fix(gax-grpc): add pick_first fallback to direct path service config

[westarle]

The Dependency Compatibility Test CI workflow has been failing on the InstantiatingGrpcChannelProviderTest tests (e.g. canUseDirectPath_happyPath) with the following error during initialization:

java.lang.IllegalStateException: Default config is invalid: Status{code=UNKNOWN, description=None of [grpclb] specified by Service Config are available., cause=null}

Root Cause & DNS Spam Issue:
As of grpc-java v1.77.0, the grpc-grpclb artifact is no longer implicitly included because the legacy grpclb load balancer plugin is deprecated (Tracked in #4012). However, InstantiatingGrpcChannelProvider still hardcoded grpclb as its primary DirectPath load balancing policy.

This missing plugin crashes tests, but the backend Google grpclb lookaside servers were also officially turned down in July 2025. Because the library still explicitly requests grpclb, it causes gRPC to continuously spam DNS with _grpclb._tcp.* SRV queries trying to find those dead servers. This leads to ~22,000 NXDOMAIN DNS errors a day on user networks (Tracked in googleapis/java-pubsub#2404).

Fix:
This PR completely removes grpclb and replaces it with pick_first in getDefaultDirectPathServiceConfig().

We deliberately choose pick_first to preserve the original 2019 design intent (maintaining exactly one connection per pooled GAX channel to emulate CFE behavior) and to provide a safe, fail-open default. While Traffic Director relies on the xds API, configuring it as a default fallback would require complex bootstrap configurations and cause crashes for clients
without that setup. pick_first stops the DNS spam entirely, ensures stability on modern gRPC versions, and resolves the test failures.

[gemini-code-assist]

Summary of Changes

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This pull request provides a crucial update to the gax-grpc library by replacing the deprecated grpclb load balancing policy with pick_first for DirectPath service configurations. This change directly resolves recurring Dependency Compatibility Test failures stemming from the grpc-grpclb artifact's removal in recent grpc-java versions. Furthermore, it eliminates a significant source of network noise by preventing gRPC from repeatedly querying for non-existent grpclb servers, thereby improving system stability and reducing DNS error logs. The adoption of pick_first ensures continued adherence to the original design principle of maintaining a single connection per GAX channel.

Highlights

• Resolved Test Failures: Fixed Dependency Compatibility Test failures in InstantiatingGrpcChannelProviderTest caused by the removal of the grpc-grpclb artifact in grpc-java v1.77.0.
• Eliminated DNS Spam: Addressed a critical issue causing gRPC to continuously spam DNS with _grpclb._tcp.* SRV queries, resulting in approximately 22,000 NXDOMAIN errors daily, by removing the explicit request for the deprecated grpclb policy.
• Updated Load Balancing Policy: Replaced the deprecated grpclb load balancing policy with pick_first in getDefaultDirectPathServiceConfig() to ensure stability and maintain the original design intent of one connection per pooled GAX channel.

Changelog

• gax-java/gax-grpc/src/main/java/com/google/api/gax/grpc/InstantiatingGrpcChannelProvider.java
  • Removed the grpclb load balancing policy from the default DirectPath service configuration.
  • Updated internal comments to reflect the change to the pick_first strategy.
• gax-java/gax-grpc/src/test/java/com/google/api/gax/grpc/InstantiatingGrpcChannelProviderTest.java
  • Updated test configurations to use pick_first instead of grpclb for DirectPath service configurations.
  • Modified assertions to validate the presence of pick_first as the load balancing policy.

Activity

• westarle opened this pull request to address critical issues with grpclb deprecation.
• The Dependency Compatibility Test CI workflow was failing, indicating automated testing activity.

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[gemini-code-assist]

Code Review

This pull request correctly addresses the deprecation of the grpclb load balancing policy by replacing it with pick_first as the default for DirectPath. This change resolves CI failures and mitigates a DNS spam issue caused by requests for the now-defunct grpclb servers. The implementation is clean and the corresponding test updates are appropriate. I have one minor suggestion to improve consistency in the test code.