Not one I can think of at the moment, but intuitively that's the kind of flexibility I would like to see should a grouping comparator become configurable for Combiners.
By pushing your reasoning further, why specify a combiner class at all instead of applying the Reducer on the map side. On Oct 31, 2011 10:02 PM, "Shevek" <[email protected]> wrote: > On 31 October 2011 13:37, Mathias Herberts <[email protected] > >wrote: > > > I don't know if it's a bug but I'd rather have the ability to set a > > Combiner specific group comparator than to have the Combiner use the > group > > comparator set for the Reducer. > > On Oct 31, 2011 9:21 PM, "Harsh J" <[email protected]> wrote: > > > > Now I'm curious. Can you argue that there's a case where it makes a > difference? Preferably one where it can't be trivially curried into the > combiner? > > S. > > > > > Shevek, > > > > > > The problem Mathias indicates here is that the Combiners do not utilize > > > the Grouping Comparators. They only use the Sort Comparators. Is that > > > probably a bug is what I wonder. > > > > > > On 31-Oct-2011, at 11:14 PM, Shevek wrote: > > > > > > > I like the ability to reuse a Java component for both sorting and > > > grouping, > > > > and to be honest, since the cases where one can do a comparison > without > > > > deserializing the raw bytes are relatively few and far between, I > tend > > to > > > > use java's Comparator interface, and wrap it in some > > > > infrastructure-specific adapter. I have a vague feeling that Hadoop > > > > sometimes calls the byte interface and sometimes the object interface > > > > anyway? ICBW, the way I've been writing code makes it irrelevant. > > > > > > > > Alternatively, I've misunderstood the (simpler) question, and the > > answer > > > is > > > > to use the setGroupingComparatorClass() API. > > > > > > > > S. > > > > > > > > On 29 October 2011 04:35, Mathias Herberts < > [email protected] > > > >wrote: > > > > > > > >> Another point concerning the Combiners, > > > >> > > > >> the grouping is currently done using the RawComparator used for > > > >> sorting the Mapper's output. Wouldn't it be useful to be able to > set a > > > >> custom CombinerGroupingComparatorClass? > > > >> > > > >> Mathias. > > > >> > > > > > > > > >
