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https://issues.apache.org/jira/browse/GORA-228?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13883873#comment-13883873
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Yasin Kılınç commented on GORA-228:
-----------------------------------
Hi all,
Thanks for the comments [~hsaputra], [~renato2099], [~alparslan.avci].
As [~hsaputra] said, my patch is not a solution for distributed clusters. The
best solution for this problem is using a distributed cache but I don't think
it is so neccesary right now. We can move on with static ConcurrentSkipListMap,
then maybe we can open an issue for using distributed cache on MemStore later.
> java.util.ConcurrentModificationException when using MemStore for concurrent
> tests
> ----------------------------------------------------------------------------------
>
> Key: GORA-228
> URL: https://issues.apache.org/jira/browse/GORA-228
> Project: Apache Gora
> Issue Type: Sub-task
> Components: gora-core
> Affects Versions: 0.3
> Reporter: Lewis John McGibbney
> Fix For: 0.4
>
> Attachments: GORA-228.patch
>
>
> Finally, a multithreaded test in [3] fails with the following
> {code}
> java.util.ConcurrentModificationException
> at
> java.util.TreeMap$NavigableSubMap$SubMapIterator.nextEntry(TreeMap.java:1594)
> at
> java.util.TreeMap$NavigableSubMap$SubMapKeyIterator.next(TreeMap.java:1655)
> at
> org.apache.gora.memory.store.MemStore$MemResult.nextInner(MemStore.java:81)
> at org.apache.gora.query.impl.ResultBase.next(ResultBase.java:112)
> at
> org.apache.nutch.storage.TestGoraStorage.readWrite(TestGoraStorage.java:74)
> at
> org.apache.nutch.storage.TestGoraStorage.access$100(TestGoraStorage.java:41)
> at
> org.apache.nutch.storage.TestGoraStorage$1.call(TestGoraStorage.java:107)
> at
> org.apache.nutch.storage.TestGoraStorage$1.call(TestGoraStorage.java:102)
> at java.util.concurrent.FutureTask$Sync.innerRun(FutureTask.java:334)
> at java.util.concurrent.FutureTask.run(FutureTask.java:166)
> at
> java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1110)
> at
> java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:603)
> at java.lang.Thread.run(Thread.java:722)
> {code}
> I believe that the final failure is due to to the use of TreeMap [5] as a
> private object in MemStore. TreeMap implementations are not synchronized. If
> multiple threads access a map concurrently, and at least one of the threads
> modifies the map structurally, it must be synchronized externally. (A
> structural modification is any operation that adds or deletes one or more
> mappings; merely changing the value associated with an existing key is not a
> structural modification.) This is typically accomplished by synchronizing on
> some object that naturally encapsulates the map. If no such object exists,
> the map should be "wrapped" using the Collections.synchronizedSortedMap
> method. This is best done at creation time, to prevent accidental
> unsynchronized access to the map e.g.
> SortedMap m = Collections.synchronizedSortedMap(new TreeMap(...));
> N.B. The NOTE on TreeMap's come right from the Oracle JavaDoc.
> [3]
> http://svn.apache.org/viewvc/nutch/branches/2.x/src/test/org/apache/nutch/storage/TestGoraStorage.java?view=markup
> [4]
> http://svn.apache.org/viewvc/nutch/branches/2.x/src/test/org/apache/nutch/util/AbstractNutchTest.java?view=markup
> [5] http://docs.oracle.com/javase/6/docs/api/java/util/TreeMap.html
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