Guys,
I was looking at this again recently and I still do not understand how
combiner could have different interface than Reducer! Hadoop forces a
user to implement combiner as a Reducer
http://developer.yahoo.com/hadoop/tutorial/module4.html#functionality
and
http://hadoop.apache.org/common/docs/current/api/org/apache/hadoop/mapreduce/Job.html#setCombinerClass%28java.lang.Class%29
In addition, the original paper does not mention any change of types.
What we have admittedly done wrong is to apply Reducer on individual
Mapper without checking if a reduce function is both /commutative/ and
/associative/! This can lead to problems:
http://philippeadjiman.com/blog/2010/01/14/hadoop-tutorial-series-issue-4-to-use-or-not-to-use-a-combiner/
So yes, I am all for adding Combiner (it should do the optional reducer
per mapper we do automatically now) but I do not see why we have to
change the interface!
Regards,
Vladimir
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