Reduce phase behavior clarification

Brian Rowe rowe at muxspace.com
Mon Jul 25 18:03:16 EDT 2011


Hello,

I see the following on the Basho wiki for Riak [1]:

  The important thing to understand is that the function defining the
reduce phase may be evaluated multiple times, and the input of later
evaluations will include the input of earlier evaluations.
  For example, a reduce phase may implement the set-union function. In
that case, the first set of inputs might be [1,2,2,3], and the output
would be [1,2,3]. When the phase receives more inputs, say [3,4,5],
the function will be called with the concatentation of the two lists:
[1,2,3,4,5].

This description isn't clear to me as the behavior I'm seeing is that
the _output_ of earlier evaluations is included as input to later
evaluations. Unfortunately, the example yields the same results
irrespective of whether it is the input or the output of the first
evaluation, so it doesn't clear anything up.

Thanks,
Brian


[1] http://wiki.basho.com/MapReduce.html




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