[GitHub] [kafka] tombentley commented on a change in pull request #9441: KAFKA-10614: Ensure group state (un)load is executed in the submitted order

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tombentley commented on a change in pull request #9441:
URL: https://github.com/apache/kafka/pull/9441#discussion_r615722539



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File path: core/src/main/scala/kafka/coordinator/group/GroupCoordinator.scala
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@@ -905,19 +907,33 @@ class GroupCoordinator(val brokerId: Int,
    *
    * @param offsetTopicPartitionId The partition we are now leading
    */
-  def onElection(offsetTopicPartitionId: Int): Unit = {
-    info(s"Elected as the group coordinator for partition 
$offsetTopicPartitionId")
-    groupManager.scheduleLoadGroupAndOffsets(offsetTopicPartitionId, 
onGroupLoaded)
+  def onElection(offsetTopicPartitionId: Int, coordinatorEpoch: Int): Unit = {
+    val currentEpoch = epochForPartitionId.get(offsetTopicPartitionId)
+    if (currentEpoch.forall(currentEpoch => coordinatorEpoch > currentEpoch)) {
+      info(s"Elected as the group coordinator for partition 
$offsetTopicPartitionId in epoch $coordinatorEpoch")
+      groupManager.scheduleLoadGroupAndOffsets(offsetTopicPartitionId, 
onGroupLoaded)
+      epochForPartitionId.put(offsetTopicPartitionId, coordinatorEpoch)
+    } else {
+      warn(s"Ignored election as group coordinator for partition 
$offsetTopicPartitionId " +
+        s"in epoch $coordinatorEpoch since current epoch is $currentEpoch")
+    }
   }
 
   /**
    * Unload cached state for the given partition and stop handling requests 
for groups which map to it.
    *
    * @param offsetTopicPartitionId The partition we are no longer leading
    */
-  def onResignation(offsetTopicPartitionId: Int): Unit = {
-    info(s"Resigned as the group coordinator for partition 
$offsetTopicPartitionId")
-    groupManager.removeGroupsForPartition(offsetTopicPartitionId, 
onGroupUnloaded)
+  def onResignation(offsetTopicPartitionId: Int, coordinatorEpoch: 
Option[Int]): Unit = {
+    val currentEpoch = epochForPartitionId.get(offsetTopicPartitionId)
+    if (currentEpoch.forall(currentEpoch => currentEpoch <= 
coordinatorEpoch.getOrElse(Int.MaxValue))) {
+      info(s"Resigned as the group coordinator for partition 
$offsetTopicPartitionId in epoch $coordinatorEpoch")
+      groupManager.removeGroupsForPartition(offsetTopicPartitionId, 
onGroupUnloaded)
+      epochForPartitionId.remove(offsetTopicPartitionId)

Review comment:
       I guess mostly I did it for symmetry with `onElection`, however you're 
right that is doesn't affect correctness. I don't entirely follow your point 
about how we could use this when the replica is being deleted though. Could you 
explain?




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