Simple explanation of MapReduce?

Question

Related to my CouchDB question.

Can anyone explain MapReduce in terms a numbnuts could understand?

Answer 1

1. Take a bunch of data
2. Perform some kind of transformation that converts every datum to another kind of datum
3. Combine those new data into yet simpler data

Step 2 is Map. Step 3 is Reduce.

For example,

1. Get time between two impulses on a pair of pressure meters on the road
2. Map those times into speeds based upon the distance of the meters
3. Reduce those speeds to an average speed

The reason MapReduce is split between Map and Reduce is because different parts can easily be done in parallel. (Especially if Reduce has certain mathematical properties.)

For a complex but good description of MapReduce, see: Google's MapReduce Programming Model -- Revisited (PDF).