fix(spanner): don't set sticky AFFINITY_KEY for multiplexed sessions

[akash329d]

Problem

Since 6.105.0 (#4239 enabled grpc-gcp by default), a Spanner client whose first traffic is a high-concurrency burst sees throughput collapse and p50 latency climb. disableGrpcGcpExtension() restores the expected scaling.

concurrency	static numChannels=32 qps / p50 / p99	dynamic channel pool qps / p50 / p99	grpc-gcp OFF qps / p50 / p99
50	723 / 67.6 / 84.9	—	723 / 67.5 / 78.9
100	1198 / 82.1 / 137.8	—	1261 / 68.1 / 309.5
200	1491 / 142.9 / 178.5	—	2841 / 67.5 / 109.7
400	973 / 402.2 / 538.8	5384 / 66 / 341	5862 / 64.8 / 80.1

(single multiplexed-session client, SELECT 1; each cell is a fresh client whose first traffic is at the target concurrency. Dynamic-pool row from a separate probe with enableDynamicChannelPool() set on the builder.)

Root cause

newCallContext sets GcpManagedChannel.AFFINITY_KEY on every data RPC under grpc-gcp. GcpManagedChannel.getChannelRef(key) binds each new key via pickLeastBusyChannel() (1754-1790), which reads activeStreamsCount and ties to channelRefs.get(0). The count isn't incremented until later in GcpClientCall.start() (:284), so a concurrent first burst all binds to channel 0 and the bindings are sticky. Subsequent RPCs funnel through one HTTP/2 connection and queue at MAX_CONCURRENT_STREAMS.

The static-numChannels path used affinity.intValue() % numChannels (≈63 distinct keys), so the collapse is permanent. The dynamic-pool path mostly self-corrects (p50 matches OFF) because most hints are per-transaction random longs — but MultiplexedSessionDatabaseClient.getSingleUseChannelHint allocates the first numChannels concurrent hints from a recycled BitSet (values 0..N-1), and those few sticky-bind during the warmup race and keep getting reused, leaving a p99 tail. (Separately: in our testing, setting enableDynamicChannelPool=true via JDBC/connection properties did not fully propagate to GcpManagedChannel, so DCP is not currently a workaround for connection-API users.)

Fix

Don't set AFFINITY_KEY when grpc-gcp is on. Multiplexed sessions are a single session, so sticky per-transaction channel affinity provides no backend-locality benefit. With no key, getChannelRef(null) does a fresh per-call least-busy pick with no sticky binding and no affinity-map growth — matches the OFF curve. (Math.floorMod alone doesn't help; the race still ties bounded keys to channel 0.)

RetryOnDifferentGrpcChannelMockServerTest previously asserted distinct AFFINITY_KEY values via an interceptor; with no key set those assertions are unobservable, so they're removed (the request-count and session assertions still cover the retry loop). Note this means the opt-in spanner.retry_deadline_exceeded_on_different_channel feature now relies on grpc-gcp's per-call least-busy pick rather than a forced distinct channel under grpc-gcp (the wedged channel will normally have a higher active-stream count, so least-busy usually picks a different one, but it's no longer guaranteed); the GAX withChannelAffinity path (used when grpc-gcp is off) is unchanged.

Repro

The trigger is the client's first traffic burst being high-concurrency (e.g., a connection pool warming many connections at once); a gentle low-C warmup spreads the keys and masks the bug. Standalone single-file reproducer (only dep google-cloud-spanner):

SpannerAffinityRepro.java

// Repro: grpc-gcp channel affinity collapses onto a few channels under// concurrent warmup with multiplexed sessions, capping per-client throughput.//// For each (grpcGcp, concurrency) pair, builds a FRESH Spanner client,// fires a high-concurrency warmup burst (this is when the race binds keys),// then measures qps/p50/p99 of SELECT 1 at that concurrency. Expect parity// at C<=100 and a large divergence (lower qps, higher p50) for grpc-gcp=ON// at C>=200.//// javac -cp 'lib/*' SpannerAffinityRepro.java// java -cp '.:lib/*' SpannerAffinityRepro PROJECT INSTANCE DATABASEimportcom.google.cloud.spanner.*;
importjava.util.*;
importjava.util.concurrent.*;
importjava.util.concurrent.atomic.AtomicInteger;
publicclassSpannerAffinityRepro {
staticfinalintNUM_CHANNELS = 32;
staticfinalint[] SWEEP = {50, 100, 200, 400};
recordRow(doubleqps, doublep50, doublep99) {}
staticRowone(Stringproj, Stringinst, StringdbId, booleangcp, intc) throwsException {
SpannerOptions.Builderb = SpannerOptions.newBuilder()
.setProjectId(proj).setNumChannels(NUM_CHANNELS);
if (!gcp) b.disableGrpcGcpExtension();
try (Spanners = b.build().getService()) {
DatabaseClientdb = s.getDatabaseClient(DatabaseId.of(proj, inst, dbId));
Statementstmt = Statement.of("SELECT 1");
Runnableq = () -> { try (ResultSetrs = db.singleUse().executeQuery(stmt)) { while (rs.next()) {} } };
ExecutorServiceex = Executors.newVirtualThreadPerTaskExecutor();
// Warmup AT the target concurrency: this is the burst during which the// ~63 affinity keys race through pickLeastBusyChannel and bind.runAt(ex, q, c, Math.max(500, c * 4), null);
intiters = Math.max(2000, c * 30);
long[] lats = newlong[iters];
longelapsed = runAt(ex, q, c, iters, lats);
ex.shutdown();
Arrays.sort(lats);
doublems = 1e-6;
returnnewRow(iters / (elapsed * 1e-9), lats[iters / 2] * ms, lats[(int) (iters * 0.99)] * ms);
}
}
staticlongrunAt(ExecutorServiceex, Runnableq, intc, intiters, long[] lats) throwsException {
AtomicIntegerleft = newAtomicInteger(iters), idx = newAtomicInteger();
CountDownLatchdone = newCountDownLatch(c);
longt0 = System.nanoTime();
for (inti = 0; i < c; i++) ex.submit(() -> {
while (true) {
ints = left.decrementAndGet(); if (s < 0) break;
longt = System.nanoTime(); q.run();
if (lats != null) lats[idx.getAndIncrement()] = System.nanoTime() - t;
}
done.countDown();
});
done.await();
returnSystem.nanoTime() - t0;
}
publicstaticvoidmain(String[] a) throwsException {
if (a.length < 3) { System.err.println("usage: PROJECT INSTANCE DATABASE"); System.exit(1); }
System.out.printf("numChannels=%d, multiplexed sessions (default), fresh client per cell%n", NUM_CHANNELS);
System.out.printf("%5s | %22s | %22s%n", "", "grpc-gcp ON (default)", "grpc-gcp OFF");
System.out.printf("%5s | %6s %6s %6s | %6s %6s %6s%n", "C", "qps", "p50ms", "p99ms", "qps", "p50ms", "p99ms");
for (intc : SWEEP) {
Rowon = one(a[0], a[1], a[2], true, c);
Rowoff = one(a[0], a[1], a[2], false, c);
System.out.printf("%5d | %6.0f %6.1f %6.1f | %6.0f %6.1f %6.1f%n",
c, on.qps, on.p50, on.p99, off.qps, off.p50, off.p99);
}
}
}

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[gemini-code-assist]

Code Review

This pull request modifies the channel affinity logic in GapicSpannerRpc.java. Specifically, it disables the setting of AFFINITY_KEY when dynamic channel pooling is disabled to prevent sticky binding issues under concurrent load, allowing the system to perform fresh least-busy channel picks instead. I have no feedback to provide.

[rahul2393]

@akash329d Can you please check if this #12726 fixes your issue?

[akash329d]

@rahul2393 IMO its a worse solution because its keeping a bunch of unnecessary complexity.

But I think it decreases the severity of the issue significantly. However does not fully fix it [tbf neither does my PR], as the initial burst race still exists. (before any statements have been run, you still have the pickLeastBusyChannel race condition). I think we need to land the GoogleCloudPlatform/grpc-gcp-java#234 to really fix this.

[rahul2393]

Closing this since we already released another fix https://repo1.maven.org/maven2/com/google/cloud/google-cloud-spanner/6.115.0/
Please re-open if you find any issue with latest version
Thanks