Which statement best describes the ordering of these values?

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.

View Answer

Answer: B

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>