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https://issues.apache.org/jira/browse/FLINK-18452?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17156587#comment-17156587
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Weike Dong commented on FLINK-18452:
------------------------------------
Yes, I agree that we can fix this bug in next major release, i.e. 1.12.0, as
currently there are no other user reports on this problem, it seems not to be
that urgent to change existing class structure.
> Flaws in RetractableTopNFunction.ComparatorWrapper#equals method prevent
> state access after restoration
> -------------------------------------------------------------------------------------------------------
>
> Key: FLINK-18452
> URL: https://issues.apache.org/jira/browse/FLINK-18452
> Project: Flink
> Issue Type: Bug
> Components: Table SQL / Runtime
> Affects Versions: 1.10.0, 1.10.1, 1.11.0
> Reporter: Weike Dong
> Assignee: Weike Dong
> Priority: Major
> Fix For: 1.12.0
>
> Attachments:
> c2ebeac8aadebad0dffa5cc255d45190594c5b2a84bda020dd30bf24b9169702.png
>
>
> We found that in SQL jobs using "Top-N" functionality provided by the blink
> planner, the job state cannot be retrieved because of "incompatible" state
> serializers (in fact they are compatible).
> The error log is displayed like below
> {panel:title=taskmanager.log}
> 2020-06-30 09:19:32.089 [Rank(strategy=[RetractStrategy],
> rankType=[ROW_NUMBER], rankRange=[rankStart=1, rankEnd=100],
> partitionBy=[appkey, serverid], orderBy=[quantity DESC], select=[appkey,
> serverid, quantity]) (1/1)] INFO org.apache.flink.runtime.taskmanager.Task
> - Rank(strategy=[RetractStrategy], rankType=[ROW_NUMBER],
> rankRange=[rankStart=1, rankEnd=100], partitionBy=[appkey, serverid],
> orderBy=[quantity DESC], select=[appkey, serverid, oid, quantity]) (1/1)
> (bd4d2e4327efac57dc70e220b8de460b) switched from RUNNING to FAILED.
> java.lang.RuntimeException: Error while getting state
> at
> org.apache.flink.runtime.state.DefaultKeyedStateStore.getState(DefaultKeyedStateStore.java:62)
> at
> org.apache.flink.streaming.api.operators.StreamingRuntimeContext.getState(StreamingRuntimeContext.java:144)
> at
> org.apache.flink.table.runtime.operators.rank.RetractableTopNFunction.open(RetractableTopNFunction.java:115)
> at
> org.apache.flink.api.common.functions.util.FunctionUtils.openFunction(FunctionUtils.java:36)
> at
> org.apache.flink.streaming.api.operators.AbstractUdfStreamOperator.open(AbstractUdfStreamOperator.java:102)
> at
> org.apache.flink.streaming.api.operators.KeyedProcessOperator.open(KeyedProcessOperator.java:57)
> at
> org.apache.flink.streaming.runtime.tasks.StreamTask.initializeStateAndOpen(StreamTask.java:990)
> at
> org.apache.flink.streaming.runtime.tasks.StreamTask.lambda$beforeInvoke$0(StreamTask.java:453)
> at
> org.apache.flink.streaming.runtime.tasks.StreamTaskActionExecutor$SynchronizedStreamTaskActionExecutor.runThrowing(StreamTaskActionExecutor.java:94)
> at
> org.apache.flink.streaming.runtime.tasks.StreamTask.beforeInvoke(StreamTask.java:448)
> at
> org.apache.flink.streaming.runtime.tasks.StreamTask.invoke(StreamTask.java:460)
> at org.apache.flink.runtime.taskmanager.Task.doRun(Task.java:708)
> at org.apache.flink.runtime.taskmanager.Task.run(Task.java:533)
> at java.lang.Thread.run(Thread.java:748)
> Caused by: org.apache.flink.util.StateMigrationException: The new state
> serializer cannot be incompatible.
> at
> org.apache.flink.contrib.streaming.state.RocksDBKeyedStateBackend.updateRestoredStateMetaInfo(RocksDBKeyedStateBackend.java:543)
> at
> org.apache.flink.contrib.streaming.state.RocksDBKeyedStateBackend.tryRegisterKvStateInformation(RocksDBKeyedStateBackend.java:491)
> at
> org.apache.flink.contrib.streaming.state.RocksDBKeyedStateBackend.createInternalState(RocksDBKeyedStateBackend.java:652)
> at
> org.apache.flink.runtime.state.KeyedStateFactory.createInternalState(KeyedStateFactory.java:47)
> at
> org.apache.flink.runtime.state.ttl.TtlStateFactory.createStateAndWrapWithTtlIfEnabled(TtlStateFactory.java:72)
> at
> org.apache.flink.runtime.state.AbstractKeyedStateBackend.getOrCreateKeyedState(AbstractKeyedStateBackend.java:279)
> at
> org.apache.flink.runtime.state.AbstractKeyedStateBackend.getPartitionedState(AbstractKeyedStateBackend.java:328)
> at
> org.apache.flink.runtime.state.DefaultKeyedStateStore.getPartitionedState(DefaultKeyedStateStore.java:124)
> at
> org.apache.flink.runtime.state.DefaultKeyedStateStore.getState(DefaultKeyedStateStore.java:60)
> ... 13 more{panel}
>
> After careful debugging, it is found to be an issue with the compatibility
> check of type serializers.
>
> In short, during checkpointing, Flink serializes _SortedMapSerializer_ by
> creating a _SortedMapSerializerSnapshot_ object, and the original comparator
> is encapsulated within the object (here we call it
> _StreamExecSortComparator$579_).
>
> At restoration, the object is read and restored as normal. However, during
> the construction of RetractableTopNFunction instance, another Comparator is
> provided by Flink as an argument (we call it _StreamExecSortComparator$626_),
> and it is later used in the _ValueStateDescriptor_ which acts like a key to
> the state store.
>
> Here comes the problem: when the newly-restored Flink program tries to access
> state (_getState_) through the previously mentioned _ValueStateDescriptor_,
> the State Backend firstly detects whether the provided comparator in state
> descriptor is compatible with the one in snapshot, eventually the logic goes
> to the _equals_ method at _RetractableTopNFunction.ComparatorWrapper_ class.
>
> In the equals method, here is a code snippet:
> {code:java}
> return
> generatedRecordComparator.getClassName().equals(oGeneratedComparator.getClassName())
> &&
>
> generatedRecordComparator.getCode().equals(oGeneratedComparator.getCode()) &&
> Arrays.equals(generatedRecordComparator.getReferences(),
> oGeneratedComparator.getReferences());
> {code}
> After debugging, we found that the class name of comparator within snapshot
> is _StreamExecSortComparator$579_, and the class name of comparator provided
> in the new job is _StreamExecSortComparator$626_, hence this method always
> returns false, even though actually they are indeed compatible (acts the
> same). Also, because the code in each generator is generated independently,
> the corresponding varaibles within the two comparators are highly likely to
> be different (_isNullA$581_ vs _isNullA$682_).
>
> Hence we believe that the implementation of equals method has serious flaws,
> and should be addressed in later releases.
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