Combiner:-

•To optimize the n/w & band width limitations combiner concept will be used in map reduce programming.

•Combiner will act as local reducer [or] mini reducer as whatever the data consumed by reducer phase the same copy will reside in the combiner hadoop d x not provide any guarantee on combiner’s execution.

•Hadoop may will combiner function zero one ore many times for a particular map output record.

Note :-

•While calling reduce method, hadoop does not provide any guarantee on values in stored order corresponding to a particular key to achieve this we used the secondary sorting.

•The combiner function does not replace the reduce function.
•combiner is specify the combiner function.