A picture of Hadoop MapReduce Processing
Monday, March 01, 2010
3:07 PM
Worker 1
    Worker 2
Worker 3
Worker 4
put      map       shuffle  reduce     get
Often, the same nodes are mappers and reducers. 
This picture is a bit deceptive, because phases begin asynchronously. 
If one mapper gets done mapping early, it will start shuffling before the others. 
Combiner can reduce data during the shuffle; reducer is called after shuffling is complete. 