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From Renato Javier Marroquín Mogrovejo (JIRA) <j...@apache.org>
Subject [jira] [Commented] (GORA-228) java.util.ConcurrentModificationException when using MemStore for concurrent tests
Date Mon, 27 Jan 2014 16:59:38 GMT

    [ https://issues.apache.org/jira/browse/GORA-228?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13882959#comment-13882959

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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