Hadoop MapReduce is a data processing framework that can be utilized to process massive amounts of data stored in HDFS. As we mentioned earlier, distributed processing of a massive amount of data in a reliable and efficient manner is not an easy task. Hadoop MapReduce aims to make it easy for users by providing a clean abstraction for programmers by providing automatic parallelization of the programs and by providing framework managed fault tolerance support.
Hadoop MapReduce groups the output key-value records of all the Map tasks of a computation by the key and distributes them to the Reduce tasks. This distribution and transmission of data to the Reduce tasks is called the Shuffle phase of the MapReduce computation. Input data to each Reduce task would also be sorted and grouped by the key. The Reduce function gets invoked for each key and the group of values of that key (reduce
<key, list_of_values>) in the sorted order of the keys. In a typical MapReduce program, users only have to implement the Map and Reduce functions and Hadoop takes care of scheduling and executing them in parallel. Hadoop will rerun any failed tasks and also provide measures to mitigate any unbalanced computations.
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