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https://issues.apache.org/jira/browse/KAFKA-6378?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16321374#comment-16321374
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Andy Bryant commented on KAFKA-6378:
------------------------------------
A sentinel value is ok where only a subset of the available values for a type
are valid, but it does seem messy to have to convert {{null}} values to the
sentinel before the join then back to {{null}} again in the merge function
afterwards.
Also it doesn't cater for the case where you can't pick a sentinel because all
values of a type are valid.
Since as Matthias pointed out {{null}} can never be a valid key explicitly
calling it out as indicating no match in the docs and updated the code so it
doesn't crash (a two line change by the looks) seems like a nice clean approach
to me.
> NullPointerException on KStream-GlobalKTable leftJoin when KeyValueMapper
> returns null
> --------------------------------------------------------------------------------------
>
> Key: KAFKA-6378
> URL: https://issues.apache.org/jira/browse/KAFKA-6378
> Project: Kafka
> Issue Type: Bug
> Components: streams
> Affects Versions: 1.0.0
> Reporter: Andy Bryant
>
> On a Stream->GlobalKTable leftJoin if the KeyValueMapper returns null, the
> stream fails with a NullPointerException (see stacktrace below). On Kafka
> 0.11.0.0 the stream processes this successfully, calling the ValueJoiner with
> the table value set to null.
> The use-case for this is joining a stream to a table containing reference
> data where the stream foreign key may be null. There is no straight-forward
> workaround in this case with Kafka 1.0.0 without having to resort to either
> generating a key that will never match or branching the stream for records
> that don't have the foreign key.
> Exception in thread "workshop-simple-example-client-StreamThread-1"
> java.lang.NullPointerException
> at java.base/java.util.Objects.requireNonNull(Objects.java:221)
> at
> org.apache.kafka.streams.state.internals.CachingKeyValueStore.get(CachingKeyValueStore.java:136)
> at
> org.apache.kafka.streams.state.internals.CachingKeyValueStore.get(CachingKeyValueStore.java:35)
> at
> org.apache.kafka.streams.state.internals.InnerMeteredKeyValueStore.get(InnerMeteredKeyValueStore.java:184)
> at
> org.apache.kafka.streams.state.internals.MeteredKeyValueBytesStore.get(MeteredKeyValueBytesStore.java:116)
> at
> org.apache.kafka.streams.kstream.internals.KTableSourceValueGetterSupplier$KTableSourceValueGetter.get(KTableSourceValueGetterSupplier.java:49)
> at
> org.apache.kafka.streams.kstream.internals.KStreamKTableJoinProcessor.process(KStreamKTableJoinProcessor.java:56)
> at
> org.apache.kafka.streams.processor.internals.ProcessorNode$1.run(ProcessorNode.java:46)
> at
> org.apache.kafka.streams.processor.internals.StreamsMetricsImpl.measureLatencyNs(StreamsMetricsImpl.java:208)
> at
> org.apache.kafka.streams.processor.internals.ProcessorNode.process(ProcessorNode.java:124)
> at
> org.apache.kafka.streams.processor.internals.ProcessorContextImpl.forward(ProcessorContextImpl.java:85)
> at
> org.apache.kafka.streams.processor.internals.SourceNode.process(SourceNode.java:80)
> at
> org.apache.kafka.streams.processor.internals.StreamTask.process(StreamTask.java:216)
> at
> org.apache.kafka.streams.processor.internals.AssignedTasks.process(AssignedTasks.java:403)
> at
> org.apache.kafka.streams.processor.internals.TaskManager.process(TaskManager.java:317)
> at
> org.apache.kafka.streams.processor.internals.StreamThread.processAndMaybeCommit(StreamThread.java:942)
> at
> org.apache.kafka.streams.processor.internals.StreamThread.runOnce(StreamThread.java:822)
> at
> org.apache.kafka.streams.processor.internals.StreamThread.runLoop(StreamThread.java:774)
> at
> org.apache.kafka.streams.processor.internals.StreamThread.run(StreamThread.java:744)
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