Details
Major Description
Combiners are often described as 'Reducers running on the Map side'.
As Reducers, Combiners are fed <K,
{V}>, where {V}is built by grouping values associated with the 'same' key.
For Reducers, the comparator used for grouping values can be set independently of that used to sort the keys (using Job.setGroupingComparatorClass).
Such a configuration is not possible for Combiners, meaning some things done in Reducers cannot be done in Combiners (such as secondary sort).
It would be handy to have a Job.setCombinerGroupingComparatorClass method that would allow the setting of the grouping comparator used when applying a Combiner.