MapReduce frame operated at <key, value> key on the upper, i.e., the frame as the input job is a set of <key, value> key-value pairs, also generating a set of <key, value> key-value pairs as the output of the job, both sets of key-value pairs may be different.
A MapReduce input and output types of operations, as shown below: As can be seen throughout the standard process, there will be three groups of <key, value> key exists for the type.
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