I think he means that it is not only applied to the output of the mapper, but to output of the combiners many times as well. It is not used at the reducer.
On Thu, Sep 27, 2012 at 9:56 AM, Sigurd Spieckermann <[email protected]> wrote: > @Jake: Could you please elaborate on how exactly the combiner can be called > before the reducer gets the data? Do you mean the combiner is called at the > datanode that instantiates reducer tasks? I thought the combiner is just > called after the map task has finished and still on that datanode. > > 2012/9/26 Jake Mannix <[email protected]> > >> It should also be noted that the Combiner does not only run for the mappers >> - >> they can be used one (or more) times after mapping, and then one or more >> times before the reducer gets the results. It's not quite so simple as to >> say that >> you get combiners used only (and always) on the outputs of each map task.
