Reduce phase behavior clarification

siculars at siculars at
Mon Jul 25 18:16:24 EDT 2011

And wouldn't the concatenation be [1,2,3,3,4,5] ? 

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-----Original Message-----
From: Bob Ippolito <bob at>
Sender: riak-users-bounces at
Date: Mon, 25 Jul 2011 15:13:36 
To: Brian Rowe<rowe at>
Cc: riak-users<riak-users at>
Subject: Re: Reduce phase behavior clarification

It sounds like there is a typo, I would assume that it should be "the
input of later evaluations will include the output of earlier
evaluations". It's not really a reduction otherwise.

On Mon, Jul 25, 2011 at 3:03 PM, Brian Rowe <rowe at> wrote:
> 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]
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