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From "Maximilian Michels (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (FLINK-4829) Accumulators are not thread safe
Date Fri, 14 Oct 2016 10:09:20 GMT

    [ https://issues.apache.org/jira/browse/FLINK-4829?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15574886#comment-15574886
] 

Maximilian Michels commented on FLINK-4829:
-------------------------------------------

Actually, they used to be a synchronized map when I did the initial implementation. That prevented
concurrent modifications. In FLINK-3880 we removed the lock but forgot that the heartbeat
could actually concurrently access the map.

> 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
>             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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