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https://issues.apache.org/jira/browse/KAFKA-19212?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
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Jun Rao resolved KAFKA-19212.
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Resolution: Fixed
merged the PR to trunk
> Fix ElectionWasClean bug when partition reassignment completes
> --------------------------------------------------------------
>
> Key: KAFKA-19212
> URL: https://issues.apache.org/jira/browse/KAFKA-19212
> Project: Kafka
> Issue Type: Bug
> Reporter: Calvin Liu
> Assignee: Calvin Liu
> Priority: Major
> Fix For: 4.1.0
>
>
> The current ElectionWasClean checks if the new leader is in the previous ISR.
> However, there is a corner case in the partition reassignment.
> The partition reassignment can change the partition replicas. If the new
> preferred leader (the first one in the new replicas) is the last one to join
> ISR, this preferred leader will be elected in the same partition change.
> For example:
> In the previous state, the partition is
> Leader: 0, Replicas (2,1,0), ISR (1,0), Adding(2), removing(0).
> Then replica 2 joins the ISR. The new partition would be like:
> Leader: 2, Replicas (2,1), ISR(1,2).
> The new leader 2 is not in the previous ISR (1,0) but it is still a clean
> election.
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