MapReduce is an algorithm invented by Google for processing large data sets in parallel on multi processor or distributed systems. It's highly scalable and improves performance.
A map function performs filtering or sorting on sub-sets of the data. A reduce function aggregate the partial results.
For example, a mapreduce algorithm is run to find tallest building in each city. Two of the map functions return the following results:
London, The Shard = 309 London, One Canada Water = 235
The reduce function returns a single result:
London, The Shard = 309
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