Thanks Matthias. I just verified this and found why there's this confusion
about tasks.
In this case:
int BoltParallelism = 3;
int BoltTaskParallelism = 2;
builder.setBolt("bolt1", new BoltA(), *BoltParallelism*)
.setNumTasks(*BoltTaskParallelism*)
BoltParallelism is indeed the number of executors and BoltTaskParallelism
is indeed the number of tasks.
BUT
int BoltParallelism = 3;
builder.setBolt("bolt1", new BoltA(), *BoltParallelism*)
When you don't specify setNumTasks, Storm creates BoltParallelism number of
tasks and creates BoltParallelism number of executors as well.
*To your reply of "No. All executors run in parallel":*
When I have 3 tasks and 3 executors, I won't have to worry about
concurrency inside the Bolt, right? Because every Bolt instance is being
run in a separate thread, so all their member variables and functions are
specific to the executor.
Also, even if I have 3 tasks and 1 executor, every task is going to be run
one after the other by the executor, so there's no worry about concurrency
here either.
So in what situation would I have to worry about concurrency? AFAIK, even
in a single bolt, the execute() function has to complete before the same
execute() is invoked again.
On Tue, May 17, 2016 at 12:54 AM, Matthias J. Sax <[email protected]> wrote:
> Answers inline.
>
> I guess you are not aware, that a worker run other thread next to the
> executors, too. For example, there are two threads (one for input; one
> for output), that work as "dispatcher" for incoming messages. There is a
> global input queue, and the dispatcher "forwards" incoming messages to
> the individual tasks queues such that the executors can all work in
> parallel. Same for output. Executors write into own output queues and a
> single "output thread" reads the data from there and take care of
> network transfer to downstream bolts.
>
> -Matthias
>
> On 05/16/2016 06:24 PM, Navin Ipe wrote:
> > Err...guys....I appreciate the ongoing discussion, but the original
> > question remains unanswered. The one I've asked at the very beginning of
> > this conversation. Some help would be appreciated.
> > Referring to the code I posted and as per Nathan's answer, you say that
> > int *BoltParallelism* actually represents the tasks
>
> No. *BoltParallslim* is the number of executor threads.
>
> which are the number
> > of instances of Bolts/Spouts? And BoltTaskParallelism is the number of
> > executors (OS threads)?
>
> No. This is the number of tasks.
>
> > If that's the case, then execute() will get called only after the
> > previous execute() call of a Bolt has completed. And nextTuple() will
> > get called only after the previous nextTuple() of a Spout has completed.
>
> For a single executor, yes.
>
> > That's a bit reassuring, since now one does not have to cater to
> > multithreading within a Spout/Bolt.
>
> No. All executors run in parallel.
>
> >
> >
> > On Mon, May 16, 2016 at 7:07 PM, Matthias J. Sax <[email protected]
> > <mailto:[email protected]>> wrote:
> >
> > Hi,
> >
> >
> > So this is not correct:
> > > and
> > > the Bolt creates a task for processing each incoming Tuple.
> >
> > Storm create exactly *BoltTaskParallelism* tasks and assigns incoming
> > messages to tasks (according to the used connection pattern --
> shuffle,
> > fieldsGrouping etc).
> >
> > Futhermore:
> >
> > > If there
> > > are not enough tasks, then the excess Tuples are made to wait in a
> > > queue of the executor.
> >
> > No. There is no thing as "not enough tasks". Each task has its own
> input
> > queue/buffer and tuple are stored there.
> >
> > The executor threads process one or multiple tasks. Thus, if a task
> is
> > currently "on hold", new tuples are just added to the task's input
> > queue. If an executor picks up on of its tasks for processing, the
> > buffered tuples of the task are processed.
> >
> >
> > -Matthias
> >
> > On 05/16/2016 09:07 AM, Adrien Carreira wrote:
> > > +1
> > >
> > > 2016-05-16 6:40 GMT+02:00 Navin Ipe <
> [email protected] <mailto:[email protected]>
> > > <mailto:[email protected]
> > <mailto:[email protected]>>>:
> > >
> > > Hi,
> > >
> > > I've seen the explanations
> > >
> > <
> http://www.michael-noll.com/blog/2012/10/16/understanding-the-parallelism-of-a-storm-topology/
> >,
> > > but none of them explain it in terms of what I see in the
> code. This
> > > is what I understood:
> > >
> > > int BoltParallelism = 3;
> > > int BoltTaskParallelism = 2;
> > > builder.setBolt("bolt1", new BoltA(), *BoltParallelism*)
> > > .setNumTasks(*BoltTaskParallelism*)
> > >
> > > BoltParallelism creates 3 instances of BoltA. These are the
> > executors.
> > > BoltTaskParallelism allows Tuples to come into BoltA very
> > fast, and
> > > the Bolt creates a task for processing each incoming Tuple. If
> > there
> > > are not enough tasks, then the excess Tuples are made to wait
> in a
> > > queue of the executor.
> > >
> > > Strange thing is that the explanation says the tasks are run
> in a
> > > single thread, so obviously I misunderstood something. Could
> you
> > > help me understand it?
> > >
> > > --
> > > Regards,
> > > Navin
> > >
> > >
> >
> >
> >
> >
> > --
> > Regards,
> > Navin
>
>
--
Regards,
Navin
