How to use MapReduce in Hadoop?

Why do we use MapReduce? and what a开发者_如何学编程re some use cases?

The classic example is counting the occurrence of words in a very large collection of documents. You can use the map step to generate a set of word counts for every document in parallel, then use the reduce step to collate all the words found in each document to a global count.

Here is a link to the documentation page you were probably looking for: http://hadoop.apache.org/common/docs/current/mapred_tutorial.html