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How is identity mapper different from chain mapper?

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Identity Mapper

Chain Mapper

This is the default mapper that is chosen when no mapper is specified in the MapReduce driver class.

It implements identity function, which directly writes all its key-value pairs into output.

It is defined in old MapReduce API (MR1) in: org.apache.Hadoop.mapred.lib.package

This class is used to run multiple mappers in a single map task.

The output of the first mapper becomes the input to the second mapper, second to third and so on.

It is defined in: org.apache.Hadoop.mapreduce.lib.chain.ChainMapperpackage

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