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https://issues.apache.org/jira/browse/KAFKA-12503?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
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A. Sophie Blee-Goldman resolved KAFKA-12503.
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Resolution: Fixed
> Resizing the thread cache in a non thread safe way can cause records to be
> redirected throughout the topology
> -------------------------------------------------------------------------------------------------------------
>
> Key: KAFKA-12503
> URL: https://issues.apache.org/jira/browse/KAFKA-12503
> Project: Kafka
> Issue Type: Bug
> Components: streams
> Affects Versions: 2.8.0
> Reporter: Walker Carlson
> Assignee: Walker Carlson
> Priority: Blocker
> Fix For: 2.8.0
>
>
> When a thread is added, removed or replaced the cache is resized. When the
> thread cache was resized it was being done so from the thread initiating
> these calls. This can cause the record to be redirected to the wrong
> processor via the call to `evict` in the cache. The evict flushes records
> downstream to the next processor after the cache. But if this is on the wrong
> thread the wrong processor receives them.
> This can cause 3 problems.
> 1) When the owner finishes processing the record it set the current node to
> null in the processor context a this then causes the other processor to throw
> an exception `StreamsException: Current node is unknown.`.
> 2) Depending on the type it can cause a class cast exception as the record is
> a different type. Mostly this happened when the value types were different
> inside of the map node from the toStream method
> 3) A silent issue is it could cause data to be processed by the wrong node
> and cause data corruption. We have not been able to confirm this last one but
> it is the most dangerous in many ways.
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