下面这段，不多评论了，简明扼要地道出了mapreduce的基本原理。
Goal: count the number of books in the library.
Map: You count up shelf #1, I count up shelf #2.
(The more people we get, the faster this part goes. )
Reduce: We all get together and add up our individual counts.
本文转自：https://www.chrisstucchio.com/blog/2011/mapreduce_explained.html