"If the reduce method is like addition, where summing subtotals of terms is equivalent to summing all individual terms, it’s more efficient to split the reduce step and do some of it before shuffle. In this case, a combiner method is used, which is often the same method as the reducer. This cuts down the number of records that need to be copied from node to node."
I would like understand who invokes the combiner method?
Is the Hadoop Framework automatically invokes or the programmer has to explicitly invoke in his/her program?