrapperConfig instances. It ensures that all shards in a group share a uniform SQLDialect, simplifying downstream transform logic.
• DataSourceProvider Interface: Defines the contract for worker-side connection routing.
• DataSourceManager / DataSourceManagerImpl: A centralized service for managing and caching connection sources across multiple physical shards.

Rationale:

To achieve a constant graph size, we must move connection details from pipeline construction time (PTransform fields) to element processing time (DoFn logic). These POJOs provide the structure for this dynamic routing.

---

Why it's Safe (Concurrency & Resource Leaks)

• Thread-Safe Caching: DataSourceManagerImpl implements a thread-safe cache for DataSource instances using ReentrantLock and a double-checked locking pattern. This ensures that even when multiple Beam bundles execute concurrently on the same worker, each physical shard connection pool is initialized exactly once.
• Resource Management: The DataSourceManager provides a closeAll() method. This is specifically designed to be called during DoFn.teardown() or finishBundle() to ensure that all opened connection pools are gracefully released, preventing resource leaks on Dataflow workers.
• Serialization Robustness: Fields within DataSourceManagerImpl are marked transient where appropriate, and the class includes null-safety checks to handle lifecycle transitions in the Dataflow environment.

Testing

The added tests verify:

• Correct aggregation of shard configurations.
• Thread-safe lazy initialization of data sources.
• Proper handling of unknown shard IDs.

[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 establishes the foundational infrastructure for multi-shard table reader schema discovery. It provides a robust framework for managing multiple JDBC data sources within a Dataflow worker, focusing on thread-safe connection handling, resource lifecycle management, and configuration validation to support complex multi-shard migration scenarios.

Highlights

• Configuration Management: Introduced JdbcIoWrapperConfigGroup to encapsulate and validate JDBC shard configurations, ensuring dialect consistency.
• Data Source Provisioning: Added DataSourceProvider and its implementation to enable dynamic, serializable routing of data sources in multi-shard environments.
• Lifecycle Management: Implemented DataSourceManager to handle thread-safe, on-demand initialization, caching, and graceful teardown of data source connections.

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 58.67%. Comparing base (72a145d) to head (8128b2a).
⚠️ Report is 8 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff              @@
##               main    #3685      +/-   ##
============================================
+ Coverage     52.47%   58.67%   +6.19%     
+ Complexity     6224     2122    -4102     
============================================
  Files          1062      505     -557     
  Lines         64136    29129   -35007     
  Branches       7089     3191    -3898     
============================================
- Hits          33654    17090   -16564     
+ Misses        28198    11061   -17137     
+ Partials       2284      978    -1306     

Components	Coverage Δ
spanner-templates	73.76% <100.00%> (+1.64%)	⬆️
spanner-import-export	∅ <ø> (∅)
spanner-live-forward-migration	80.77% <ø> (-0.09%)	⬇️
spanner-live-reverse-replication	77.46% <ø> (-0.07%)	⬇️
spanner-bulk-migration	89.32% <100.00%> (+0.01%)	⬆️
gcs-spanner-dv	86.67% <ø> (+0.94%)	⬆️

Files with missing lines	Coverage Δ
...dbc/iowrapper/config/JdbcIoWrapperConfigGroup.java	100.00% <100.00%> (ø)
...o/jdbc/uniformsplitter/DataSourceProviderImpl.java	100.00% <100.00%> (ø)
...formsplitter/transforms/DataSourceManagerImpl.java	100.00% <100.00%> (ø)

... and 585 files with indirect coverage changes

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[sm745052]

LGTM

[sm745052]

LGTM