Posted On: Mar 18, 2020
A Combiner or a semi-reducer is just an optional class in the MapReduce. It mainly summarizes the map output records with the same key. The combiner accepts the input from the map class. The output of the combiner will be sent over the network to the reducer task as the input. So the combiner works between the Map and Reduce class to reduce the amount of data transfer between them. For the combiner to work properly, the key-value type should be the same between the mapper and the reducer class. Also, the combiner doesn’t have a predefined interface as it implements the reduce() method from the interface of the reducer.
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