Answer / Akhilesh Kumar Kushwaha

Combiners are a feature of the MapReduce programming model that allows intermediate results from map tasks to be partially reduced before they are written to disk. This can help reduce the amount of data that needs to be shuffled between the map and reduce phases, thereby improving the overall performance of the job. Combiners should be used when the reduce function is commutative and associative, meaning that the order in which map output pairs are processed does not affect the final result and the operation can be performed in multiple steps without changing the outcome. For example, if you have a MapReduce job to find the sum of values for each key, using a combiner would help reduce the number of intermediate calculations before the actual reduction.

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