MapReduce (cont)
MapReduce makes use of (key,value) pairs
- key values identify parts of computation
Map(key1,val1) → list(key2,val2)
- applied in parallel to all (key1,val1) pairs
- results with common key2 are collected in group for "reduction"
Reduce(key2,list(val2)) → val3
- collects all values tagged with key2
- combines them to produce result(s) val3
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