Enable client side gRPC health check by default

[ahrtr]

Followup to #16278

Previously the default gRPC service config for the resolver is {"loadBalancingPolicy": "round_robin"}.

etcd/client/v3/internal/resolver/resolver.go

Line 44 in 7ab7612

r.serviceConfig = cc.ParseServiceConfig(`{"loadBalancingPolicy": "round_robin"}`)

Now I propose to change the default service config to {"loadBalancingPolicy": "round_robin"}, "healthCheckConfig": {"serviceName": ""}. The benefit is that

• any applications which depend on etcd client sdk don't need to care about the low level grpc service config.
• Is it also a best practice to always enable client side health check if the loadBalancingPolicy is round_robin? @dfawley But I am not whether is there any performance penalty? Probably we need to run rw-heatmaps to double confirm.

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[ahrtr]

Please anyone feel free to work on this on top of this PR.

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Codecov Report

Attention: Patch coverage is 57.14286% with 3 lines in your changes missing coverage. Please review.

Project coverage is 68.73%. Comparing base (7ab7612) to head (fa2078f).
Report is 4 commits behind head on main.

❗ Current head fa2078f differs from pull request most recent head a9f846b

Please upload reports for the commit a9f846b to get more accurate results.

Files with missing lines	Patch %	Lines
server/etcdmain/grpc_proxy.go	0.00%	3 Missing ⚠️

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Additional details and impacted files

Files with missing lines	Coverage Δ
client/v3/internal/resolver/resolver.go	84.00% <100.00%> (ø)
server/etcdserver/api/v3rpc/interceptor.go	73.71% <100.00%> (+1.31%)	⬆️
server/etcdmain/grpc_proxy.go	14.91% <0.00%> (-0.14%)	⬇️

... and 20 files with indirect coverage changes

@@            Coverage Diff             @@
##             main   #18882      +/-   ##
==========================================
+ Coverage   68.72%   68.73%   +0.01%     
==========================================
  Files         420      420              
  Lines       35532    35537       +5     
==========================================
+ Hits        24418    24428      +10     
- Misses       9681     9687       +6     
+ Partials     1433     1422      -11     

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[serathius]

How this change is related to health checks?

[ahrtr]

gRPC client will call the Watch RPC on the health check service automatically for the stream RPC. Refer to https://grpc.io/docs/guides/health-checking/#enabling-client-health-checking

Most likely we will do similar change for isRPCSupportedForLearner as well,

etcd/server/etcdserver/api/v3rpc/util.go

Lines 142 to 151 in 7ab7612

	func isRPCSupportedForLearner(req any) bool {
	switch r := req.(type) {
	case *pb.StatusRequest:
	return true
	case *pb.RangeRequest:
	return r.Serializable
	default:
	return false
	}
	}

[ahrtr]

The client side sees a grpc error, not sure why.

$ ./bin/etcdctl get k1
2024/11/12 13:49:23 ERROR: [core] [Channel #1 SubChannel #2]Health check is requested but health check function is not set.

I am pretty sure that we have registered the health service and set the healthpb.HealthCheckResponse_SERVING status. @dfawley @easwars @aranjans can you please share some thought on this?

etcd/server/etcdserver/api/v3rpc/grpc.go

Lines 77 to 79 in 7ab7612

	hsrv := health.NewServer()
	healthNotifier := newHealthNotifier(hsrv, s)
	healthpb.RegisterHealthServer(grpcServer, hsrv)

etcd/server/etcdserver/api/v3rpc/health.go

Line 68 in 7ab7612

hc.hs.SetServingStatus(allGRPCServices, healthpb.HealthCheckResponse_SERVING)

[chaochn47]

$ ./bin/etcdctl get k1
2024/11/12 13:49:23 ERROR: [core] [Channel #1 SubChannel #2]Health check is requested but health check function is not set.

@ahrtr Side effect of importing import _ "google.golang.org/grpc/health" would register the health check function here and mentioned in the feature example

[lavacat]

@ahrtr

The client side sees a grpc error, not sure why.

Maybe you need an import that sets internal.HealthCheckFunc

Any danger to roll this out enabled by default? Do we need a config option? I think, yes. The change is minimal but if there are any bugs in grpc health impl, we might need a way to disable.

[ahrtr]

Thanks both. It's a bad pattern to have a blank-import in a non-main package,

internal/resolver/resolver.go:18:2: blank-imports: a blank import should be only in a main or test package, or have a comment justifying it (revive)
	_ "google.golang.org/grpc/health"

[ahrtr]

/retest

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@ahrtr: The following test failed, say /retest to rerun all failed tests or /retest-required to rerun all mandatory failed tests:

Test name	Commit	Details	Required	Rerun command
pull-etcd-integration-1-cpu-amd64	a9f846b	link	true	/test pull-etcd-integration-1-cpu-amd64

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[easwars]

@arjan-bal

[ahrtr]

Any danger to roll this out enabled by default? Do we need a config option?

YES, we definitely need a config option for this; otherwise it will be a breaking change. If the server side health check isn't enabled, the client will will get an error something like below,

2024/11/13 07:39:59 ERROR: [core] [Channel #1 SubChannel #2]Subchannel health check is unimplemented at server side, thus health check is disabled

Please anyone feel free to continue to work on this task on top of this PR,

• add a config flag and a function something like WithGRPCHealthCheckEnabled for the client side;
• run benchmark test to evaluate the performance impact.

[lavacat]

Please anyone feel free to continue to work on this task on top of this PR

assigned to myself

[ahrtr]

assigned to myself

thx