MapReduce

출처: http://en.wikipedia.org/wiki/Mapreduce

"Map" step: The master node takes the input, divides it into smaller sub-problems, and distributes them to worker nodes. A worker node may do this again in turn, leading to a multi-level tree structure. The worker node processes the smaller problem, and passes the answer back to its master node.

"Reduce" step: The master node then collects the answers to all the sub-problems and combines them in some way to form the output – the answer to the problem it was originally trying to solve.

한장의 그림으로 요약하는 MapReduce

출처: http://www.searchworkings.org/blog/-/blogs/introduction-to-hadoop/