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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
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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14 changes: 14 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 @@ -184,6 +184,7 @@ private static final class Worker implements Runnable {
private volatile boolean continueWork = true;
private final ArrayList<SpanData> batch;
private final long maxQueueSize;
private final AtomicInteger droppedSpanCount = new AtomicInteger(0);

private Worker(
SpanExporter spanExporter,
Expand Down Expand Up @@ -212,6 +213,7 @@ private void addSpan(ReadableSpan span) {
spanProcessorInstrumentation.buildQueueMetricsOnce(maxQueueSize, queue::size);
if (!queue.offer(span)) {
spanProcessorInstrumentation.dropSpans(1);
droppedSpanCount.incrementAndGet();
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} else {
if (queueSize.incrementAndGet() >= spansNeeded.get()) {
signal.offer(true);
Expand Down Expand Up @@ -315,6 +317,18 @@ private void exportCurrentBatch() {
return;
}

int dropped = droppedSpanCount.getAndSet(0);
if (dropped > 0) {
logger.log(
Level.WARNING,
"BatchSpanProcessor dropped "
+ dropped
+ " span(s) since the last export because the queue is full"
+ " (maxQueueSize="
+ maxQueueSize
+ ")");
}

String error = null;
try {
CompletableResultCode result = spanExporter.export(Collections.unmodifiableList(batch));
Expand Down
31 changes: 31 additions & 0 deletions sdk/trace/src/test/java/io/opentelemetry/sdk/trace/export/BatchSpanProcessorTest.java
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Original file line number Diff line number Diff line change
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)
.setScheduleDelay(Duration.ofMillis(1))
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@anuraaga anuraaga Mar 19, 2026

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Since we don't need any exports, may as well set to a huge number to reduce potential flakiness. And maybe it makes sense then to drop more than one span

.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"));
}

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