A75: Implement the new LB policy topology for the non-aggregate cluster by mswierq · Pull Request #9234 · grpc/grpc-go

mswierq · 2026-07-15T11:20:49Z

Implements A75 for the non-aggregate cluster.

This change implements the new LB policy topology for the non-aggregate clusters only, the aggregate cluster uses the legacy approach. Refactoring of the new topology for the aggregate cluster case will be delivered in a follow-up PR. This change does not break non-aggregate to aggregate (and vice versa) switch.

RELEASE NOTES:

xds: refactored LB policy topology for the non-aggregate clusters according to A75

…ers only

codecov · 2026-07-15T11:26:28Z

Codecov Report

❌ Patch coverage is 91.66667% with 8 lines in your changes missing coverage. Please review.
✅ Project coverage is 83.25%. Comparing base (44332db) to head (f2fc6b5).
⚠️ Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
internal/xds/balancer/cdsbalancer/cdsbalancer.go	87.87%	3 Missing and 1 partial ⚠️
internal/xds/balancer/cdsbalancer/configbuilder.go	93.65%	2 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #9234      +/-   ##
==========================================
+ Coverage   83.20%   83.25%   +0.04%     
==========================================
  Files         423      423              
  Lines       35321    35208     -113     
==========================================
- Hits        29389    29311      -78     
+ Misses       4425     4402      -23     
+ Partials     1507     1495      -12

Files with missing lines	Coverage Δ
internal/xds/balancer/cdsbalancer/cdsbalancer.go	`69.66% <87.87%> (+7.54%)`	⬆️
internal/xds/balancer/cdsbalancer/configbuilder.go	`92.15% <93.65%> (+0.26%)`	⬆️

... and 26 files with indirect coverage changes

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easwars · 2026-07-17T16:44:52Z

/gemini review

gemini-code-assist

Code Review

This pull request updates the CDS balancer to support the gRFC A75 topology for single (non-aggregate) clusters, routing them through an outlier detection policy instead of a priority policy. The changes include updating the configuration builder to construct the new balancer tree (outlier_detection -> cluster_impl -> priority -> wrr_locality -> round_robin) and updating corresponding tests. The review feedback highlights several critical areas in the new code paths where missing configuration data (such as nil cluster configs, endpoint configs, or empty priorities) could lead to nil pointer dereferences or out-of-bounds panics, and suggests adding defensive checks to ensure robustness.

Pranjali-2501 · 2026-08-01T19:48:22Z

+		parser, ok := builder.(balancer.ConfigParser)
+		if !ok {
+			return nil, nil, fmt.Errorf("xds: balancer builder for %v does not implement ConfigParser", name)
+		}


Since you have added the logic to retrieve the childConfigParser here, we can remove it from `Build()``.

this has been removed as a result of using gracefulswitch.Balancer

setting childConfigParser in Build has been removed

Pranjali-2501 · 2026-08-01T19:56:01Z

+	newChildBalancer = func(name string, cc balancer.ClientConn, opts balancer.BuildOptions) (balancer.Balancer, balancer.ConfigParser, error) {
+		builder := balancer.Get(name)


Can we use gracefulswitch.Balancer here and make cdsBalancer.childLB of type *gracefulswitch.Balancer?

newChildBalancer function and its output have been removed as gracefulswitch.Balancer and gracefulswitch.ParseConfig implementation already covers the same functionality

I wanted to start a discussion on this before making any change and want to hear your thoughts on this.

Now based on my offline discussions with @easwars, We don't need to use gracefulSwitch here because it is very rare that the type of cluster get changed.

Could you please revert back the changes you made to use gracefulSwitch.

I see, I have reverted it

Pranjali-2501 · 2026-08-01T20:19:10Z

+		if len(b.priorities) == 0 {
+			return fmt.Errorf("no priorities configured for non-aggregate cluster %q", clusterName)
+		}


why do we need this check here?

I think we can directly pass b.priorities instead of passing its first element in buildSingleClusterConfigJSON.

I also think handleClusterUpdate() will never set b.priorities to nil.

reverted + getting the first element is inside ConfigJSON function

This reverts commit fbed93a.

…b-policy-tree-refactoring

Pranjali-2501 · 2026-08-12T07:26:11Z

 // management server sends two updates for the same leaf cluster resource. The
-// test verifies that the load balancing configuration pushed to the priority LB
-// policy contains the expected discovery mechanism corresponding to the leaf
+// test verifies that the load balancing configuration pushed to the top-level LB


Wrap the comment in 80 column.

Pranjali-2501 · 2026-08-12T07:26:17Z

 // the priority LB policy contains the discovery mechanisms for both child
 // clusters. The test then updates the root cluster resource requested by the
 // cds LB policy to a leaf cluster of type EDS and verifies the load balancing
-// configuration pushed to the priority LB policy contains a single discovery
-// mechanism.
+// configuration pushed to the outlier detection LB policy contains the leaf cluster config.


Same here, please wrap the comment in 80 column. Here and elsewhere.

Pranjali-2501 · 2026-08-12T07:28:45Z

+	if isAggregate {
+		childCfgBytes, endpoints, err = buildAggregateClusterConfigJSON(b.priorities, &b.xdsLBPolicy)
+	} else {
+		childCfgBytes, endpoints, err = buildLeafClusterConfigJSON(b.priorities, &b.xdsLBPolicy)
+	}


I think the variable topLBName can be set here only instead of setting it in another if-else block.

after reverting gracefulswitch this actually has to stay that way, because we need to check first if the top policy type has changed due to update (e.g. update from non-aggregate to aggregate cluster)

Pranjali-2501 · 2026-08-12T07:28:59Z

+			select {
+			case <-exitIdleCh:
+			default:


Why do we need this here?

This might be called more than once in test cases where balancers are being reconfigured, e.g. transition from non-aggregate to aggregate cluster (TestAggregatedClusterSuccess_SwitchBetweenLeafAndAggregate).

However, those channels are not used in the mentioned aggregate test case. We may consider having these exitIdleCh and closeCh channels configurable and remove this default option. WDYT?

Pranjali-2501 · 2026-08-12T07:29:05Z

+			select {
+			case <-closeCh:


Same here, Close() will not going to be called more that once for a balancer.

Look at my previous comment

Pranjali-2501 · 2026-08-12T07:29:21Z

+func (s) TestBuildAggregateClusterConfigJSON(t *testing.T) {
 	testLRSServerConfig, err := bootstrap.ServerConfigForTesting(bootstrap.ServerConfigTestingOptions{


You have added a similar test for leaf cluster TestBuildLeafClusterConfigJSON. I think we can merge both in a table-driven test.

merged into a table-driven test case

Pranjali-2501 · 2026-08-12T07:29:26Z

 	return retEndpoint
 }

+func (s) TestBuildLeafClusterConfigJSON(t *testing.T) {


Add a docstring about what the test is doing. Here and elsewhere.

…b-policy-tree-refactoring

This reverts commit 51e986e.

A75: Implement the new LB policy topology for the non-aggregate clust…

533dafb

…ers only

easwars requested a review from Pranjali-2501 July 15, 2026 18:13

easwars assigned Pranjali-2501 Jul 15, 2026

easwars added Type: Feature New features or improvements in behavior Area: xDS Includes everything xDS related, including LB policies used with xDS. labels Jul 15, 2026

easwars added this to the 1.84 Release milestone Jul 15, 2026

mswierq added 2 commits July 17, 2026 09:36

Merge branch 'master' into a75/non-aggregate-lb-policy-tree-refactoring

5516779

A75: Add missing test cases

9b3c703

gemini-code-assist Bot reviewed Jul 17, 2026

View reviewed changes

Address Gemini review - fix nil checks

fbed93a

Pranjali-2501 reviewed Aug 1, 2026

View reviewed changes

Pranjali-2501 reviewed Aug 3, 2026

View reviewed changes

Comment thread internal/xds/balancer/cdsbalancer/configbuilder.go Outdated

Comment thread internal/xds/balancer/cdsbalancer/configbuilder.go

Comment thread internal/xds/balancer/cdsbalancer/cdsbalancer.go

Comment thread internal/xds/balancer/cdsbalancer/configbuilder.go Outdated

Pranjali-2501 assigned mswierq and unassigned Pranjali-2501 Aug 3, 2026

mswierq added 6 commits August 3, 2026 12:26

Revert "Address Gemini review - fix nil checks"

c1ab2d9

This reverts commit fbed93a.

Fix comments and legacy code cleanup

785be23

Use gracefulswitch.Balancer

51e986e

Fix functions and variables names

e7c0475

Do not create clusterImpl for each priority

d6a83ca

Merge remote-tracking branch 'origin/master' into a75/non-aggregate-l…

31d6b4c

…b-policy-tree-refactoring

mswierq requested a review from Pranjali-2501 August 4, 2026 11:13

easwars assigned Pranjali-2501 and unassigned mswierq Aug 11, 2026

Pranjali-2501 reviewed Aug 12, 2026

View reviewed changes

Pranjali-2501 assigned mswierq and unassigned Pranjali-2501 Aug 12, 2026

mswierq added 2 commits August 12, 2026 09:42

Merge remote-tracking branch 'origin/master' into a75/non-aggregate-l…

53b4209

…b-policy-tree-refactoring

Address feedback

9f43e37

mswierq requested a review from Pranjali-2501 August 12, 2026 14:54

easwars assigned Pranjali-2501 and unassigned mswierq Aug 12, 2026

Revert "Use gracefulswitch.Balancer"

f2fc6b5

This reverts commit 51e986e.

		newChildBalancer = func(name string, cc balancer.ClientConn, opts balancer.BuildOptions) (balancer.Balancer, balancer.ConfigParser, error) {
		builder := balancer.Get(name)

		func (s) TestBuildAggregateClusterConfigJSON(t *testing.T) {
		testLRSServerConfig, err := bootstrap.ServerConfigForTesting(bootstrap.ServerConfigTestingOptions{

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mswierq commented Jul 15, 2026 •

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