Suppose you have a big text file that contains order_ID, employee_name, and sale_amount, separated by tabs. You goal is to calculate sum of all sales by employees.

0 Alice 3625

1 Bob 5174

2 Alice 893

3 Alice 2139

4 Diana 3581

5 Carlos 1039

6 Bob 4823

7 Alice 5834

8 Carlos 392

9 Diana 1804

...

Describe how Hadoop MapReduce carries out such a task, including what steps are involved, their input/out, when do data reading, writing, transferring occur, and when does parallel processing occur.