Default number of reducers

Question

In Hadoop, if we have not set number of reducers, then how many number of reducers will be created?

Like number of mappers is dependent on (total data size)

Answer 1

By default the no of reducers is set to 1.

You can change it by adding a parameter

mapred.reduce.tasks in the command line or in the Driver code or in the conf file that you pass.

e.g: Command Line Argument: bin/hadoop jar ... -Dmapred.reduce.tasks= or, in Driver code as: conf.setNumReduceTasks(int num);

Recommended read: https://wiki.apache.org/hadoop/HowManyMapsAndReduces