Hadoop Map/Reduce vs built-In Map/Reduce

Question

What are the key differences to do map/reduce work on MongoDB using Hadoop map/reduce vs built-in map/reduce of Mongo?

When do I pick which map/reduce engine? what a

Answer 1

As of MongoDB 2.4 MapReduce jobs are no longer single threaded.

Also, see the Aggregation Framework for a higher-performance, declarative way to perform aggregates and other analytical workloads in MongoDB.