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https://issues.apache.org/jira/browse/KAFKA-3924?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15420049#comment-15420049
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Maysam Yabandeh commented on KAFKA-3924:
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Thanks [~aozeritsky]. That makes sense.
So to summarize the shutdown hook would wait for all request handlers to
terminate and we have a particular request handler that cannot finish until the
fetcher thread (which has invoked the shutdown hook) releases a lock.
How about we have the fetcher thread throwing an exit exception, catch it
outside the synchronized block (which would have all the locks released) and
then call System.exit(1) in the catch clause? This seems to be a good pattern
for invoking System.exit(1) in the rest of the source code as well.
> 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
> Fix For: 0.10.0.1
>
> Attachments: deadlock-stack
>
>
> 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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