Big Data Web Scale

MapReduce is a programming model and an associated implementation for processing and generating large data sets. Users specify a map function that processes a key/value pair to generate a set of intermediate key/value pairs, and a reduce function that merges all intermediate values associated with the same intermediate key.

(Source: Jeffrey Dean and Sanjay Ghemawat. Mapreduce: Simplified data processing on large clusters. In Proc. Symposium on Operating Systems Design and Implementation (OSDI’04), 2004)