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

Author: VamshikrishnaMonagari

sdk/trace/src/main/java/io/opentelemetry/sdk/trace/export/BatchSpanProcessor.java

@@ -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,
@@ -212,6 +213,7 @@ private void addSpan(ReadableSpan span) {
       spanProcessorInstrumentation.buildQueueMetricsOnce(maxQueueSize, queue::size);
       if (!queue.offer(span)) {
         spanProcessorInstrumentation.dropSpans(1);
+        droppedSpanCount.incrementAndGet();
       } else {
         if (queueSize.incrementAndGet() >= spansNeeded.get()) {
           signal.offer(true);
@@ -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));

sdk/trace/src/test/java/io/opentelemetry/sdk/trace/export/BatchSpanProcessorTest.java

@@ -18,6 +18,7 @@
 import static org.mockito.Mockito.verify;
 import static org.mockito.Mockito.when;
 
+import io.github.netmikey.logunit.api.LogCapturer;
 import io.opentelemetry.api.internal.GuardedBy;
 import io.opentelemetry.api.trace.Span;
 import io.opentelemetry.api.trace.Tracer;
@@ -42,11 +43,13 @@
 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;
 import org.mockito.junit.jupiter.MockitoSettings;
 import org.mockito.quality.Strictness;
+import org.slf4j.event.Level;
 
 @SuppressWarnings("PreferJavaTimeOverload")
 @ExtendWith(MockitoExtension.class)
@@ -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
@@ -232,6 +239,32 @@ 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();
+
+    // Fill the queue with the first span, then drop 2 more.
+    createEndedSpan(SPAN_NAME_1);
+    createEndedSpan(SPAN_NAME_2);
+    createEndedSpan(SPAN_NAME_2);
+
+    // Force export to trigger the drop count log.
+    sdkTracerProvider.forceFlush().join(10, TimeUnit.SECONDS);
+
+    logs.assertContains(
+        loggingEvent -> loggingEvent.getLevel().equals(Level.WARN),
+        "BatchSpanProcessor dropped 2 span(s) since the last export"
+            + " because the queue is full (maxQueueSize=1)");
+  }
+
   @Test
   void forceExport() {
     WaitingSpanExporter waitingSpanExporter =