Question

Say we are doing processing on a Node. The keys waiting for processing is 2,1,3.
In preprocessing, keys will be sorted to 1,2,3.
And processing sequence will be:

begin processing 1

processing 1 done

begin processing 2

processing 2 done

begin processing 3

processing 3 done

Can I emit some thing with key 2, while processing key 1? Than the thing emitted will be processed when processing key 2.

I think this have no conflict with the concept of parallel processing, because keys on same node will be processed in sequence.

Answer 1

No, because the partitioning step has already happened, so any output from your reduce steps will go to the destination folder, not back into the input folder

partitioned input => reducers => output

You could always run a second mapreduce job with an identity mapper and the same reducer.