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From Shevek <had...@anarres.org>
Subject Re: max value for a dataset
Date Mon, 20 Apr 2009 11:26:11 GMT
On Sat, 2009-04-18 at 09:57 -0700, jason hadoop wrote:
> The traditional approach would be a Mapper class that maintained a member
> variable that you kept the max value record, and in the close method of your
> mapper you output a single record containing that value.

Perhaps you can forgive the question from a heathen, but why is this
first mapper not also a reducer? It seems to me that it is performing a
reduce operation, and that maps should (philosophically speaking) not
maintain data from one input to the next, since the order (and location)
of inputs is not well defined. The program to compute a maximum should
then be a tree of reduction operations, with no maps at all.

Of course in this instance, what you propose works, but it does seem
puzzling. Perhaps the answer is simple architectural limitation?


> The map method of course compares the current record against the max and
> stores current in max when current is larger than max.
> Then each map output is a single record and the reduce behaves very
> similarly, in that the close method outputs the final max record. A single
> reduce would be the simplest.
> On your question a Mapper and Reducer defines 3 entry points, configure,
> called once on on task start, the map/reduce called once for each record,
> and close, called once after the last call to map/reduce.
> at least through 0.19, the close is not provided with the output collector
> or the reporter, so you need to save them in the map/reduce method.
> On Sat, Apr 18, 2009 at 9:28 AM, Farhan Husain <russoue@gmail.com> wrote:
> > How do you identify that map task is ending within the map method? Is it
> > possible to know which is the last call to map method?
> >
> > On Sat, Apr 18, 2009 at 10:59 AM, Edward Capriolo <edlinuxguru@gmail.com
> > >wrote:
> >
> > > I jumped into Hadoop at the 'deep end'. I know pig, hive, and hbase
> > > support the ability to max(). I am writing my own max() over a simple
> > > one column dataset.
> > >
> > > The best solution I came up with was using MapRunner. With maprunner I
> > > can store the highest value in a private member variable. I can read
> > > through the entire data set and only have to emit one value per mapper
> > > upon completion of the map data. Then I can specify one reducer and
> > > carry out the same operation.
> > >
> > > Does anyone have a better tactic. I thought a counter could do this
> > > but are they atomic?
> > >
> >

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