[jira] [Commented] (KAFKA-3271) Notification upon unclean leader election

["Monal Daxini (JIRA)" <ji...@apache.org>]

    [ https://issues.apache.org/jira/browse/KAFKA-3271?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15159732#comment-15159732 ] 

Monal Daxini commented on KAFKA-3271:
-------------------------------------

If there is an api to query a broker (leader of a partition) what the offset is when it became a leader is good enough, even if the consumer is not notified. When an invalid offset exception is encountered a client can make a leader offset metadata request for the specific topic and partition.

> Notification upon unclean leader election
> -----------------------------------------
>
>                 Key: KAFKA-3271
>                 URL: https://issues.apache.org/jira/browse/KAFKA-3271
>             Project: Kafka
>          Issue Type: New Feature
>          Components: clients, core
>            Reporter: Yasuhiro Matsuda
>            Priority: Minor
>
> It is a legitimate restriction that unclean leader election results in some message loss. That said, it is always good to try to minimize the message loss. A notification of unclean leader election can reduce message loss in the following scenario.
> 1. The latest offset is L.
> 2. A consumer is at C, where C < L
> 3. A slow broker (not in ISR) is at S, where S < C
> 4. All brokers in ISR die.
> 5. The slow broker becomes a leader by unclean leader election.
> 6. Now the offset of S.
> 7. The new messages get offsets S, S+1, S+2, and so on.
> Currently the consumer won't receive new messages of offsets between S and C. However, if the consumer is notified when unclean leader election happened and resets its offset to S, it can receive new messages between S and C.



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