[KAFKA-8663] partition assignment would be better original_assignment + new_reassignment during reassignments

• Type: Improvement
• Status: Resolved
• Priority: Minor
• Resolution: Won't Fix
• Affects Version/s: 1.1.1, 2.3.0
• Component/s: controller, core
• Assignee: Unassigned
• Reporter: GEORGE LI
• Created: 13/Jul/19 23:45
• Updated: 19/Jul/19 20:39
• Resolved: 19/Jul/19 20:39

Description

From my observation/experience during reassignment, the partition assignment replica ordering gets changed. because it's OAR + RAR (original replicas + reassignment replicas) set union.

However, it seems like the preferred leaders changed during the reassignments. Normally if there is no cluster preferred leader election, the leader is still the old leader. But if during the reassignments, there is a leader election, the leadership changes. This caused some side effects. Let's look at this example.

Topic:georgeli_test	PartitionCount:8	ReplicationFactor:3	Configs:
	Topic: georgeli_test	Partition: 0	Leader: 1026	Replicas: 1026,1028,1025	Isr: 1026,1028,1025

reassignment (1026,1028,1025) => (1027,1025,1028)

Topic:georgeli_test	PartitionCount:8	ReplicationFactor:4	Configs:leader.replication.throttled.replicas=0:1026,0:1028,0:1025,follower.replication.throttled.replicas=0:1027
	Topic: georgeli_test	Partition: 0	Leader: 1026	Replicas: 1027,1025,1028,1026	Isr: 1026,1028,1025

Notice the above: Leader remains 1026. but Replicas: 1027,1025,1028,1026. If we run preferred leader election, it will try 1027 first, then 1025. After 1027 is in ISR, then the final assignment will be (1027,1025,1028).

My proposal for a minor improvement is to keep the original ordering replicas during the reassignment (could be long for big topic/partitions). and after all replicas in ISR, then finally set the partition assignment to New reassignment.

      val newAndOldReplicas = (reassignedPartitionContext.newReplicas ++ controllerContext.partitionReplicaAssignment(topicPartition)).toSet
      //1. Update AR in ZK with OAR + RAR.
      updateAssignedReplicasForPartition(topicPartition, newAndOldReplicas.toSeq)

above code changed to below to keep the original ordering first during reassignment:

      val newAndOldReplicas = (controllerContext.partitionReplicaAssignment(topicPartition) ++ reassignedPartitionContext.newReplicas).toSet