The MapReduce framework operates exclusively on

The key and value classes have to implement:

• the Writable interface (to be serializable)
• the WritableComparable interface (to facilitate sorting)

See also MapReduce - InputFormat

Input and Output types of a MapReduce job forms a pipeline:

(input) <k1, v1> -> map -> <k2, v2> -> combine -> <k2, v2> -> reduce -> <k3, v3> (output)

where:

• MapReduce - Map (Mapper)
• MapReduce - Shuffling (Combine)
• MapReduce - Reducer