A combiner reduces:
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.
Answer: B
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?
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