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Reducer 1 question


I  have seen  some of my jobs are using only  one reducer in the end.  Does it mean we have one output file?  Or it depends?


Expert Contributor



The Reducer process the output of the mapper. After processing the data, it produces a new set of output. At last HDFS stores this output data. Reducer takes a set of an intermediate key value pair produced by the mapper as the input and runs a Reducer function on each of them.

The mappers and reducers depends on the data that is processing.


You can manually set the number of reducers with below property but i think it is not recommended.

set mapred.reduce.tasks=xx;



Chethan YM

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