impl(bigtable): add StubManager class

[scotthart]

No description provided.

[gemini-code-assist]

Summary of Changes

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This pull request introduces a foundational StubManager class designed to streamline the handling of BigtableStub objects within the Bigtable client library. This enhancement provides a flexible mechanism for managing stub lifecycle and selection, allowing for either a global stub or instance-specific stubs, which is crucial for optimizing resource utilization and request routing in various Bigtable interaction patterns.

Highlights

• New StubManager Class: Introduced a new StubManager class to centralize the management of BigtableStub instances. This class supports two primary modes: kNoAffinity for a single shared stub and kInstanceAffinity for managing stubs specific to Bigtable instances.
• Instance Affinity Support: The StubManager can maintain a collection of BigtableStub instances, each potentially bound to a specific Bigtable instance. It includes logic to retrieve an existing stub for a given instance or create a new one on demand using a provided creation function.
• Build System Integration: The new StubManager source and header files, along with its unit tests, have been integrated into the CMake and Bazel build configurations for the google-cloud-cpp-bigtable library.
• Comprehensive Unit Tests: Added a dedicated unit test file (stub_manager_test.cc) to thoroughly validate the functionality of the StubManager class, covering both its no-affinity and instance-affinity behaviors.

Changelog

• google/cloud/bigtable/CMakeLists.txt
  • Added internal/stub_manager.cc and internal/stub_manager.h to the google_cloud_cpp_bigtable library sources.
  • Added internal/stub_manager_test.cc to the bigtable_client_unit_tests.
• google/cloud/bigtable/bigtable_client_unit_tests.bzl
  • Included internal/stub_manager_test.cc in the list of unit tests for Bazel builds.
• google/cloud/bigtable/google_cloud_cpp_bigtable.bzl
  • Added internal/stub_manager.h to the library headers for Bazel builds.
  • Added internal/stub_manager.cc to the library sources for Bazel builds.
• google/cloud/bigtable/internal/stub_manager.cc
  • Implemented the StubManager class, including constructors for no-affinity and instance-affinity modes, and the GetStub method for retrieving stubs.
• google/cloud/bigtable/internal/stub_manager.h
  • Declared the StubManager class, defining its public interface, internal state (mode, stub creation function, affinity stubs), and an enum for priming.
• google/cloud/bigtable/internal/stub_manager_test.cc
  • Created unit tests for StubManager, verifying its behavior in both kNoAffinity and kInstanceAffinity configurations, including dynamic stub creation.

Activity

• No human activity has been recorded on this pull request yet.

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[gemini-code-assist]

Code Review

This pull request introduces a StubManager class to manage BigtableStub instances. This class supports two modes: kNoAffinity with a single shared stub, and kInstanceAffinity which maintains a map of stubs per instance, creating them on demand. The implementation is sound and includes corresponding unit tests. I have one suggestion to improve the implementation of GetStub for better performance and conciseness.

[codecov]

Codecov Report

❌ Patch coverage is 98.71795% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 92.63%. Comparing base (ced8a7c) to head (ec1ada1).
⚠️ Report is 3 commits behind head on main.

Files with missing lines	Patch %	Lines
...oogle/cloud/bigtable/internal/stub_manager_test.cc	98.36%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #16022   +/-   ##
=======================================
  Coverage   92.63%   92.63%           
=======================================
  Files        2333     2335    +2     
  Lines      214631   214709   +78     
=======================================
+ Hits       198827   198906   +79     
+ Misses      15804    15803    -1     

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