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From "Matthias J. Sax (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (KAFKA-4366) KafkaStreams.close() blocks indefinitely
Date Wed, 06 Sep 2017 00:25:00 GMT

    [ https://issues.apache.org/jira/browse/KAFKA-4366?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16154625#comment-16154625
] 

Matthias J. Sax commented on KAFKA-4366:
----------------------------------------

- You will not loose messages.
- {{KafkaStreams}} wait until all {{StreamThread}}s did shut down -- if this does not happen
within timeout, {{close()}} just returns (cf. https://github.com/apache/kafka/blob/0.11.0.0/streams/src/main/java/org/apache/kafka/streams/KafkaStreams.java#L520)
- if {{close(0)}} is called, _no_ timeout is applied and {{KafkaStreams}} waits forever --
thus, timeout must be at least 1ms to be effective (an described in the JavaDocs): {{A timeout
of 0 means to wait forever.}}

> KafkaStreams.close() blocks indefinitely
> ----------------------------------------
>
>                 Key: KAFKA-4366
>                 URL: https://issues.apache.org/jira/browse/KAFKA-4366
>             Project: Kafka
>          Issue Type: Bug
>          Components: streams
>    Affects Versions: 0.10.0.1, 0.10.1.0
>            Reporter: Michal Borowiecki
>            Assignee: Damian Guy
>             Fix For: 0.10.2.0
>
>         Attachments: threadDump.log
>
>
> KafkaStreams.close() method calls join on all its threads without a timeout, meaning
indefinitely, which makes it prone to deadlocks and unfit to be used in shutdown hooks.
> (KafkaStreams::close is used in numerous examples by confluent: https://github.com/confluentinc/examples/tree/kafka-0.10.0.1-cp-3.0.1/kafka-streams/src/main/java/io/confluent/examples/streams
and https://www.confluent.io/blog/introducing-kafka-streams-stream-processing-made-simple/
so we assumed it to be recommended practice)
> A deadlock happens, for instance, if System.exit() is called from within the uncaughtExceptionHandler.
(We need to call System.exit() from the uncaughtExceptionHandler because KAFKA-4355 issue
shuts down the StreamThread and to recover we want the process to exit, as our infrastructure
will then start it up again.)
> The System.exit call (from the uncaughtExceptionHandler, which runs in the StreamThread)
will execute the shutdown hook in a new thread and wait for that thread to join. If the shutdown
hook calls KafkaStreams.close, it will in turn block waiting for the StreamThread to join,
hence the deadlock.
> Runtime.addShutdownHook javadocs state:
> {quote}
> Shutdown hooks run at a delicate time in the life cycle of a virtual machine and should
therefore be coded defensively. They should, in particular, be written to be thread-safe and
to avoid deadlocks insofar as possible
> {quote}
> and
> {quote}
> Shutdown hooks should also finish their work quickly.
> {quote}
> Therefore the current implementation of KafkaStreams.close() which waits forever for
threads to join is completely unsuitable for use in a shutdown hook. 



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