A simple framework to do map-reduce work

component:

Master: responsible for spliting job to small partitions

      dispatch each task to registed workers
      
      and redispatch task for failed workers
      
      finally, merge all the result into one file

Worker: know how to do Map and how to do Reduce

      communicate with master via RPC in both way

main: start up master and workers

key issues:

1. How workers register to master

2. How master dispatch task to worker

3. How master handles the worker failure

4. How master and worker communicate information