Identity Mapper and Reducer just like the concept of Identity function in mathematics i.e. do not transform the input and return it as it is in output form. Identity Mapper takes the input key/value pair and spits it out without any processing.
The case of identity reducer is a bit different. It does not mean that the reduce step will not take place. It will take place and the related sorting and shuffling will also be performed but there will be no aggregation. So you can use identity reducer if you want to sort your data that is coming from map but don't care for any grouping and also fine with multiple reducer outputs (unlike using 1 reducer.)
On Thu, Sep 5, 2013 at 9:43 AM, mallik arjun <[EMAIL PROTECTED]> wrote:
> hi all, > > please tell me what is the difference between mapper and identtiy mapper > , reducer and identity reducer. > > thanks in advance. >
What is the difference between Hdfs and DistributedFileSystem?