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Owen O'Malley commented on HADOOP-3586:
---------------------------------------
Sorry, my comment is out of context. Arkady had asked on the core-dev list:
{quote}
For my education, can someone tell:
was "combiner guaranteed to run once and only once for each map" a documented part of the
contract ?
{quote}
> keep combiner backward compatible with earlier versions of hadoop
> -----------------------------------------------------------------
>
> Key: HADOOP-3586
> URL: https://issues.apache.org/jira/browse/HADOOP-3586
> Project: Hadoop Core
> Issue Type: Bug
> Components: mapred
> Reporter: Olga Natkovich
> Assignee: Chris Douglas
> Priority: Blocker
> Fix For: 0.18.0
>
> Attachments: 3586-0.patch
>
>
> In hadoop 16 and earlier, the combiner was guaranteed to run once and only once for each
map. In 17 this compatibility was slightly broken: the combiner does not run if a single <K,V>
occupies the entire sort buffer. In 18, this is further changed to where the combiner can
be called multiple times on both map and reduce sides.
> This breaks Pig's current implementation of the combiner and it is not easy to fix in
a short period of time.
> We would like to ask that for a way for an application to ask for a backward compatible
behavior for some period of time until it can adjust to the new behavior.
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