From f2495a67d206534b6ee79b083ce49b3131165192 Mon Sep 17 00:00:00 2001
From: Yuanli Han <44718283+yuanlihan@users.noreply.github.com>
Date: Tue, 29 Mar 2022 00:21:06 +0800
Subject: [PATCH] fix messageGap metric (#12337)

---
 .../druid/segment/realtime/FireDepartmentMetrics.java       | 3 ++-
 .../druid/segment/realtime/FireDepartmentMetricsTest.java   | 6 ++++++
 2 files changed, 8 insertions(+), 1 deletion(-)

diff --git a/server/src/main/java/org/apache/druid/segment/realtime/FireDepartmentMetrics.java b/server/src/main/java/org/apache/druid/segment/realtime/FireDepartmentMetrics.java
index 9fd286428d0..0811a3434f7 100644
--- a/server/src/main/java/org/apache/druid/segment/realtime/FireDepartmentMetrics.java
+++ b/server/src/main/java/org/apache/druid/segment/realtime/FireDepartmentMetrics.java
@@ -263,7 +263,8 @@ public class FireDepartmentMetrics
     retVal.messageMaxTimestamp.set(messageMaxTimestamp.get());
     retVal.messageProcessingCompletionTime.set(messageProcessingCompletionTime.get());
     retVal.messageProcessingCompletionTime.compareAndSet(DEFAULT_PROCESSING_COMPLETION_TIME, System.currentTimeMillis());
-    retVal.messageGap.set(retVal.messageProcessingCompletionTime.get() - messageMaxTimestamp.get());
+    long maxTimestamp = retVal.messageMaxTimestamp.get();
+    retVal.messageGap.set(maxTimestamp > 0 ? retVal.messageProcessingCompletionTime.get() - maxTimestamp : 0L);
     return retVal;
   }
 }
diff --git a/server/src/test/java/org/apache/druid/segment/realtime/FireDepartmentMetricsTest.java b/server/src/test/java/org/apache/druid/segment/realtime/FireDepartmentMetricsTest.java
index 0c1025161b1..17d4356acd3 100644
--- a/server/src/test/java/org/apache/druid/segment/realtime/FireDepartmentMetricsTest.java
+++ b/server/src/test/java/org/apache/druid/segment/realtime/FireDepartmentMetricsTest.java
@@ -33,6 +33,12 @@ public class FireDepartmentMetricsTest
     metrics = new FireDepartmentMetrics();
   }
 
+  @Test
+  public void testSnapshotBeforeProcessing()
+  {
+    Assert.assertEquals(0L, metrics.snapshot().messageGap());
+  }
+
   @Test
   public void testSnapshotAfterProcessingOver()
   {