Will you be able to reuse your existing Reduces as your combiner in this case and why or why not?

You want to count the number of occurrences for each unique word in the supplied input data. You’ve decided to implement this by having your mapper tokenize each word and emit a literal value 1, and then have your reducer increment a counter for each literal 1 it receives. After successful implementing this, it occurs to you that you could optimize this by specifying a combiner.

Will you be able to reuse your existing Reduces as your combiner in this case and why or why not?
A . Yes, because the sum operation is both associative and commutative and the input and output types to the reduce method match.
B . No, because the sum operation in the reducer is incompatible with the operation of a Combiner.
C . No, because the Reducer and Combiner are separate interfaces.
D . No, because the Combiner is incompatible with a mapper which doesn’t use the same data type for both the key and value.
E . Yes, because Java is a polymorphic object-oriented language and thus reducer code can be reused as a combiner.

View Answer

Answer: A

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?