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https://issues.apache.org/jira/browse/BEAM-2979?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16225831#comment-16225831
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Raghu Angadi commented on BEAM-2979:
------------------------------------
I think it is good to have. There is no work around for Flink users that set
watermark function.
> Race condition between KafkaIO.UnboundedKafkaReader.getWatermark() and
> KafkaIO.UnboundedKafkaReader.advance()
> -------------------------------------------------------------------------------------------------------------
>
> Key: BEAM-2979
> URL: https://issues.apache.org/jira/browse/BEAM-2979
> Project: Beam
> Issue Type: Bug
> Components: sdk-java-core
> Affects Versions: 2.0.0, 2.1.0
> Reporter: Wesley Tanaka
> Assignee: Raghu Angadi
> Priority: Blocker
> Fix For: 2.2.0
>
>
> getWatermark() looks like this:
> {noformat}
> @Override
> public Instant getWatermark() {
> if (curRecord == null) {
> LOG.debug("{}: getWatermark() : no records have been read yet.",
> name);
> return initialWatermark;
> }
> return source.spec.getWatermarkFn() != null
> ? source.spec.getWatermarkFn().apply(curRecord) : curTimestamp;
> }
> {noformat}
> advance() has code in it that looks like this:
> {noformat}
> curRecord = null; // user coders below might throw.
> // apply user deserializers.
> // TODO: write records that can't be deserialized to a
> "dead-letter" additional output.
> KafkaRecord<K, V> record = new KafkaRecord<K, V>(
> rawRecord.topic(),
> rawRecord.partition(),
> rawRecord.offset(),
> consumerSpEL.getRecordTimestamp(rawRecord),
> keyDeserializerInstance.deserialize(rawRecord.topic(),
> rawRecord.key()),
> valueDeserializerInstance.deserialize(rawRecord.topic(),
> rawRecord.value()));
> curTimestamp = (source.spec.getTimestampFn() == null)
> ? Instant.now() : source.spec.getTimestampFn().apply(record);
> curRecord = record;
> {noformat}
> There's a race condition between these two blocks of code which is exposed at
> the very least in the FlinkRunner, which calls getWatermark() periodically
> from a timer.
> The symptom of the race condition is a stack trace that looks like this (SDK
> 2.0.0):
> {noformat}
> Caused by: org.apache.flink.runtime.client.JobExecutionException: Job
> execution failed.
> at
> org.apache.flink.runtime.jobmanager.JobManager$$anonfun$handleMessage$1$$anonfun$applyOrElse$7.apply$mcV$sp(JobManager.scala:910)
> at
> org.apache.flink.runtime.jobmanager.JobManager$$anonfun$handleMessage$1$$anonfun$applyOrElse$7.apply(JobManager.scala:853)
> at
> org.apache.flink.runtime.jobmanager.JobManager$$anonfun$handleMessage$1$$anonfun$applyOrElse$7.apply(JobManager.scala:853)
> at
> scala.concurrent.impl.Future$PromiseCompletingRunnable.liftedTree1$1(Future.scala:24)
> at
> scala.concurrent.impl.Future$PromiseCompletingRunnable.run(Future.scala:24)
> at akka.dispatch.TaskInvocation.run(AbstractDispatcher.scala:40)
> at
> akka.dispatch.ForkJoinExecutorConfigurator$AkkaForkJoinTask.exec(AbstractDispatcher.scala:397)
> at scala.concurrent.forkjoin.ForkJoinTask.doExec(ForkJoinTask.java:260)
> at
> scala.concurrent.forkjoin.ForkJoinPool$WorkQueue.runTask(ForkJoinPool.java:1339)
> at
> scala.concurrent.forkjoin.ForkJoinPool.runWorker(ForkJoinPool.java:1979)
> at
> scala.concurrent.forkjoin.ForkJoinWorkerThread.run(ForkJoinWorkerThread.java:107)
> Caused by: TimerException{java.lang.NullPointerException}
> at
> org.apache.flink.streaming.runtime.tasks.SystemProcessingTimeService$TriggerTask.run(SystemProcessingTimeService.java:220)
> at
> java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:511)
> at java.util.concurrent.FutureTask.run(FutureTask.java:266)
> at
> java.util.concurrent.ScheduledThreadPoolExecutor$ScheduledFutureTask.access$201(ScheduledThreadPoolExecutor.java:180)
> at
> java.util.concurrent.ScheduledThreadPoolExecutor$ScheduledFutureTask.run(ScheduledThreadPoolExecutor.java:293)
> at
> java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1142)
> at
> java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:617)
> at java.lang.Thread.run(Thread.java:745)
> Caused by: java.lang.NullPointerException
> at org.apache.beam.sdk.io.kafka.KafkaIO$Read$1.apply(KafkaIO.java:568)
> at org.apache.beam.sdk.io.kafka.KafkaIO$Read$1.apply(KafkaIO.java:565)
> at
> org.apache.beam.sdk.io.kafka.KafkaIO$UnboundedKafkaReader.getWatermark(KafkaIO.java:1210)
> at
> org.apache.beam.runners.flink.translation.wrappers.streaming.io.UnboundedSourceWrapper.onProcessingTime(UnboundedSourceWrapper.java:431)
> at
> org.apache.flink.streaming.runtime.tasks.SystemProcessingTimeService$TriggerTask.run(SystemProcessingTimeService.java:218)
> ... 7 more
> {noformat}
> Based on inspecting the code, what is probably happening is that while
> advance() is executing:
> * Flink runner calls getWatermark()
> * getWatermark evaluates (curRecord == null) and it is false
> * advance() proceeds to set curRecord = null
> * The flink runner thread calls getWatermarkFn().apply(curRecord) which
> passes a null record into the custom watermark function
> * If that watermark function had been set with withWatermarkFn() (as
> suggested in the javadoc), then it's using the closure created in
> unwrapKafkaAndThen()
> * That calls record.getKV() and we get the NullPointerException
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