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Date: Mon, 4 Aug 2008 08:18:46 -0700 (PDT)
From: "Alejandro Abdelnur (JIRA)" <jira@apache.org>
To: core-dev@hadoop.apache.org
Subject: [jira] Commented: (HADOOP-3702) add support for chaining Maps in a
 single Map and after a Reduce [M*/RM*]
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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 ] 

Alejandro Abdelnur commented on HADOOP-3702:
--------------------------------------------

*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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