Do not emit negative lag because of stale offsets (#14292)

The latest topic offsets are polled frequently and used to determine the lag based on the current offsets. However, when the offsets are stale (which can happen due to connection issues commonly), we may see a negative lag .

This PR prevents emission of metrics when the offsets are stale and at least one of the partitions has a negative lag.

indexing-service/src/main/java/org/apache/druid/indexing/seekablestream/supervisor/SeekableStreamSupervisor.java

@@ -4220,6 +4220,21 @@ protected void emitLag()
           return;
         }
 
+        // Try emitting lag even with stale metrics provided that none of the partitions has negative lag
+        final long staleMillis = sequenceLastUpdated == null
+            ? 0
+            : DateTimes.nowUtc().getMillis()
+              - (tuningConfig.getOffsetFetchPeriod().getMillis() + sequenceLastUpdated.getMillis());
+        if (staleMillis > 0 && partitionLags.values().stream().anyMatch(x -> x < 0)) {
+          // Log at most once every twenty supervisor runs to reduce noise in the logs
+          if ((staleMillis / getIoConfig().getPeriod().getMillis()) % 20 == 0) {
+            log.warn("Lag is negative and will not be emitted because topic offsets have become stale. "
+                     + "This will not impact data processing. "
+                     + "Offsets may become stale because of connectivity issues.");
+          }
+          return;
+        }
+
         LagStats lagStats = computeLags(partitionLags);
         Map<String, Object> metricTags = spec.getContextValue(DruidMetrics.TAGS);
         for (Map.Entry<PartitionIdType, Long> entry : partitionLags.entrySet()) {

indexing-service/src/test/java/org/apache/druid/indexing/seekablestream/supervisor/SeekableStreamSupervisorStateTest.java

@@ -62,6 +62,7 @@
 import org.apache.druid.indexing.seekablestream.supervisor.SeekableStreamSupervisorStateManager.SeekableStreamExceptionEvent;
 import org.apache.druid.indexing.seekablestream.supervisor.SeekableStreamSupervisorStateManager.SeekableStreamState;
 import org.apache.druid.indexing.seekablestream.supervisor.autoscaler.AutoScalerConfig;
+import org.apache.druid.java.util.common.DateTimes;
 import org.apache.druid.java.util.common.ISE;
 import org.apache.druid.java.util.common.StringUtils;
 import org.apache.druid.java.util.common.granularity.Granularities;
@@ -1035,6 +1036,30 @@ public void testGetStats()
     );
   }
 
+  @Test
+  public void testStaleOffsetsNegativeLagNotEmitted() throws Exception
+  {
+    expectEmitterSupervisor(false);
+
+    CountDownLatch latch = new CountDownLatch(1);
+
+    final TestEmittingTestSeekableStreamSupervisor supervisor = new TestEmittingTestSeekableStreamSupervisor(
+        latch,
+        TestEmittingTestSeekableStreamSupervisor.LAG,
+        // Record lag must not be emitted
+        ImmutableMap.of("0", 10L, "1", -100L),
+        null
+    );
+    supervisor.start();
+    // Forcibly set the offsets to be stale
+    supervisor.sequenceLastUpdated = DateTimes.nowUtc().minus(Integer.MAX_VALUE);
+
+    latch.await();
+
+    supervisor.emitLag();
+    Assert.assertEquals(0, emitter.getEvents().size());
+  }
+
   private List<Event> filterMetrics(List<Event> events, List<String> whitelist)
   {
     List<Event> result = events.stream()