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. Many real world tasks are expressible in this model, as shown in the paper.

Programs written in this functional style are automatically parallelized and executed on a large cluster of commodity machines. The run-time system takes care of the details of partitioning the input data,  scheduling the program's execution across a set of machines, handling machine failures, and managing the required inter-machine communication. This allows programmers without any experience with parallel and distributed systems to easily utilize the resources of a large distributed system.

Example: A process to count the appearances of each different word in a set of documents

void map(String name, String document):

  // name: document name

  // document: document contents

  for each word w in document:

    EmitIntermediate(w, 1);

void reduce(String word, Iterator partialCounts):

  // word: a word

  // partialCounts: a list of aggregated partial counts

  int result = 0;

  for each pc in partialCounts:

    result += ParseInt(pc);

  Emit(result);

Here, each document is split in words, and each word is counted initially with a "1" value by the Map function, using the word as the result key. The framework puts together all the pairs with the same key and feeds them to the same call to Reduce, thus this function just needs to sum all of its input values to find the total appearances of that word.

Sarang, K