Friday, January 26, 2018

Phases in Map Reduce

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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