Reverse Sorting Reducer Keys
hadoop, java, mapreduce
Solution
In Hadoop 1.X, you can specify a custom comparator class for your outputs using `JobConf.setOutputKeyComparatorClass`.
Your comparator must implement the `RawComparator` interface.
With Hadoop 2.X, this is done by using `Job.setSortComparatorClass`, still with an implementation of `RawComparator`.
Problem
What is the best approach to get the Map Output keys to a reducer in reverse order? By default the reducer receives all keys in ascending order of keys. Any help or comments widely appreciated. In simple words, in the normal scenario, if a map emits keys 1,4,3,5,2 the reducer receives the same as 1,2,3,4,5. I would like the reducer to receive 5,4,3,2,1 instead.