chore: remove session pool and only use multiplexed sessions

[rahul2393]

This PR removes the legacy SessionPool from the Java Spanner client, transitioning to multiplexed sessions as the only session management mechanism. Multiplexed sessions allow a single session to support multiple parallel transactions, eliminating the need for session pooling.

[generated-files-bot]

Warning: This pull request is touching the following templated files:

• .github/workflows/ci.yaml

[gemini-code-assist]

Summary of Changes

Hello @rahul2393, 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 fundamentally refactors the session management within the Java Spanner client. By removing the traditional session pool and fully embracing multiplexed sessions, the client's architecture is streamlined. This change simplifies the codebase, reduces overhead associated with managing a pool of individual sessions, and aligns the client with a more efficient, modern approach to handling concurrent database interactions.

Highlights

• Session Pool Removal: The legacy SessionPool mechanism has been entirely removed from the Java Spanner client, simplifying session management.
• Exclusive Multiplexed Sessions: The client now exclusively uses multiplexed sessions, which allow a single session to handle multiple parallel transactions, eliminating the need for traditional session pooling.
• API Simplification: Client implementations like BatchClientImpl and DatabaseClientImpl have been streamlined, removing logic and parameters related to the old session pool and direct session creation.
• Deprecation of SessionPoolOptions: The SessionPoolOptions class and related methods in SessionClient have been deprecated, with a clear indication that session pool configurations are no longer relevant.
• Test Infrastructure Cleanup: Numerous tests, benchmarks, and CI configurations related to the legacy session pool and its specific behaviors (e.g., integration-regular-sessions) have been removed or updated.

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Ignored Files

• Ignored by pattern: .github/workflows/** (2)
  • .github/workflows/ci.yaml
  • .github/workflows/integration-tests-against-emulator-with-regular-session.yaml

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Code Review

This pull request is a significant and well-executed refactoring that removes the legacy SessionPool and transitions the Java Spanner client to exclusively use multiplexed sessions. The changes are consistent across the entire codebase, including production code, tests, benchmarks, and protobuf definitions, resulting in a much simpler and cleaner implementation. Obsolete files have been removed, and relevant public APIs have been clearly deprecated. I have one minor suggestion for removing some dead code that was left over from the refactoring. Overall, this is an excellent improvement.

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google-cloud-spanner/src/main/java/com/google/cloud/spanner/DatabaseClientImpl.java (250-257)

These methods are correctly removed as part of this refactoring. It seems that the fields nthRequest, dbId, and clientIdToOrdinalMap, along with the dbIdFromClientId() method, are now also unused. Consider removing them in a follow-up to complete the cleanup of dead code related to the old request ID generation.