What is Reducer in MapReduce?

As the name suggests, the reducer reduces the set of intermediate values. The intermediate values are reduced to the smaller set of values that share a key.

The Reducer has three phases. They are the shuffle, sort, and the reduce phase.

The shuffle and sort phase occur concurrently to get the input from the mapper phase and organize it. The reduce phase occurs after the shuffle. and sort to aggregate the key-value pairs. After it reduces the data, the output is written to the FileSystem.