Why do we use MapReduce? and what are some use cases?
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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