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https://issues.apache.org/jira/browse/GORA-228?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13882959#comment-13882959
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Renato Javier Marroquín Mogrovejo commented on GORA-228:
--------------------------------------------------------
Hi [~alparslan.avci],
Thanks a lot for your patch! This is great indeed.
What you are describing for HBaseStore is right i.e. it is middleware between
the main application and datastore, and making MemStore work the same way as
others makes sense to me as well.
But when using MemStore, wouldn't it be better to have several in-memory
objects than having a single one being shared by all the others? In this case,
we'd have a dataStore object having its own ConcurrentSkipListMap, not all of
them sharing a single one. My use case was trying to use Gora inside embedded
systems, so instead of holding an object in memory for a long period of time, I
could create several objects and dispose them after used. Wdyt?
> 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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