title: when is an identity mapper/reducer used?
body: <p>I had two conceptual doubts related to mapreduce and hadoop.I know a simple one iteration map-reduce program, know what a mapper,reducer, shuffler is..
  But still want to know about the following questions</p>
  
  <p>1)when is iterative map reduce done?</p>
  
  <p>2)i know identity mapper/reducer gives same output as the fed input.
  But when do we use an identity mapper/reducer?</p>