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From "Alejandro Abdelnur (JIRA)" <j...@apache.org>
Subject [jira] Commented: (HADOOP-3702) add support for chaining Maps in a single Map and after a Reduce [M*/RM*]
Date Fri, 05 Sep 2008 12:11:44 GMT

    [ https://issues.apache.org/jira/browse/HADOOP-3702?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12628622#action_12628622
] 

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

With *option 3* the _//should fail_ line fails.

Your comment _However the output classes can indeed be more specific than declared. An example
of this is a mapper which does not change the input key, and directly outputs it._ is correct,
nasty but correct.

This bring then another question, currently when adding a mapper to the chain the {{addMapper}}
method checks that the input key/value classes match (being equal, a ==) the output key/value
classes of the previous mapper. Should this be changed to an {{isAssignable()}}? Or is it
too much?




> 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
>             Fix For: 0.19.0
>
>         Attachments: Hadoop-3702.patch, patch3702.txt, patch3702.txt, 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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