Data Transformation
Tuesday, February 22, 2011
11:50 AM
There are two distinct reasons for Map/Reduce
Retrieval: retrieving data that is already stored. 
Transformation: summarizing data in a new and more useful form. 
Actually, the two prevalent forms of Map/Reduce have different strengths:
The (implicit) MR in Google AppEngine is intended for data retrieval. 
The (explicit) MR in Hadoop is intended mainly for data transformation. 
(Hadoop presumes that you will eventually retrieve data by another mechanism!)