mapreduce - hadoop: difference between 0 reducer and identity reducer?

Question

I am just trying to confirm my understanding of difference between 0 reducer and identity reducer.

• 0 reducer means reduce step will be skipped and mapper output will be the final out
• Identity reducer means then shuffling/sorting will still take place?

Answer 1

You understanding is correct. I would define it as following: If you do not need sorting of map results - you set 0 reduced,and the job is called map only.
If you need to sort the mapping results, but do not need any aggregation - you choose identity reducer.
And to complete the picture we have a third case : we do need aggregation and, in this case we need reducer.