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Response from Brent Douglas:<br>
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Hi Vladimir,
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<div>I'm not sure this is the same thing being discussed however if
it is not I had intended to request this anyhow. When I looked
into using infinispans's map reduce facility this is the the task
I came up with:</div>
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<div><a href="http://pastebin.com/7GGjVnVt">http://pastebin.com/7GGjVnVt</a></div>
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<div>I would prefer to specify it as:</div>
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<div><a href="http://pastebin.com/HTSq3g66">http://pastebin.com/HTSq3g66</a></div>
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<div>I'm pretty sure this is the not intended use case but it
distributes the creation of my reports which is what I want. It's
not really a big deal for me as I can get around this limitation
by creating a wrapper class such as in the first example but it
would be nice if I did not have to. Is this a reasonable request?</div>
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<div>Also, and this is probably just wrong, when I use this I bundle
up the task and execute it via JMS. Would it be reasonable to make
Collator extend Serializable?</div>
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<div>Brent</div>
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On 12-01-30 11:47 AM, Vladimir Blagojevic wrote:
<blockquote cite="mid:4F26AD6E.3030206@redhat.com" type="cite">
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Guys,<br>
<br>
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 <a
moz-do-not-send="true" class="moz-txt-link-freetext"
href="http://developer.yahoo.com/hadoop/tutorial/module4.html#functionality">http://developer.yahoo.com/hadoop/tutorial/module4.html#functionality</a>
and <a moz-do-not-send="true" class="moz-txt-link-freetext"
href="http://hadoop.apache.org/common/docs/current/api/org/apache/hadoop/mapreduce/Job.html#setCombinerClass%28java.lang.Class%29">http://hadoop.apache.org/common/docs/current/api/org/apache/hadoop/mapreduce/Job.html#setCombinerClass%28java.lang.Class%29</a>
In addition, the original paper does not mention any change of
types.<br>
<br>
What we have admittedly done wrong is to apply Reducer on
individual Mapper
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without checking if a reduce function is both <i>commutative</i>
and <i>associative</i>! This can lead to problems:
<a moz-do-not-send="true" class="moz-txt-link-freetext"
href="http://philippeadjiman.com/blog/2010/01/14/hadoop-tutorial-series-issue-4-to-use-or-not-to-use-a-combiner/">http://philippeadjiman.com/blog/2010/01/14/hadoop-tutorial-series-issue-4-to-use-or-not-to-use-a-combiner/</a><br>
<br>
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!<br>
<br>
<br>
Regards,<br>
Vladimir<br>
<br>
<br>
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