Both Combiner and Reducer are optional, but most frequently used in MapReduce. There are three main differences such as: combiner will get only one input from one Mapper. While Reducer will get multiple mappers from different mappers. If aggregation required used reducer, but if the function follows commutative (a.b=b.a) and associative a.(b.c)= (a.b).c law, use combiner. Input and output keys and values types must same in combiner, but reducer can follows any type input, any output format.
