[jira] [Commented] (KAFKA-3924) Data loss due to halting when LEO is larger than leader's LEO

["ASF GitHub Bot (JIRA)" <ji...@apache.org>]

    [ https://issues.apache.org/jira/browse/KAFKA-3924?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15383035#comment-15383035 ] 

ASF GitHub Bot commented on KAFKA-3924:
---------------------------------------

GitHub user maysamyabandeh opened a pull request:

    https://github.com/apache/kafka/pull/1634

    KAFKA-3924: Replacing halt with exit upon LEO mismatch to trigger gra…

    …ceful shutdown
    
    The patch is pretty simple and the justification is explained in https://issues.apache.org/jira/browse/KAFKA-3924
    
    I could not find Andrew Olson, who seems to be the contributor of this part of the code, in github so I am not sure whom I should ask to review the patch.
    
     the contribution is my original work and that i license the work to the project under the project's open source license.

You can merge this pull request into a Git repository by running:

    $ git pull https://github.com/maysamyabandeh/kafka KAFKA-3924

Alternatively you can review and apply these changes as the patch at:

    https://github.com/apache/kafka/pull/1634.patch

To close this pull request, make a commit to your master/trunk branch
with (at least) the following in the commit message:

    This closes #1634
    
----
commit cc59b8845219e21f150fa584581d066d5db1d9c6
Author: Maysam Yabandeh <my...@dropbox.com>
Date:   2016-07-18T20:41:00Z

    KAFKA-3924: Replacing halt with exit upon LEO mismatch to trigger graceful shutdown

----


> Data loss due to halting when LEO is larger than leader's LEO
> -------------------------------------------------------------
>
>                 Key: KAFKA-3924
>                 URL: https://issues.apache.org/jira/browse/KAFKA-3924
>             Project: Kafka
>          Issue Type: Bug
>          Components: core
>    Affects Versions: 0.10.0.0
>            Reporter: Maysam Yabandeh
>
> Currently the follower broker panics when its LEO is larger than its leader's LEO,  and assuming that this is an impossible state to reach, halts the process to prevent any further damage.
> {code}
>     if (leaderEndOffset < replica.logEndOffset.messageOffset) {
>       // Prior to truncating the follower's log, ensure that doing so is not disallowed by the configuration for unclean leader election.
>       // This situation could only happen if the unclean election configuration for a topic changes while a replica is down. Otherwise,
>       // we should never encounter this situation since a non-ISR leader cannot be elected if disallowed by the broker configuration.
>       if (!LogConfig.fromProps(brokerConfig.originals, AdminUtils.fetchEntityConfig(replicaMgr.zkUtils,
>         ConfigType.Topic, topicAndPartition.topic)).uncleanLeaderElectionEnable) {
>         // Log a fatal error and shutdown the broker to ensure that data loss does not unexpectedly occur.
>         fatal("...")
>         Runtime.getRuntime.halt(1)
>       }
> {code}
> Firstly this assumption is invalid and there are legitimate cases (examples below) that this case could actually occur. Secondly halt results into the broker losing its un-flushed data, and if multiple brokers halt simultaneously there is a chance that both leader and followers of a partition are among the halted brokers, which would result into permanent data loss.
> Given that this is a legit case, we suggest to replace it with a graceful shutdown to avoid propagating data loss to the entire cluster.
> Details:
> One legit case that this could actually occur is when a troubled broker shrinks its partitions right before crashing (KAFKA-3410 and KAFKA-3861). In this case the broker has lost some data but the controller cannot still elects the others as the leader. If the crashed broker comes back up, the controller elects it as the leader, and as a result all other brokers who are now following it halt since they have LEOs larger than that of shrunk topics in the restarted broker.  We actually had a case that bringing up a crashed broker simultaneously took down the entire cluster and as explained above this could result into data loss.
> The other legit case is when multiple brokers ungracefully shutdown at the same time. In this case both of the leader and the followers lose their un-flushed data but one of them has HW larger than the other. Controller elects the one who comes back up sooner as the leader and if its LEO is less than its future follower, the follower will halt (and probably lose more data). Simultaneous ungrateful shutdown could happen due to hardware issue (e.g., rack power failure), operator errors, or software issue (e.g., the case above that is further explained in KAFKA-3410 and KAFKA-3861 and causes simultaneous halts in multiple brokers)



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