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https://issues.apache.org/jira/browse/FLINK-4829?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15575423#comment-15575423
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ASF GitHub Bot commented on FLINK-4829:
---------------------------------------

Github user mxm commented on the issue:

    https://github.com/apache/flink/pull/2634
  
    That's correct. I guess it still makes sense to merge this PR.


> Accumulators are not thread safe
> --------------------------------
>
>                 Key: FLINK-4829
>                 URL: https://issues.apache.org/jira/browse/FLINK-4829
>             Project: Flink
>          Issue Type: Bug
>          Components: Local Runtime
>    Affects Versions: 1.2.0
>            Reporter: Till Rohrmann
>            Assignee: Maximilian Michels
>             Fix For: 1.2.0
>
>
> Flink's {{Accumulators}} are not thread safe. With the introduction of live 
> accumulator snapshots which are sent to the {{JobManager}}, we've introduced 
> a concurrent access to accumulators without properly guard them against 
> concurrent modifications. So if an accumulator snapshot is taken for an 
> accumulator which is at the same time modified, it can cause an 
> {{ConcurrentModificationException}} as it was reported by an user: 
> {code}
> WARN  org.apache.flink.runtime.accumulators.AccumulatorRegistry     - Failed 
> to serialize accumulators for task.
> java.util.ConcurrentModificationException
>         at java.util.TreeMap$PrivateEntryIterator.nextEntry(TreeMap.java:1211)
>         at java.util.TreeMap$EntryIterator.next(TreeMap.java:1247)
>         at java.util.TreeMap$EntryIterator.next(TreeMap.java:1242)
>         at java.util.TreeMap.writeObject(TreeMap.java:2436)
>         at sun.reflect.GeneratedMethodAccessor491.invoke(Unknown Source) 
>         at 
> sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
>         at java.lang.reflect.Method.invoke(Method.java:498)
>         at 
> java.io.ObjectStreamClass.invokeWriteObject(ObjectStreamClass.java:1028)
>         at 
> java.io.ObjectOutputStream.writeSerialData(ObjectOutputStream.java:1496)
>         at 
> java.io.ObjectOutputStream.writeOrdinaryObject(ObjectOutputStream.java:1432)
>         at 
> java.io.ObjectOutputStream.writeObject0(ObjectOutputStream.java:1178)
>         at 
> java.io.ObjectOutputStream.defaultWriteFields(ObjectOutputStream.java:1548)
>         at 
> java.io.ObjectOutputStream.writeSerialData(ObjectOutputStream.java:1509)
>         at 
> java.io.ObjectOutputStream.writeOrdinaryObject(ObjectOutputStream.java:1432)
>         at 
> java.io.ObjectOutputStream.writeObject0(ObjectOutputStream.java:1178)
>         at java.io.ObjectOutputStream.writeObject(ObjectOutputStream.java:348)
>         at java.util.HashMap.internalWriteEntries(HashMap.java:1785)
>         at java.util.HashMap.writeObject(HashMap.java:1362)
>         at sun.reflect.GeneratedMethodAccessor189.invoke(Unknown Source) 
>         at 
> sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
>         at java.lang.reflect.Method.invoke(Method.java:498)
>         at 
> java.io.ObjectStreamClass.invokeWriteObject(ObjectStreamClass.java:1028)
>         at 
> java.io.ObjectOutputStream.writeSerialData(ObjectOutputStream.java:1496)
>         at 
> java.io.ObjectOutputStream.writeOrdinaryObject(ObjectOutputStream.java:1432)
>         at 
> java.io.ObjectOutputStream.writeObject0(ObjectOutputStream.java:1178)
>         at java.io.ObjectOutputStream.writeObject(ObjectOutputStream.java:348)
>         at 
> org.apache.flink.util.InstantiationUtil.serializeObject(InstantiationUtil.java:301)
>         at 
> org.apache.flink.util.SerializedValue.<init>(SerializedValue.java:52)
>         at 
> org.apache.flink.runtime.accumulators.AccumulatorSnapshot.<init>(AccumulatorSnapshot.java:58)
>         at 
> org.apache.flink.runtime.accumulators.AccumulatorRegistry.getSnapshot(AccumulatorRegistry.java:75)
>         at 
> org.apache.flink.runtime.taskmanager.TaskManager$$anonfun$sendHeartbeatToJobManager$2.apply(TaskManager.scala:1286)
> {code}



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