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From Jinal Shah <jinalshah2...@gmail.com>
Subject Re: Execution Control
Date Thu, 06 Feb 2014 17:00:54 GMT
Hi Josh and Micah,

In both the scenerios I can easily do a Pipeline.run() and get going from
there. But my main question would be why should I do a pipeline.run() in
between just to make the planner run something in a sequential format
rather than the way it would have planned otherwise. What I'm getting at is
that there should some mechanism that will tell the Planner to do something
in a certain way to some extend like you can take example of Apache Hive,
till 0.7 release Hive use to provide a mechanism called HINT which would
tell the query planner to run something as indicated in the HINT rather
than the way it would have been otherwise. I know that you might say it
might not create optimized plan but at this point the consumer is more
focused on the way it should be planned rather than the optimization.

May be there might be option already there in Crunch that I might have not
explored but just wanted to put my point out there. If there is an option I
would love to learn about it.


On Thu, Feb 6, 2014 at 10:44 AM, Josh Wills <josh.wills@gmail.com> wrote:

> Hey Jinal,
>
> On scenario 2, the easiest way to do this is to force a run() between the
> write and the second read, ala:
>
> HBase.read()
> doSomeChangesOnData
> HBase.write()
> Pipeline.run()
> HBase.read()
>
> If that isn't possible for some reason, you'll need to add an output file
> to the first phase that can be used to indicate that the HBase.write is
> complete, and then have the second read depend on that file existing before
> it can run, which can be done via ParallelDoOptions, e.g.,
>
> SourceTarget marker = ...;
> HBase.read()
> doSomeChangesOnData
> dummyDoFnToCreateMarker
> HBase.write()
> marker.write()
> HBase.read().parallelDo(DoFn, PType,
> ParallelDoOptions.builder().sourceTarget(marker).build());
>
> but that's obviously uglier and more complicated.
>
> J
>
>
> On Wed, Feb 5, 2014 at 7:14 PM, Jinal Shah <jinalshah2007@gmail.com>
> wrote:
>
> > Hi Josh,
> >
> > Here is a small example of what I am looking for. So here is what I'm
> doing
> >
> > Scenario 1:
> >
> > PCollection<Something> s = FunctionDoingSomething();
> > pipeline.write(s, path);
> > doSomeFilteringOn(s);
> >
> > I want that when I do some filtering this should be done in the map phase
> > instead it is doing it in the Reduce phase due to which I have to
> introduce
> > a pipeline.run() and now this is what the code looks like
> >
> > PCollection<Something> s = FunctionDoingSomething();
> > pipeline.write(s, path);
> > pipeline.run()
> > doSomeFilteringOn(s);
> >
> > Scenerio 2:
> >
> > I'm doing an operation on HBase and here is how it looks.
> >
> > Hbase.read()
> > doSomeChangesOnData
> > HBase.write()
> > HBase.read()
> >
> > Now Crunch at this points considers both the reads as separate and tries
> to
> > run it in parallel so now before I even write my changes it reads those
> > changes so I have to again put a pipeline.run() in order to break it
> into 2
> > separate flow and execute them in sequence.
> >
> > So I'm asking is there any way to send an HINT to the Planner that how it
> > create the Plan instead of it deciding by itself or someway to have more
> > control how to make a planner understand in certain situations.
> >
> > Thanks
> > Jinal
> >
> >
> > On Thu, Jan 30, 2014 at 11:10 AM, Josh Wills <jwills@cloudera.com>
> wrote:
> >
> > > On Thu, Jan 30, 2014 at 7:09 AM, Jinal Shah <jinalshah2007@gmail.com>
> > > wrote:
> > >
> > > > Hi everyone,
> > > >
> > > > This is Jinal Shah, I'm new to the group. I had a question about
> > > Execution
> > > > Control in Crunch. Is there any way we can force Crunch to do certain
> > > > operations in parallel or certain operations in sequential ways. For
> > > > example, let's say if we want the pipeline to executed a particular
> > DoFn
> > > > function in the Map phase instead of the Reduce phase or vice-versa.
> Or
> > > > Execute a particular Flow only after a particular flow is completed
> as
> > > > oppose to running it in parallel.
> > > >
> > >
> > > Forcing a DoFn to operate in a map or reduce phase is tough for the
> > planner
> > > to do right now; we sort of rely on the developer to have a mental
> model
> > of
> > > how the jobs will proceed. The place where you usually want to force a
> > DoFn
> > > to execute in the reduce vs. the map phase is when you have dependent
> > > groupByKey operations, and you can use cache() or materialize() on the
> > > intermediate output that you want to split on, and the planner will
> > respect
> > > that.
> > >
> > > On the latter question, the thing to look for is
> > > org.apache.crunch.ParallelDoOptions, which isn't something I've doc'd
> in
> > > the user guide yet (it's on the todo list, I promise.) You can give a
> > > parallelDo call an additional argument that specifies one or more
> > > SourceTargets that have to exist before a particular DoFn is allowed to
> > > run. In this way, you can force aspects of the pipeline to be
> sequential
> > > instead of parallel. We make use of ParallelDoOptions inside of the
> > > MapsideJoinStrategy code, to ensure that the data set that we'll be
> > loading
> > > in-memory actually exists in the file system before we run the code
> that
> > > reads it into memory.
> > >
> > >
> > > >
> > > > Maybe this might be asked before so sorry if it came again. If you
> guys
> > > > have further question on the details do let me know
> > > >
> > > >
> > > > Thanks everyone and Have a great day.
> > > >
> > > > Thanks
> > > > Jinal
> > > >
> > >
> > >
> > >
> > > --
> > > Director of Data Science
> > > Cloudera <http://www.cloudera.com>
> > > Twitter: @josh_wills <http://twitter.com/josh_wills>
> > >
> >
>

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