In Hadoop, there are three primary phases of a Reducer:
1. Shuffle: In this phase, Reducer copies the sorted output from each Mapper.
2. Sort: In this phase, Hadoop framework sorts the input to Reducer by same key. It uses
merge sort in this phase. Sometimes, shuffle and sort phases occur at the same time.
3. Reduce: This is the phase in which output values associated with a key are reduced to give output result. Output from Reducer is not re-sorted.