Re: [Boost-users] Does anybody interested in active object

Subject: Re: [Boost-users] Does anybody interested in active object
From: Marat Abrarov (abrarov_at_[hidden])
Date: 2010-11-08 03:53:14

To Dean Michael Berris:

Thank you for your patience and detailed answers.

> Nope, sorry, s11n_example::connection is not an active object. What it actually is, is an execution
> context -- or sometimes people call it a continuation, a generator, or a state machine. Because you're
> using the execution context relevant with a particular connection, it doesn't imply that the
> operations are performed on a different thread.

Ok. May be I'm wrong trying to name it "active object".
"Continuation", "state machine", "execution context" - this is very close to what I had in mind. 
Therefore, I conclude that we understand each other (despite some language barriers with my hand).
I thought all of "Continuation/state machine/execution context" was related to the pattern of "active object", hmm...

> If you have unlimited number of cores and no limits on the number of threads you can spawn for each
> active object you create, then yes.

I think the main feature of "Continuation/state machine/execution context" is not saving threads,
but organizing flexible and comfortable way of communication between 
state machines (yes - it is tasks and queues, but wrapped in async method call with callback).
That is what I try to implement by means of Boost.Asio.
It was chosen "historically" (started from network programming), 
but for the now times I see it is the best choice :) 

> Otherwise, you can stick with continuations, asynchronous completion handlers, and just plain data
> parallelism.

At the "samples" I develop (thanks to Boost.Asio examples - the are the first) the main goals were:
1) share execution context (thread pool) between different "state machines".
2) (make possible to) divide execution contexts of different "state machines" and application layers
   when it is needed (session and session_manager - not layers but are close to them).
3) develop a simple way of implementing async method call (for the "state machines") with required callback.
   (with required usage of "custom memory allocation" which can make usage of async methods as cheap as possible)

Regards,
Marat Abrarov.

> Message: 3
> Date: Mon, 8 Nov 2010 10:29:02 +0800
> From: Dean Michael Berris <mikhailberis_at_[hidden]>
> To: boost-users_at_[hidden]
> Subject: Re: [Boost-users] Does anybody interested in active object
> Message-ID:
> 	<AANLkTimW0ozMYQ_GfstJw4mV111=2JTc0JSh+0YzLdYF_at_[hidden]>
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> 
> On Mon, Nov 8, 2010 at 7:11 AM, Marat Abrarov <abrarov_at_[hidden]> wrote:
> > To Dean Michael Berris:
> >> You'd be better of doing data parallelism with Boost.MPI
> >
> > No. Because I'm speaking about thread-based parallelism (with shared
> > memory access).
> > Processed-based parallelism and IPC supplement but don't replace
> > thread-based parallelism in general.
> >
> 
> Well, which is why I said 'data parallelism'. This implies a whole different kind of programming,
> which doesn't typically include threads and synchronization. ;)
> 
> >
> >> and/or Boost.Asio's io_service
> >> run on multiple threads, and having independent tasks multiplexed
> >> across a number of processors (either through processes or threads)
> >
> > Tie this tasks with the data they share and You will have an active object.
> 
> Nope, sorry, that's not an active object. An Active Object requires that you have a at least one
> thread of execution dedicated to the active object, and that you have a synchronous interface to that
> object.
> 
> > If the tasks don't have shared data - they don't need an active object.
> 
> Active Objects really have nothing to do with shared data. It's a design pattern that prescribes a way
> of isolating the synchronization and serialization of operations applied to an object that is assumed
> to be shared across multiple threads of execution. The way the active object does it is by lining up
> the operations to be performed asynchronously and perform those in a different thread of execution.
> 
> You can implement an active object without involving shared data -- for example you want to model a
> processor, or a math kernel, which does all its work on a single thread because maybe you have a
> library underneath that is not thread-safe. In situations like these it makes sense to have an active
> object implementation.
> 
> > I think - active object is built on task-based parallelism but it does
> > not mean that "active object" should replace "task-based parallelism"
> > everywhere.
> >
> 
> I think you have it wrong. The active object pattern is a design pattern that doesn't know about task-
> based parallelism. Task-based parallelism is a different pattern and different paradigm altogether.
> The active object pattern by itself is just a design pattern, nothing more.
> 
> >
> >> But what you're describing is not the active object pattern. ;)
> >
> > http://en.wikipedia.org/wiki/Active_object
> > http://en.wikipedia.org/wiki/Active_object_(Symbian_OS)
> > What about ma::echo::server::session class - is it an active object?
> >
> 
> I haven't looked at your examples, because I was only commenting on the issue regarding massive
> parallelism. Maybe when I have enough time I'll look through your examples. ;)
> 
> >
> > http://www.boost.org/doc/libs/1_44_0/doc/html/boost_asio/example/chat/
> > chat_c
> > lient.cpp
> > Is chat_client an active object? If "yes" then why?
> > chat_client::write have no completion callback - and this is not
> > enough sometimes (I can say "always", if You don't implementing
> > "fire-and-forget" logic which is rather seldom).
> >
> 
> No, this is not an active object. An active object has its own lifetime thread. In this situation the
> lifetime thread is shared between Boost.Asio's IO subsystem and all the ASIO related handlers.
> 
> Also, you don't need a completion callback for a class to model the Active Object pattern. You can
> create methods that return 'void' and just execute the actions asynchronously in its own lifetime
> thread.
> 
> >
> > http://www.boost.org/doc/libs/1_44_0/doc/html/boost_asio/example/seria
> > lizati
> > on/connection.hpp
> > s11n_example::connection is an active object and have completion callback...
> > but it's much simpler then ma::echo::server::session_manager - I try
> > to suggest "samples" that can help to build such a kind of complex
> > active objects.
> >
> 
> Nope, sorry, s11n_example::connection is not an active object. What it actually is, is an execution
> context -- or sometimes people call it a continuation, a generator, or a state machine. Because you're
> using the execution context relevant with a particular connection, it doesn't imply that the
> operations are performed on a different thread.
> 
> All this is doing is encapsulating callbacks and the protocol implementation into an object. It
> exposes a means of asynchronously scheduling operations, but it doesn't hide these details from the
> interface. The goal of the active object pattern is to hide the asynchronous nature of the
> implementation from the user of the object.
> 
> >
> > http://www.drdobbs.com/go-parallel/blog/archives/2009/09/parallelism_s
> > ho.htm
> > l
> >> We are surrounded by parallelism even more so that serial activities.
> >> Social and biological systems are immersed in parallelism and these
> >> systems have worked out the kinks over millions of years.
> >> In biological systems, independent units perform simple computations
> >> or activities independently and in parallel.
> > and
> >> IMPLICATIONS FOR COMPLEX REALTIME SYSTEMS:
> >> Complex real-time systems would be a sea of state machine objects
> >> interacting with each other. There will be low level state machines
> >> which would work together to implement high level state machine behavior.
> >
> > Here's the place for active object pattern.
> >
> 
> If you have unlimited number of cores and no limits on the number of threads you can spawn for each
> active object you create, then yes.
> Otherwise, you can stick with continuations, asynchronous completion handlers, and just plain data
> parallelism.
> 
> The place for an active object is where you have to expose a synchronous interface to a resource which
> underneath performs the actions asynchronously. This is like the half-sync half-async pattern which,
> once you understand, is what the active object implementation provides.
> 
> --
> Dean Michael Berris
> deanberris.com