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How MapReduce Works

There is a single Master computer and many Worker computers.

The Master divides the input data into bite-size chunks of 64 MB and assigns the data chunks to workers. If possible, Master chooses a worker that already has the data on its hard drive in the Google File System, or is close to a computer with the data; this minimizes network traffic.

Think of the data chunks as being like a sack of beans: lots of pieces of data, all more or less alike.