grpclb: pick_first delegation #12568

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shivaspeaks wants to merge 1 commit into grpc:master from shivaspeaks:port-grpclb

+207 −234

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shivaspeaks commented Dec 15, 2025

Summary of Changes
This pull request refactors the grpclb load balancer's PICK_FIRST mode to delegate its logic to a standard pick_first load balancing policy.

The key changes are as follows:

grpclb/build.gradle

Added dependency on grpc-util module to access ForwardingLoadBalancerHelper

grpclb/src/main/java/io/grpc/grpclb/GrpclbState.java

New imports:
LoadBalancer, LoadBalancerProvider, LoadBalancerRegistry, ResolvedAddresses, FixedResultPicker, ForwardingLoadBalancerHelper
New fields for PICK_FIRST delegation:
- pickFirstLbProvider - Provider for creating child pick_first LB
- pickFirstLb - The child LoadBalancer instance
- pickFirstLbState / pickFirstLbPicker - Track child LB's state and picker
- currentPickFirstLoadRecorder - Load recorder for token attachment
Key behavioral changes:
- updateServerList() PICK_FIRST case: Instead of creating a single subchannel, it now:
  - Creates the child pick_first LB once and then updates it with new addresses on subsequent updates.
  - Passes addresses to child LB via acceptResolvedAddresses()
- maybeUpdatePicker() PICK_FIRST case: Uses child LB's state and picker wrapped with ChildLbPickerEntry
- RoundRobinEntry.picked() signature change: Changed from picked(Metadata) to picked(PickSubchannelArgs) to allow child picker delegation
- New ChildLbPickerEntry class: Wraps child LB's picker and attaches TokenAttachingTracerFactory for token propagation
- New PickFirstLbHelper class: Forwarding helper that intercepts updateBalancingState() to store child state and trigger grpclb picker updates
- Updated shutdown(), requestConnection(), maybeUseFallbackBackends(): Handle the new child LB delegation model

grpclb/src/test/java/io/grpc/grpclb/GrpclbLoadBalancerTest.java

Updated tests to reflect the new delegation behavior:
- Initial state is now CONNECTING (not IDLE) since standard pick_first eagerly connects
- Tests now verify the child LB is created only once and then handles address updates internally.
- Adjusted verification expectations for the new flow
Key Behavioral Changes:
- Delegation to child LB: The grpclb state no longer directly manages subchannels for PICK_FIRST mode; the child pick_first LB handles this internally.


          grpclb: pick_first delegation

bd7aa4c

shivaspeaks requested review from ejona86 and kannanjgithub

December 15, 2025 07:18

kannanjgithub requested changes

View reviewed changes

Contributor

kannanjgithub left a comment

Review comments.

grpclb/src/test/java/io/grpc/grpclb/GrpclbLoadBalancerTest.java

    
                  // One subchannel is created by the child LB

                  assertThat(mockSubchannels).hasSize(1);

                  Subchannel subchannel = mockSubchannels.poll();

                  assertThat(picker0.dropList).containsExactly(null, null);

Contributor

kannanjgithub Dec 16, 2025

Add assertion for picker0.dropList size 1 and is instance of ChildLbPickerEntry.

grpclb/src/test/java/io/grpc/grpclb/GrpclbLoadBalancerTest.java

    
                  // Only one subchannel is created

                  // One subchannel is created by the child LB

                  assertThat(mockSubchannels).hasSize(1);

                  Subchannel subchannel = mockSubchannels.poll();

Contributor

kannanjgithub Dec 16, 2025

Add

verify(subchannel).requestConnection();

grpclb/src/test/java/io/grpc/grpclb/GrpclbLoadBalancerTest.java

    
                  inOrder.verify(helper).updateBalancingState(eq(READY), pickerCaptor.capture());

                  RoundRobinPicker picker3 = (RoundRobinPicker) pickerCaptor.getValue();

                  assertThat(picker3.dropList).containsExactly(null, null);

                  assertThat(picker3.pickList).containsExactly(

Contributor

kannanjgithub Dec 16, 2025

Similarly here, don't remove the assertion for the pickList with the fake subchannel, assert using accessor in ChildLbPickerEntry for use by the test.

grpclb/src/test/java/io/grpc/grpclb/GrpclbLoadBalancerTest.java

Comment on lines -1945 to 1952

    
                  // TRANSIENT_FAILURE

                  Status error = Status.UNAVAILABLE.withDescription("Simulated connection error");

                  deliverSubchannelState(subchannel, ConnectivityStateInfo.forTransientFailure(error));

                  inOrder.verify(helper).updateBalancingState(eq(TRANSIENT_FAILURE), pickerCaptor.capture());

                  RoundRobinPicker picker2 = (RoundRobinPicker) pickerCaptor.getValue();

                  assertThat(picker2.dropList).containsExactly(null, null);

                  assertThat(picker2.pickList).containsExactly(new ErrorEntry(error));

Contributor

kannanjgithub Dec 16, 2025

Do not remove the testing for simulated transient failure.
Now we cannot directly do

assertThat(picker2.pickList).containsExactly(new ErrorEntry(error));

as the ErorrEntry is wrapped in a ChildLbPickerEntry. Create accessor for the wrapped entry for use in the unit test so we can assert using that.

grpclb/src/test/java/io/grpc/grpclb/GrpclbLoadBalancerTest.java

Comment on lines -2219 to 2230

    
                  assertThat(mockSubchannels).isEmpty();

                  verify(subchannel).updateAddresses(

                      eq(Arrays.asList(

                          new EquivalentAddressGroup(backends1.get(0).addr, eagAttrsWithToken("token0001")),

                          new EquivalentAddressGroup(backends1.get(1).addr,

                              eagAttrsWithToken("token0002")))));

                  inOrder.verify(helper).updateBalancingState(eq(READY), pickerCaptor.capture());

                  RoundRobinPicker picker2 = (RoundRobinPicker) pickerCaptor.getValue();

                  assertThat(picker2.dropList).containsExactly(null, null);

                  assertThat(picker2.pickList).containsExactly(

                      new BackendEntry(subchannel, new TokenAttachingTracerFactory(getLoadRecorder())));

                  // the same child LB is updated with new addresses. We expect

                  // only one subchannel to be created for the whole test. The child LB will call

                  // updateAddresses() internally, so we remove the verifications for recreation.

                  verify(helper, times(1)).createSubchannel(any(CreateSubchannelArgs.class));

Contributor

kannanjgithub Dec 16, 2025

This PF behavior may be done by a different code now but the behavior should still be the same. Why remove the test for it?

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