Reduce phase only on one node?

John Butler php_john at hotmail.com
Fri Jul 23 17:18:14 EDT 2010


Hello,
I went through the Riak Fast Track and overall I'm excited about the possibilities of Riak. However, I did see one thing that concerns me in regard to MapReduce jobs.  From the Riak docs and this page here: http://seancribbs.com/tech/2010/02/06/why-riak-should-power-your-next-rails-app/ it seems to suggest that while Map jobs are performed in parallel Reduce jobs are not. 
Wouldn't be better if each node performed the reduce function on it's own set of data before sending it to the main coordinator? Imagine if you are mapping over a high number of objects (millions) and are trying to aggregate the data grouped by some demographic (such as state). If all the mappers send the data to coordinating node to be reduced then you are going to send millions of items from each data node all to one coordinating node. If instead each node called reduce first, then sent it to the coordinating node, then each node would be at most sending over 50 objects (for 50 states). Not only would this greatly reduce network traffic, it would be able to execute in parallel greatly improving performance. Since all reduce jobs are communicative, associative and idempotent I see no reason why this couldn't happen (of course I'm not familiar with the Riak internals to strongly assert this).
Maybe I misunderstood and this is already happening. If it's not happening, why and is that on the roadmap to be added later?
-John 		 	   		  
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