[FLINK-10960] CEP: Job Failure when .times(2) is used

• Type: Bug
• Status: Closed
• Priority: Critical
• Resolution: Workaround
• Affects Version/s: 1.6.2
• Component/s: Library / CEP
• Release Note: It seems this issue was as David described, due to restoring into a smaller state machine than had existed before
• Assignee: Unassigned
• Reporter: Thomas Wozniakowski
• Created: 21/Nov/18 11:23
• Updated: 02/Jan/20 16:30
• Resolved: 03/Jan/19 15:34

Description

Hi Guys,

Encountered a strange one today. We use the CEP library in a configurable way where we plug a config file into the Flink Job JAR and it programmatically sets up a bunch of CEP operators matching the config file.

I encountered a strange bug when I was testing with some artificially low numbers in our testing environment today. The CEP code we're using (modified slightly) is:

Pattern.begin(EVENT_SEQUENCE, AfterMatchSkipStrategy.skipPastLastEvent())
            .times(config.getNumberOfUniqueEvents())
            .where(uniquenessCheckOnAlreadyMatchedEvents())
            .within(seconds(config.getWithinSeconds()));

When using the numberOfUniqueEvents: 2, I started seeing the following error killing the job whenever a match was detected:

ava.lang.RuntimeException: Exception occurred while processing valve output watermark:
at org.apache.flink.streaming.runtime.io.StreamInputProcessor$ForwardingValveOutputHandler.handleWatermark(StreamInputProcessor.java:265)
at org.apache.flink.streaming.runtime.streamstatus.StatusWatermarkValve.findAndOutputNewMinWatermarkAcrossAlignedChannels(StatusWatermarkValve.java:189)
at org.apache.flink.streaming.runtime.streamstatus.StatusWatermarkValve.inputWatermark(StatusWatermarkValve.java:111)
at org.apache.flink.streaming.runtime.io.StreamInputProcessor.processInput(StreamInputProcessor.java:184)
at org.apache.flink.streaming.runtime.tasks.OneInputStreamTask.run(OneInputStreamTask.java:105)
at org.apache.flink.streaming.runtime.tasks.StreamTask.invoke(StreamTask.java:300)
at org.apache.flink.runtime.taskmanager.Task.run(Task.java:711)
at java.lang.Thread.run(Thread.java:745)
Caused by: org.apache.flink.util.FlinkRuntimeException: State eventSequence:2 does not exist in the NFA. NFA has states [Final State $endState$ [
]), Normal State eventSequence [
StateTransition(TAKE, from eventSequenceto $endState$, with condition),
StateTransition(IGNORE, from eventSequenceto eventSequence, with condition),
]), Start State eventSequence:0 [
StateTransition(TAKE, from eventSequence:0to eventSequence, with condition),
])]
at org.apache.flink.cep.nfa.NFA.isStartState(NFA.java:144)
at org.apache.flink.cep.nfa.NFA.isStateTimedOut(NFA.java:270)
at org.apache.flink.cep.nfa.NFA.advanceTime(NFA.java:244)
at org.apache.flink.cep.operator.AbstractKeyedCEPPatternOperator.advanceTime(AbstractKeyedCEPPatternOperator.java:389)
at org.apache.flink.cep.operator.AbstractKeyedCEPPatternOperator.onEventTime(AbstractKeyedCEPPatternOperator.java:293)
at org.apache.flink.streaming.api.operators.InternalTimerServiceImpl.advanceWatermark(InternalTimerServiceImpl.java:251)
at org.apache.flink.streaming.api.operators.InternalTimeServiceManager.advanceWatermark(InternalTimeServiceManager.java:128)
at org.apache.flink.streaming.api.operators.AbstractStreamOperator.processWatermark(AbstractStreamOperator.java:746)
at org.apache.flink.streaming.runtime.io.StreamInputProcessor$ForwardingValveOutputHandler.handleWatermark(StreamInputProcessor.java:262)

Changing the config to numberOfUniqueEvents: 3 fixed the problem. Changing it back to 2 brought the problem back. It seems to be specifically related to the value of 2.

This is not a blocking issue for me because we typically use much higher numbers than this in production anyway, but I figured you guys might want to know about this issue.

Let me know if you need any more information.

Tom