Designing for MapReduce
Tuesday, February 23, 2010
7:35 PM
Designing for MapReduce
MapReduce comes with some constraints on how programs may be written. 
Mappers can change data and/or return results. 
Reducers just report and have no local data. 
Can index or restructure data with a map. 
Cannot control data locality: mapper may be split without your knowledge if its data gets too big.
Cannot presume that two kinds of data are co-resident on a mapper node.  
Joining two kinds of data often requires two MapReduce queries. 