I wrote a custom key class (implements WritableComparable) and implemented
the compareTo() method inside this class. Everything works fine when I run
the m/r job with 1 reduce task (via setNumReduceTasks). Keys are sorted
correctly in the output files.
But when I increase the number of reduce tasks, keys don't get aggregated
properly; same keys seem to end up in separate output files
(output/part-00000, output/part-00001, etc). This should not happen because
right before reduce() gets called, all (k,v) pairs from all map outputs with
the same 'k' are aggregated and the reduce function just iterates over the
values (v1, v2, etc)?
Do I need to implement anything else inside my custom key class other than
compareTo? I also tried implementing equals() but that didn't help either.
Then I came across setOutputKeyComparator(). So I added a custom Comparator
class inside the key class and tried setting this on the JobConf object. But
that didn't work either. What could be wrong?
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