1. The application provides the input files, a map function, a reduce function and the reduce tasks (nReduce)
2. A master starts an RPC server, and waits for workers to register. Then assign tasks to workers, and handle worker failures
3. The master considers each input file to be one map task, and calls doMap() to process each map task, then produces n intermediate files.
4. The master next calls doReduce() to process reduce task. Reduce task r collects the r'th intermediate file from each map task.
5. The master merges all the n files produced by 4 into a single output.
6. The master sends a Shutdown RPC to each worker and shuts down its own RPC server.
