Map Reduce has 2 phases
- Input to each functions are key value pairs
- Map is a Mapper function and Reduce is the Reducer function
Mapper phase
- First phase in the execution of map-reduce program.
- Data in each split is passed to a mapping function to produce output values.
- Several Map tasks are executed
Reduce phase
- Consumes output of mapping phase
- It consolidates the relevant record from match phase output.
- Several Reduce tasks are executed
Timeline of a map reduce job
Reduce phase may start before the end of map phase.
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