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https://issues.apache.org/jira/browse/KAFKA-18625?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
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Jun Rao resolved KAFKA-18625.
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Resolution: Not A Problem
> consumer client could get duplicated records if assigned partitions change
> quickly
> ----------------------------------------------------------------------------------
>
> Key: KAFKA-18625
> URL: https://issues.apache.org/jira/browse/KAFKA-18625
> Project: Kafka
> Issue Type: Bug
> Components: consumer
> Reporter: Jun Rao
> Assignee: TengYao Chi
> Priority: Major
>
> When a partition is unassigned to a consumer, we don't clear the buffered
> records in the client immediately. When the client calls poll(),
> [FetchCollector.fetchRecords()|https://github.com/apache/kafka/blob/trunk/clients/src/main/java/org/apache/kafka/clients/consumer/internals/FetchCollector.java#L156-L158]
> will call {{nextInLineFetch.drain()}} to drain the fetched data for that
> partition if the partition is unassigned. In the common case, the buffered
> data for unassigned partition will be drained before the partition is
> assigned back again.
> However, in the rare case, in theory, the following seems possible (1)
> partition1 is assigned to client1; (2) a CompletedFetch for partition1 is
> buffered in client1; (3) partition1 is reassigned to client2 and unassigned
> to client1; (4) client2 consumes the same data buffered in step (2); (5)
> partition1 is reassigned back to client1; (6) client1 calls poll() and
> consumes the data buffered in step (2), causing duplicated data to be
> returned to the client.
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