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

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hachikuji commented on pull request #9441:
URL: https://github.com/apache/kafka/pull/9441#issuecomment-709432283


   @tombentley Thanks for the patch. I think the problem is in the callback inside `KafkaApis.handleLeaderAndIsrRequest`. With two concurrent `LeaderAndIsr` requests, there is no guarantee about the ordering of the callbacks even though `ReplicaManager` checks epoch. They can get reordered like the following:
   
   1. Call `ReplicaManager.becomeLeaderOrFollower` with epoch=1 in thread 1
   2. Call `ReplicaManager.becomeLeaderOrFollower` with epoch=2 in thread 2
   3. Call `onLeadershipChange` callback in thread 2
   4. Call `onLeadershipChange` callback in thread 1
   
   At least that's my understanding of the issue. I think we need to find a way to push the epoch validation into `GroupCoordinator`. 


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