Reducer is the second phase of processing in Hadoop which comes after Mapper phase. Reducer takes the output of the mapper as the input and processes and produces a new set of output, which will be stored in the HDFS.
Reducer first processes the intermediate values for a particular key generated by the map function and then generates the output (zero or more key-value pair).
The user decides the number of reducers. By default, the number of reducers is 1.
If you want to learn Hadoop, I recommend this Hadoop Certification program by Intellipaat.
Also, watch this YouTube tutorial on Hadoop:
Back