feat: Add E2E fallback to the spanner client.

[kinsaurralde]

Adding E2E fallback will allow spanner clients to fallback to cloudpath when directpath is unavailable.

Also moves the logic to create defaultChannelProviderBuilder into a helper function.

[gemini-code-assist]

Summary of Changes

Hello @kinsaurralde, 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 significant reliability enhancement to the Spanner client by adding an end-to-end fallback mechanism. This ensures that client connections remain robust by automatically falling back to a cloudpath connection if the primary directpath method encounters issues. The change also includes a refactoring of the channel provider builder logic into a helper method, making the codebase cleaner and more maintainable.

Highlights

• End-to-End Fallback: Implemented an end-to-end fallback mechanism for the Spanner client, allowing it to automatically switch to cloudpath if directpath is unavailable.
• Channel Provider Refactoring: Moved the logic for creating the default InstantiatingGrpcChannelProvider.Builder into a dedicated helper function for improved code organization and reusability.
• Environment Variable Control: Introduced an environment variable (GOOGLE_SPANNER_ENABLE_GCP_FALLBACK) to enable or disable the new fallback feature.

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[gemini-code-assist]

Code Review

This pull request introduces E2E fallback for the Spanner client, allowing it to fall back to CloudPath when DirectPath is unavailable. This is a significant feature for improving connection reliability. The implementation also includes a good refactoring that moves the logic for creating the default channel provider builder into a helper function.

My review includes a few suggestions to improve code quality and maintainability:

• Using the primitive boolean instead of the Boolean wrapper for a method parameter to avoid potential NullPointerException.
• Adding a comment to explain a particularly complex piece of code for creating the fallback channel builder, which will help future maintainers.
• Converting an anonymous inner class to a more concise lambda expression.

Overall, the changes are well-structured, and the new fallback mechanism is a valuable addition.

[gemini-code-assist]

The anonymous ClientInterceptor can be simplified to a lambda expression for better readability and conciseness.

            cloudPathBuilder.intercept(
                (method, callOptions, next) ->
                    next.newCall(
                        method,
                        callOptions.withCallCredentials(MoreCallCredentials.from(credentials))));

[rahul2393]

/gemini review

[gemini-code-assist]

Code Review

This pull request introduces an end-to-end fallback mechanism for the Spanner client, enabling it to switch from DirectPath to CloudPath when DirectPath is unavailable. The implementation is well-structured, leveraging a custom FallbackChannelBuilder to integrate with GAX. Additionally, the logic for creating the channel provider has been thoughtfully refactored into a reusable helper function. The new tests effectively validate the fallback functionality. I've included one suggestion to refactor the test helpers to improve maintainability by reducing code duplication. Overall, this is a valuable and well-executed feature.

I am having trouble creating individual review comments. Click here to see my feedback.

google-cloud-spanner/src/test/java/com/google/cloud/spanner/spi/v1/GapicSpannerRpcTest.java (1083-1100)

To reduce code duplication, you can merge TestableGapicSpannerRpc and TestableGapicSpannerRpcWithLowerMinFailedCalls into a single helper class that accepts the minFailedCalls value in its constructor. This makes the test setup more flexible and maintainable.

After applying this suggestion, you can remove the TestableGapicSpannerRpcWithLowerMinFailedCalls class and update the tests to instantiate the new TestableGapicSpannerRpc class like this:

• new TestableGapicSpannerRpc(options, 10) for testFallbackIntegration_doesNotSwitchWhenThresholdNotMet
• new TestableGapicSpannerRpc(options, 1) for testFallbackIntegration_switchesToFallbackOnFailure

  static class TestableGapicSpannerRpc extends GapicSpannerRpc {
    private final int minFailedCallsForTest;

    public TestableGapicSpannerRpc(SpannerOptions options, int minFailedCallsForTest) {
      super(options);
      this.minFailedCallsForTest = minFailedCallsForTest;
    }

    @Override
    GcpFallbackChannelOptions createFallbackChannelOptions(
        GcpFallbackOpenTelemetry fallbackTelemetry, int minFailedCalls) {
      // Override default 1-minute period to 10ms for instant testing
      return GcpFallbackChannelOptions.newBuilder()
          .setPrimaryChannelName("directpath")
          .setFallbackChannelName("cloudpath")
          .setMinFailedCalls(this.minFailedCallsForTest)
          .setPeriod(Duration.ofMillis(10))
          .setGcpFallbackOpenTelemetry(fallbackTelemetry)
          .build();
    }
  }