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https://issues.apache.org/jira/browse/KAFKA-19485?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
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Shivsundar R resolved KAFKA-19485.
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    Fix Version/s: 4.2.0
       Resolution: Fixed

> Acknowledgements should not be sent on initial epoch of ShareFetch
> ------------------------------------------------------------------
>
>                 Key: KAFKA-19485
>                 URL: https://issues.apache.org/jira/browse/KAFKA-19485
>             Project: Kafka
>          Issue Type: Sub-task
>            Reporter: Shivsundar R
>            Assignee: Shivsundar R
>            Priority: Major
>             Fix For: 4.2.0
>
>
> There are some race scenarios in multi-broker environments where 
> acknowledgements are sent on an initial epoch in a ShareSession.
> This could arise if
>  # Consumer subscribed to a partition whose leader was node-0.
>  # Broker restart happens and node-0 is elected leader again. Broker starts a 
> new ShareSession.
>  # Background thread sends a fetch request with non-zero epoch.
>  # Broker responds with SHARE_SESSION_NOT_FOUND.
>  # Client updates session epoch to 0 once it receives this error. 
>  # Application thread processing the previous fetch, completes and sends acks 
> to piggyback on next fetch.
>  # Consumer is no longer *subscribed* to this partition (from heartbeat 
> response) due to rebalance/change in assignment.
>  # Next fetch will send the piggyback acknowledgements on the fetch for 
> previously subscribed partitions resulting in error from broker 
> ("Acknowledgments sent on initial epoch").
> *Fix :*  Add a check before sending out acknowledgments if we are on an 
> initial epoch and add necessary tests.



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