Brent,

I see your point but this change would require quite a big retooling of MapReduce API and we can not do that in 5.2 minor release. Lets leave that for 6.0.

For 5.2 I want to add method MapReduceTask combinedWith(Reducer<KOut,VOut> combiner). That way would properly deal with commutative and associative reducer and eliminate potential problems we currently do not address: http://philippeadjiman.com/blog/2010/01/14/hadoop-tutorial-series-issue-4-to-use-or-not-to-use-a-combiner/

Regards,
Vladimir

On 12-01-31 2:46 PM, Vladimir Blagojevic wrote:
Response from Brent Douglas:

Hi Vladimir,

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:

http://pastebin.com/7GGjVnVt

I would prefer to specify it as:

http://pastebin.com/HTSq3g66

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?

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?

Brent


On 12-01-30 11:47 AM, Vladimir Blagojevic wrote:
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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