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https://issues.apache.org/jira/browse/KAFKA-6999?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16537967#comment-16537967
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ASF GitHub Bot commented on KAFKA-6999:
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dongjinleekr opened a new pull request #5351: KAFKA-6999: Document read-write
lock usage of caching enabled stores
URL: https://github.com/apache/kafka/pull/5351
Add description for the deadlock vulnerability of `ReadOnlyKeyValueStore`.
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> Document read-write lock usage of caching enabled stores
> --------------------------------------------------------
>
> Key: KAFKA-6999
> URL: https://issues.apache.org/jira/browse/KAFKA-6999
> Project: Kafka
> Issue Type: Improvement
> Components: streams
> Reporter: Matthias J. Sax
> Priority: Minor
>
> From the mailing list
> {quote}Hello again fellow Kafkans,
>
> Yesterday we observed a production deadlock take down one of our instances.
> Upon digging, it's clear that our usage of Kafka is the proximate cause, but
> the danger of our approach is not clear at all just from the Javadocs.
>
> We have stream processors that read off an incoming KStream, possibly
> cross-reference some data from an auxiliary table via
> ReadOnlyKeyValueStore.get()
>
> This is done via custom logic rather than a direct KTable join because which
> index is consulted may change depending on the shape of incoming data.
>
> The ROKVS docs state,
> * A key value store that only supports read operations.
> * Implementations should be thread-safe as concurrent reads and writes
> * are expected.
>
> They do **not** indicate that the CachingKVS layer uses a ReadWriteLock. If
> you have one CachingKVS flush a record cause a read from another CKVS, you
> are suddenly vulnerable to classic lock order reversals! Surprise!
>
> A partial stack trace highlighting the problem, with many uninteresting
> frames removed, is inline at the bottom of this mail.
>
> You could probably rightly point to us allowing a "observer" pattern to
> callback from within streams processing code is dangerous. We might move this
> off to an auxiliary thread to alleviate this problem.
>
> But it still remains -- when you go an read that ROKVS documentation, it sure
> doesn't prepare you to this possibility!
> {quote}
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