Provide a way to broadcast an RDD (instead of just a variable made of the RDD) so that a job can access

In some cases, such as Hive's way of doing map-side join, it would be benefcial to allow client program to broadcast RDDs rather than just variables made of these RDDs. Broadcasting a variable made of RDDs requires all RDD data be collected to the driver and that the variable be shipped to the cluster after being made. It would be more performing if driver just broadcasts the RDDs and uses the corresponding data in jobs (such building hashmaps at executors).

Tez has a broadcast edge which can ship data from previous stage to the next stage, which doesn't require driver side processing.