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A combiner reduces:

A combiner reduces:

PrepAway - Latest Free Exam Questions & Answers

A.
The number of values across different keys in the iterator supplied to a single reduce method
call.

B.
The amount of intermediate data that must be transferred between the mapper and reducer.

C.
The number of input files a mapper must process.

D.
The number of output files a reducer must produce.

Explanation:
Combiners are used to increase the efficiency of a MapReduce program. They are
used to aggregate intermediate map output locally on individual mapper outputs. Combiners can
help you reduce the amount of data that needs to be transferred across to the reducers. You can
use your reducer code as a combiner if the operation performed is commutative and associative.
The execution of combiner is not guaranteed, Hadoop may or may not execute a combiner. Also, if

required it may execute it more then 1 times. Therefore your MapReduce jobs should not depend
on the combiners execution.
Reference: 24 Interview Questions & Answers for Hadoop MapReduce developers, What are
combiners? When should I use a combiner in my MapReduce Job?

4 Comments on “A combiner reduces:

  1. Haresh says:

    Ans is B Only…Combiner performs local aggregation of values for each distinct key..not across the keys (considering word count example here).




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