From ff11a8b5c19d3ad030c057c0240426a9f099eb06 Mon Sep 17 00:00:00 2001
From: Vamshikrishna Monagari <monagarivamshi9@gmail.com>
Date: Sun, 8 Mar 2026 23:05:42 -0400
Subject: [PATCH] Alert through logs for dropped spans in BatchSpanProcessor

---
 .../sdk/trace/export/BatchSpanProcessor.java  |  7 +++++
 .../trace/export/BatchSpanProcessorTest.java  | 31 +++++++++++++++++++
 2 files changed, 38 insertions(+)

diff --git a/sdk/trace/src/main/java/io/opentelemetry/sdk/trace/export/BatchSpanProcessor.java b/sdk/trace/src/main/java/io/opentelemetry/sdk/trace/export/BatchSpanProcessor.java
index 2febbb46667..4fb4302329e 100644
--- a/sdk/trace/src/main/java/io/opentelemetry/sdk/trace/export/BatchSpanProcessor.java
+++ b/sdk/trace/src/main/java/io/opentelemetry/sdk/trace/export/BatchSpanProcessor.java
@@ -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;
@@ -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);
 
   private static final String WORKER_THREAD_NAME =
       BatchSpanProcessor.class.getSimpleName() + "_WorkerThread";
@@ -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,
+            "BatchSpanProcessor dropped a span because the queue is full (maxQueueSize="
+                + maxQueueSize
+                + ")");
       } else {
         if (queueSize.incrementAndGet() >= spansNeeded.get()) {
           signal.offer(true);
diff --git a/sdk/trace/src/test/java/io/opentelemetry/sdk/trace/export/BatchSpanProcessorTest.java b/sdk/trace/src/test/java/io/opentelemetry/sdk/trace/export/BatchSpanProcessorTest.java
index 98bc45705b3..fecf7005f18 100644
--- a/sdk/trace/src/test/java/io/opentelemetry/sdk/trace/export/BatchSpanProcessorTest.java
+++ b/sdk/trace/src/test/java/io/opentelemetry/sdk/trace/export/BatchSpanProcessorTest.java
@@ -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;
@@ -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;
@@ -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;
@@ -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 +239,30 @@ 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(
+                    loggingEvent -> loggingEvent.getLevel().equals(Level.WARN),
+                    "BatchSpanProcessor dropped a span"));
+  }
+
   @Test
   void forceExport() {
     WaitingSpanExporter waitingSpanExporter =