Introduction and MapReduce - SNAP - Stanford University

Often a map task will produce many pairs of
the form (k,v 1), (k,v 2), … for the same key k
E.g., popular words in the Word Count example
Can save network time by
pre-aggregating values at
the mapper:
combine(k, list(v 1)) v 2
Combiner is usually same
as the reduce function
Works only if reduce
function is commutative and associative
1/8/2012 Jure Leskovec, Stanford CS246: Mining Massive Datasets, http://cs246.stanford.edu 50