In a MapReduce job, the reducer receives all values associated with same key. Which statement
best describes the ordering of these values?
A.
The values are in sorted order.
B.
The values are arbitrarily ordered, and the ordering may vary from run to run of the same
MapReduce job.
C.
The values are arbitrary ordered, but multiple runs of the same MapReduce job will always
have the same ordering.
D.
Since the values come from mapper outputs, the reducers will receive contiguous sections of
sorted values.
Explanation:
Note:
* Input to the Reducer is the sorted output of the mappers.
* The framework calls the application’s Reduce function once for each unique key in the sorted
order.
* Example:
For the given sample input the first map emits:
< Hello, 1>
< World, 1>
< Bye, 1>
< World, 1>
The second map emits:
< Hello, 1>
< Hadoop, 1>
< Goodbye, 1>
< Hadoop, 1>