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From Billie J Rinaldi <billie.j.rina...@ugov.gov>
Subject Re: upgrade from 1.3 to 1.4
Date Tue, 03 Jan 2012 17:53:32 GMT
On Tuesday, January 3, 2012 11:56:50 AM, "Aaron Cordova" <aaron@cordovas.org> wrote:
> BTW I love the label 'Combiner' instead of aggregator. I've been
> describing their capabilities as equivalent to MapReduce combiners to
> new Accumulo users, so this makes it even more clear.
> 
> Are they easier to code up too? It'd be sweet if one could basically
> just define a function such as
> 
> public void combine(Key k, Iterator<Value> v)
> 
> or some such simple signature

That's basically it.  Here's the method you implement:
public Value reduce(Key k, Iterator<Value> v);

We've also done something exciting with the TypedValueCombiner<V>.  It uses an Encoder<V>
which has decode and encode methods to translate from a Value's byte array to type V and back.
 Then you implement this method to do the combining:
public V typedReduce(Key k, Iterator<V> v);

We provide a LongCombiner which is a TypedValueCombiner<Long> with three built-in encoding
types: STRING, VARLEN, and FIXEDLEN.  Separating the encoding from the combining has made
writing new combiners really easy.  See the MaxCombiner and MinCombiner in addition to the
SummingCombiner.

Billie

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