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https://issues.apache.org/jira/browse/KAFKA-12920?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17359812#comment-17359812
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Bruno Cadonna commented on KAFKA-12920:
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Is this regression? Should this be a blocker for 3.0?
> Consumer's cooperative sticky assignor need to clear generation / assignment
> data upon `onPartitionsLost`
> ---------------------------------------------------------------------------------------------------------
>
> Key: KAFKA-12920
> URL: https://issues.apache.org/jira/browse/KAFKA-12920
> Project: Kafka
> Issue Type: Bug
> Reporter: Guozhang Wang
> Priority: Major
> Labels: bug, consumer
>
> Consumer's cooperative-sticky assignor does not track the owned partitions
> inside the assignor --- i.e. when it reset its state in event of
> ``onPartitionsLost``, the ``memberAssignment`` and ``generation`` inside the
> assignor would not be cleared. This would cause a member to join with empty
> generation on the protocol while with non-empty user-data encoding the old
> assignment still (and hence pass the validation check on broker side during
> JoinGroup), and eventually cause a single partition to be assigned to
> multiple consumers within a generation.
> We should let the assignor to also clear its assignment/generation when
> ``onPartitionsLost`` is triggered in order to avoid this scenario.
> Note that 1) for the regular sticky assignor the generation would still have
> an older value, and this would cause the previously owned partitions to be
> discarded during the assignment, and 2) for Streams' sticky assignor, it’s
> encoding would indeed be cleared along with ``onPartitionsLost``. Hence only
> Consumer's cooperative-sticky assignor have this issue to solve.
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