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https://issues.apache.org/jira/browse/MAPREDUCE-1347?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13030545#comment-13030545
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Todd Lipcon commented on MAPREDUCE-1347:
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Maybe best to use Guava's MapMaker.makeComputingMap?
http://guava-libraries.googlecode.com/svn/trunk/javadoc/com/google/common/collect/MapMaker.html#makeComputingMap(com.google.common.base.Function)
Guava hasn't made it into trunk ivy yet, but there was a thread on the mailing
list a while back where everyone seemed to agree it was a good idea.
> Missing synchronization in MultipleOutputFormat
> -----------------------------------------------
>
> Key: MAPREDUCE-1347
> URL: https://issues.apache.org/jira/browse/MAPREDUCE-1347
> Project: Hadoop Map/Reduce
> Issue Type: Bug
> Affects Versions: 0.20.2, 0.21.0, 0.22.0
> Reporter: Todd Lipcon
> Assignee: Harsh J Chouraria
> Attachments: mapreduce.1347.r1.diff
>
>
> MultipleOutputFormat's RecordWriter implementation doesn't use
> synchronization when accessing the recordWriters member. When using
> multithreaded mappers or reducers, this can result in problems where two
> threads will both try to create the same file, causing
> AlreadyBeingCreatedException. Doing this more fine-grained than just
> synchronizing the whole method is probably a good idea, so that multithreaded
> mappers can actually achieve parallelism writing into separate output streams.
> From what I can tell, the new API's MultipleOutputs seems not to have this
> issue.
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