Map/Reduce-side Aggregator

Aggregator is a set of functions used to aggregate distributed data sets:

createCombiner: V => C
mergeValue: (C, V) => C
mergeCombiners: (C, C) => C
Aggregator is created in combineByKeyWithClassTag transformations to create ShuffledRDDs and is eventually passed on to ShuffleDependency. It is also used in ExternalSorter.

updateMetrics Internal Method

FIXME

combineValuesByKey Method

FIXME

combineCombinersByKey Method

FIXME