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What is a Combiner?

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The Combiner is a ‘mini-reduce’ process which operates only on data generated by a mapper. The Combiner will receive as input all data emitted by the Mapper instances on a given node. The output from the Combiner is then sent to the Reducers, instead of the output from the Mappers

Consider case scenario: In M/R system, - HDFS block size is 64 MB

- Input format is FileInputFormat

– We have 3 files of size 64K, 65Mb and 127Mb

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