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https://issues.apache.org/jira/browse/MAPREDUCE-5051?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13596218#comment-13596218
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Robert Joseph Evans commented on MAPREDUCE-5051:
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Damien, The combiner only runs as part of the shuffle phase. The shuffle
phase only runs when there is a reducer that needs the data to be shuffled. So
your indexing works just fine if all of the indexes for a given key are not in
the same file?
If you want just a combiner to run with no reducers configured, you are going
to have to write something for that yourself.
> Combiner not used when NUM_REDUCES=0
> ------------------------------------
>
> Key: MAPREDUCE-5051
> URL: https://issues.apache.org/jira/browse/MAPREDUCE-5051
> Project: Hadoop Map/Reduce
> Issue Type: Bug
> Components: mrv1
> Affects Versions: 2.0.2-alpha
> Environment: CDH4.1.2 MR1
> Reporter: Damien Hardy
>
> We have a M/R job that use Mapper + Combiner but have nothing to do in
> Reducer :
> Bulk indexing of HBase data in ElasticSearch,
> Map output is K / V : #bulk / json_data_to_be_indexed.
> So job is launched maps work, combiners index and a reducer is created for
> nothing (sometimes waiting for other M/R job to free a tasktracker slot for
> reducer cf. MAPREDUCE-5019 )
> When we put ```job.setNumReduceTasks(0);``` in our job .run(), mapper are
> started but combiner are not used.
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