The computation takes a set of input key/value pairs, and produces a set of output key/value pairs. The user of theMapReduce library expresses the computation as two functions: Map and Reduce. Map, written by the user, takes an input pair and pro- duces a set of intermediate key/value pairs. The MapRe- duce library groups together all intermediate values asso- ciated with the same intermediate key I and passes them to the Reduce function. The Reduce function, also written by the user, accepts an intermediate key I and a set of values for that key. It merges together these values to form a possibly smaller set of values. Typically just zero or one.
