Mappers and reducers are independent they dont talk each other. When the functions that are commutative(a.b = b.a) and associative {a.(b.c) = (a.b).c} we goes to combiner to optimize the mapreduce process. Many mapreduce jobs are limited by the bandwidth, so by default Hadoop framework minimizes the data bandwidth network wastage. To achieve it’s goal, Mapreduce allows user defined “Cominer function” to run on the map output. It’s an MapReduce optimization technique, but it’s optional.
