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https://issues.apache.org/jira/browse/HADOOP-3702?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12619541#action_12619541
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Alejandro Abdelnur commented on HADOOP-3702:
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*On #1*, the ambiguity for the existing {write(OutputStream)}} and the
{{write(DataOutput)}} introduced by the {{Writable}} interface arises when a
{{DataOutputStream}} object is passed to the {{write(...)}} method, the
compiler cannot resolve which one to use. (I've done this in one of the
previous patches, my solution was changing the existing {{write()}} to
{{writeXml()}} but it started breaking in different places in contrib as the
method is used outside of core.
*On #2*, it doesn't make much sense to generify the {{Chain*}} classes as they
are not directly exposed to the M/R developer, they are artifacts used to
enable chaining, the M/R developer doesn't code against them. If they would be
generified it wold not add any value as at compile time nothing could be
checked for them.
*On #3*, {{ChainOutputCollector}} is lightweight class, see my answer to Chris'
regarding a similar question.
> add support for chaining Maps in a single Map and after a Reduce [M*/RM*]
> -------------------------------------------------------------------------
>
> Key: HADOOP-3702
> URL: https://issues.apache.org/jira/browse/HADOOP-3702
> Project: Hadoop Core
> Issue Type: New Feature
> Components: mapred
> Environment: all
> Reporter: Alejandro Abdelnur
> Assignee: Alejandro Abdelnur
> Attachments: patch3702.txt, patch3702.txt, patch3702.txt,
> patch3702.txt, patch3702.txt, patch3702.txt, patch3702.txt, patch3702.txt,
> patch3702.txt, patch3702.txt
>
>
> On the same input, we usually need to run multiple Maps one after the other
> without no Reduce. We also have to run multiple Maps after the Reduce.
> If all pre-Reduce Maps are chained together and run as a single Map a
> significant amount of Disk I/O will be avoided.
> Similarly all post-Reduce Maps can be chained together and run in the Reduce
> phase after the Reduce.
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