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Add rate‑limited log warning when BatchSpanProcessor drops spans #8167

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Merged
jkwatson merged 5 commits into from
Mar 25, 2026
+47 −0
Merged

Add rate‑limited log warning when BatchSpanProcessor drops spans #8167

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ff11a8b
Alert through logs for dropped spans in BatchSpanProcessor
VamshikrishnaMonagari Mar 9, 2026
912e997
Fix variable name for throttling logger in BatchSpanProcessor
VamshikrishnaMonagari Mar 11, 2026
d14a2cc
Replace ThrottlingLogger with per-batch drop count logging in BatchSp…
VamshikrishnaMonagari Mar 18, 2026
ad025e3
Address test review feedback: use forceFlush and assert full message
VamshikrishnaMonagari Mar 22, 2026
903425f
Fix checkstyle import ordering in BatchSpanProcessorTest
VamshikrishnaMonagari Mar 24, 2026
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7 changes: 7 additions & 0 deletions sdk/trace/src/main/java/io/opentelemetry/sdk/trace/export/BatchSpanProcessor.java
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Original file line number Diff line number Diff line change
Expand Up @@ -11,6 +11,7 @@
import io.opentelemetry.sdk.common.InternalTelemetryVersion;
import io.opentelemetry.sdk.common.internal.ComponentId;
import io.opentelemetry.sdk.common.internal.DaemonThreadFactory;
import io.opentelemetry.sdk.common.internal.ThrottlingLogger;
import io.opentelemetry.sdk.common.internal.ThrowableUtil;
import io.opentelemetry.sdk.trace.ReadWriteSpan;
import io.opentelemetry.sdk.trace.ReadableSpan;
Expand Down Expand Up @@ -46,6 +47,7 @@ public final class BatchSpanProcessor implements SpanProcessor {
ComponentId.generateLazy("batching_span_processor");

private static final Logger logger = Logger.getLogger(BatchSpanProcessor.class.getName());
private static final ThrottlingLogger throttledLogger = new ThrottlingLogger(logger);
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@robsunday robsunday Mar 10, 2026

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throttlingLogger would be a better name.

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@VamshikrishnaMonagari VamshikrishnaMonagari Mar 11, 2026

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Good catch! I've updated the variable name from throttledLogger to throttlingLogger to better reflect that this logger performs throttling (matching the active form of the class name ThrottlingLogger).


private static final String WORKER_THREAD_NAME =
BatchSpanProcessor.class.getSimpleName() + "_WorkerThread";
Expand Down Expand Up @@ -212,6 +214,11 @@ private void addSpan(ReadableSpan span) {
spanProcessorInstrumentation.buildQueueMetricsOnce(maxQueueSize, queue::size);
if (!queue.offer(span)) {
spanProcessorInstrumentation.dropSpans(1);
throttledLogger.log(
Level.WARNING,
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@robsunday robsunday Mar 10, 2026

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Isn't warning too high level? I think INFO would be enough.

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@VamshikrishnaMonagari VamshikrishnaMonagari Mar 11, 2026

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Good point. I think WARNING is appropriate here because dropped spans represent actual data loss, which impacts observability. This warrants immediate attention from operators in production. Since we're using ThrottlingLogger, the output is already rate-limited to prevent log spam.

However, if the team prefers INFO, I'm happy to adjust. Please let me know.

"BatchSpanProcessor dropped a span because the queue is full (maxQueueSize="
+ maxQueueSize
+ ")");
} else {
if (queueSize.incrementAndGet() >= spansNeeded.get()) {
signal.offer(true);
Expand Down
31 changes: 31 additions & 0 deletions sdk/trace/src/test/java/io/opentelemetry/sdk/trace/export/BatchSpanProcessorTest.java
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Expand Up @@ -21,6 +21,7 @@
import io.opentelemetry.api.internal.GuardedBy;
import io.opentelemetry.api.trace.Span;
import io.opentelemetry.api.trace.Tracer;
import io.github.netmikey.logunit.api.LogCapturer;
import io.opentelemetry.internal.testing.slf4j.SuppressLogger;
import io.opentelemetry.sdk.common.CompletableResultCode;
import io.opentelemetry.sdk.trace.ReadableSpan;
Expand All @@ -29,6 +30,7 @@
import io.opentelemetry.sdk.trace.samplers.Sampler;
import io.opentelemetry.sdk.trace.samplers.SamplingResult;
import java.time.Duration;
import org.slf4j.event.Level;
import java.util.ArrayList;
import java.util.Arrays;
import java.util.Collection;
Expand All @@ -42,6 +44,7 @@
import org.junit.jupiter.api.Test;
import org.junit.jupiter.api.Timeout;
import org.junit.jupiter.api.extension.ExtendWith;
import org.junit.jupiter.api.extension.RegisterExtension;
import org.mockito.ArgumentMatchers;
import org.mockito.Mock;
import org.mockito.junit.jupiter.MockitoExtension;
Expand All @@ -62,9 +65,13 @@ class BatchSpanProcessorTest {
@Mock private Sampler mockSampler;
@Mock private SpanExporter mockSpanExporter;

@RegisterExtension
LogCapturer logs = LogCapturer.create().captureForType(BatchSpanProcessor.class);

@BeforeEach
void setUp() {
when(mockSpanExporter.shutdown()).thenReturn(CompletableResultCode.ofSuccess());
when(mockSpanExporter.export(anyList())).thenReturn(CompletableResultCode.ofSuccess());
}

@AfterEach
Expand Down Expand Up @@ -232,6 +239,30 @@ void exportMoreSpansThanTheBufferSize() {
span6.toSpanData()));
}

@Test
void droppedSpanIsLogged() {
sdkTracerProvider =
SdkTracerProvider.builder()
.addSpanProcessor(
BatchSpanProcessor.builder(mockSpanExporter)
.setMaxQueueSize(1)
.setMaxExportBatchSize(1_000)
.setScheduleDelay(Duration.ofDays(1))
.build())
.build();

// Add two spans quickly to trigger a drop when the queue is full.
createEndedSpan(SPAN_NAME_1);
createEndedSpan(SPAN_NAME_2);

await()
.untilAsserted(
() ->
logs.assertContains(
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@anuraaga anuraaga Mar 19, 2026

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Seems reasonable to assert the whole message, especially since it has some dynamic parts

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@VamshikrishnaMonagari VamshikrishnaMonagari Mar 22, 2026

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Thanks for the review, @anuraaga! Good catches on the test. I've updated it.

@jkwatson @jack-berg -- on the question about SpanProcessorInstrumentation, I agree with @anuraaga that keeping the tracking in BSP makes more sense since the instrumentation counters are synchronous and adding state for this there would complicate things without much benefit.

Would appreciate another look when you get a chance!

loggingEvent -> loggingEvent.getLevel().equals(Level.WARN),
"BatchSpanProcessor dropped a span"));
}

@Test
void forceExport() {
WaitingSpanExporter waitingSpanExporter =
Expand Down