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From "Owen O'Malley (JIRA)" <j...@apache.org>
Subject [jira] Commented: (HADOOP-3594) Guaranteeing that combiner is called at least once
Date Wed, 18 Jun 2008 18:52:45 GMT

    [ https://issues.apache.org/jira/browse/HADOOP-3594?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12606035#action_12606035

Owen O'Malley commented on HADOOP-3594:

The functionality must be moved into either the map or the reduce if you want it to be done
exactly once.

> Guaranteeing that combiner is called at least once
> --------------------------------------------------
>                 Key: HADOOP-3594
>                 URL: https://issues.apache.org/jira/browse/HADOOP-3594
>             Project: Hadoop Core
>          Issue Type: Bug
>            Reporter: Olga Natkovich
>             Fix For: 0.18.0
> In 18, hadoop decides how many times to call combiner on both map and reduce sides. The
possible number is between 0 and N. 
> While having multiple invocations can be useful, not invoking combiner at all can have
serious consequences for a range of functions called algebraic (http://classweb.gmu.edu/kersch/inft864/Readings/Shoshani/DataCube/DataCubeTechReport.pdf).
The main properties of such functions is that the intermediate and final computations are
different and that the first invokation transforms the data to a different form. A most common
example of this is AVERAGE function. While it is possible to workaround this issue by annotating
each tuple, it seems that it would be much easier and faster if hadoop always guaranteed at
least a single invocation.
> Not having this guarantee will break all sorts of existing combiners.

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