On Mar 20, 2008, at 3:56 PM, Otis Gospodnetic wrote:
Hi,
The MapReduce tutorial mentions Combiners only in passing. Is
there a default Combiner or default combining behaviour?
No, there is *no* default combiner at all. It has to be explicitly
set in the JobConf to take effect.
Arun
Concretely, I want to make sure that records are not getting
combined behind the scenes in some way without me seeing it, and
causing me to lose data. For instance, if there is a default
Combiner or default combining behaviour that collapses multiple
records with identical keys and values into a single record, I'd
like to avoid that. Instead of blindly collapsing identical
records, I'd want to aggregate their values and emit the aggregate.
Thanks,
Otis
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