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From Chandan Biswas <cbiswas1...@gmail.com>
Subject Re: Process of CombineFn<S,T> returns <S,U>?
Date Fri, 18 Oct 2013 01:18:53 GMT
I have PTable<String,Comment>. and getting after reduce PTable<String, Book>

T--> Comment{ String comment, String author}, U--> Book{String id, String
lengthiestComment, int noOfComments}

But wanted to some aggregations in the map side based on some logic instead
of all aggregations at reduce side.
Yes in worst case, data flow over the n/w will remain same, but sorting
will be improved.

Thanks,
Chandan


On Thu, Oct 17, 2013 at 6:46 PM, Josh Wills <jwills@cloudera.com> wrote:

> On Thu, Oct 17, 2013 at 4:41 PM, Chandan Biswas <cbiswas1983@gmail.com
> >wrote:
>
> > Yeah, I agree with Micah that it will not eliminate the reduce phase
> > entirely. But the dummy object of U suggested by Josh (or converting to U
> > type in map for every record)  will not improve performance because same
> > amounts of records will be sorted and aggregated in the reduce phase.
>
>
> I don't think that's true-- the records of type U will be combined on the
> map-side, which would reduce the amount of data that is pushed over the
> network and improve performance.
>
> Can you give any additional details about what T and U are in this
> scenario? :)
>
>
>
> > But
> > my point is, can we improve it by applying a combiner where the combineFn
> > provides output as different type. If we have same type, we can use the
> > combiner to do some aggregation in map side which improves performance.
> > But, can we have some mechanism by which the same advantage can be
> achieved
> > when combineFn emits different type. I think, emitting same type by
> > CombineFn has restricted its use. Can we have new CombineFn that allows
> us
> > to output different type not only same type as input?
> >
> >
> > On Thu, Oct 17, 2013 at 5:05 PM, Josh Wills <jwills@cloudera.com> wrote:
> >
> > > Yeah, my experience in these kinds of situations is that you need to
> come
> > > up with a "dummy" or singleton version of U for the case where there is
> > > only a single T and do that conversion on the map side of the job,
> before
> > > the combiner runs. I think Chao had an issue like this awhile ago,
> where
> > he
> > > had a PTable<String, Double> and wanted to write a combiner that would
> > > return a PTable<String, Collection<Double>>. The solution was to
> convert
> > > the map-side object to a PTable<String, Collection<Double>>, where
the
> > > value on the map-side was a singleton list containing just that double
> > > value. Does that sort of trick work here?
> > >
> > >
> > > On Thu, Oct 17, 2013 at 2:57 PM, Micah Whitacre <mkwhit@gmail.com>
> > wrote:
> > >
> > > > Ok so the feature you are trying to achieve is the proactive
> > combination
> > > of
> > > > data before performing the GBK like the javadoc describes.
>  Essentially
> > > in
> > > > that situation the CombineFn is being used as a Combiner[1] to
> combine
> > > the
> > > > data local to that mapper before doing the GBK and then further
> > combining
> > > > the data in the reduce operation.  It will not necessarily eliminate
> > the
> > > > need for all processing in the reduce.
> > > >
> > > > If you want to use this functionality you will need to do the
> > following:
> > > >
> > > > PTable<S, T> map to PTable<S, U>
> > > > PTable<S, U> gbk to PGT<S, U>
> > > > PGT<S, U> combine PTable<S, U>
> > > >
> > > > This will take advantage of any optimization provided by the
> CombineFn.
> > > >
> > > > [1] - http://wiki.apache.org/hadoop/HadoopMapReduce
> > > >
> > > >
> > > >
> > > > On Thu, Oct 17, 2013 at 4:30 PM, Chandan Biswas <
> cbiswas1983@gmail.com
> > > > >wrote:
> > > >
> > > > > Hello Micah,
> > > > > Yes we are using MapFn now. That aggregation and computation is
> being
> > > > done
> > > > > in reduce phase. As CombineFn after GBK runs into map side, then
> > those
> > > > most
> > > > > computations can be done in map side which are now running in
> reduce
> > > > phase.
> > > > > Some smaller aggregations and computations can be done on reduce
> > phase.
> > > > > My point was to do some aggregation (and create a new object) in
> map
> > > > phase
> > > > > instead of in reduce phase.
> > > > >
> > > > > Thanks,
> > > > > Chandan
> > > > >
> > > > >
> > > > > On Thu, Oct 17, 2013 at 3:48 PM, Micah Whitacre <mkwhit@gmail.com>
> > > > wrote:
> > > > >
> > > > > > Chandan,
> > > > > >    I think what you are wanting will just be a simple MapFn
> instead
> > > of
> > > > a
> > > > > > CombineFn.  The doc of the CombineFn[1] sounds like what you
want
> > > with
> > > > > the
> > > > > > statement "A special
> > > > > > DoFn<
> > > > http://crunch.apache.org/apidocs/0.7.0/org/apache/crunch/DoFn.html>
> > > > > > implementation
> > > > > > that converts an
> > > > > > Iterable<
> > > > > >
> > > > >
> > > >
> > >
> >
> http://download.oracle.com/javase/6/docs/api/java/lang/Iterable.html?is-external=true
> > > > > > >
> > > > > > of
> > > > > > values into a single value" but it is expecting the value to
be
> of
> > > the
> > > > > same
> > > > > > time.  Since you are wanting to combine the values into a
> different
> > > > form
> > > > > it
> > > > > > should be fairly trivial to write a MapFn that converts the
> > > Iterable<T>
> > > > > ->
> > > > > > U.
> > > > > >
> > > > > > [1] -
> > > > > >
> > > >
> > http://crunch.apache.org/apidocs/0.7.0/org/apache/crunch/CombineFn.html
> > > > > >
> > > > > >
> > > > > > On Thu, Oct 17, 2013 at 3:30 PM, Chandan Biswas <
> > > cbiswas1983@gmail.com
> > > > > > >wrote:
> > > > > >
> > > > > > > I was trying to refactoring some stuffs and trying to use
> > > combineFn.
> > > > > > > But when I went into deeper, found that I can't do it as
Crunch
> > > > doesn't
> > > > > > > allow it the functionality I needed. For example, I have
a
> > > > > > > PGroupedTable<S,T>. I wanted to apply CombineFn<S,T>
on it and
> > > wanted
> > > > > to
> > > > > > > get PCollection<S,U> instead of T. Right now, CombineFn
allows
> > only
> > > > > same
> > > > > > > type as return value. The use case of this need is that
there
> > will
> > > be
> > > > > > some
> > > > > > > time saving in sorting. It's natural that when aggregating
some
> > > > objects
> > > > > > at
> > > > > > > map side can create a new different type object.
> > > > > > >
> > > > > > > Any thought on it? Am I missing any thing? If this can
be
> written
> > > in
> > > > > > > different way using existing way please let me know.
> > > > > > >
> > > > > > > Thanks
> > > > > > > Chandan
> > > > > > >
> > > > > >
> > > > >
> > > >
> > >
> > >
> > >
> > > --
> > > Director of Data Science
> > > Cloudera <http://www.cloudera.com>
> > > Twitter: @josh_wills <http://twitter.com/josh_wills>
> > >
> >
>
>
>
> --
> Director of Data Science
> Cloudera <http://www.cloudera.com>
> Twitter: @josh_wills <http://twitter.com/josh_wills>
>

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