Posted On: Mar 02, 2020
The Identity mapper is one of the pre-defined mapper class that can be used with any key/value pairs of data. It is a generic class and also the default mapper class provided by the Hadoop. When no mapper class is specified in the MR Driver class, the Identity Mapper class is invoked automatically when a Map-Reduce job is assigned.
The ChainMapper is also one of the pre-defined mapper class that allows using multiple mapper class within a single Map task. All the mappers are run in a chain fashioned, that is the output of the first mapper becomes the input of the second mapper and so on. The output of the last mapper class is written to the intermediate files.
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