[SAGA-RG] SAGA Message API Extension

[Andre Merzky]

Hi Werner, 


Quoting [Werner Benger] (Jan 18 2007):
> 
> Hi Andre,
> 
>   I have two other remarks, which might be orthogonal to the current
> draft, but might still be good to have it mentioned there:
> 
>   * Structured messages:
> 
>   The current draft just talks about transporting an array of bytes,
>   but in practice we might want to transfer floats/doubles/ints etc.
>   While this *might* be implemented on top of the current msg API,
>   this would be a waste if the low-level protocol implementation
>   (e.g. MPI) already would support such types (including byte ordering
>   conversion). As such, it were useful to have the option to use
>   such mechanisms from a low-level protocol if supported. If not,
>   then it would need to be taken care of on top of the current level.

Ah, good point - that at least needs clarification in the
spec!  

Yes, you are right: the focus on opaque messages is a
limitation for many use cases.  OTOH, support for primitive
types such such as ints or floats don't by you that much,
and for more complex structures... - well, who knows better
than you that agreeing on a data model is a reeaaally
difficult job? ;-)

So, basically the message API tries to avoid that topic for
the main reason that it seems difficult to define.  I would
wholeheartly support any activity which tries to define
domain or use case specific flavours of the API.  That would
be a simple excercise: you would only need to redefine the
set_data method on the msg class accordingly.

So, the question is: is a very limited support for primitive
data types something (really) useful?


>   * Interfacing Event Loops:
> 
>   If we want to use this API from within a larger application instead
>   of just self-standing programs, we might want to use mechanisms such
>   as socket callbacks for event handling (eg. the QSocketNotifier or
>   under X11 using XtAppAddInput). Would be good to have some support
>   to allow this, even though it might be optional.

Right, thats an important point, in particular for the
visualization use cases.  Its actually in the spec, but well
hidden :-)  The endpoint class definition says:

  class endpoint : implements saga::object
                   implements saga::async
                   implements saga::monitoring
                   [...]
  
saga::async is actually an empty interface, but what that
means is that the class will contain several versions of
every class method: a synchronous one, and 3 additional
ones.  In C++ the rendering would look like:

  // connection setup
  saga::endpoint ep;
  ep.serve ();

  // normal, synchronous version
  saga::msg m = ep.recv ();

  // task version 1: synchronous
  saga::task t1 = ep.recv <saga::task::Sync> (msg);
  
  // task version 2: asynchronous
  saga::task t2 = ep.recv <saga::task::ASync> (msg);
  
  // task version 3: task
  saga::task t3 = ep.recv <saga::task::Task>  (msg);

These three versions of the recv method all return a task,
which only differs in its state: t1 is Done, t2 is Running,
and t3 is New (not yet running).  You can get notification
on when a task is Done etc.


Additionally, the spec defines some metrics on the endpoint,
among them:

      // Metrics:
      //   name:  Message
      //   desc:  fires if a message arrives
      //   mode:  Read
      //   unit:  1
      //   type:  String
      //   value: ""
      //   notes: - the value is the endpoint URL of the 
      //            sending party, if known.

These metrics are used by the monitoring interface, which is
also implemented by the endpoint.  With that, you can add
callbacks to an endpoint which gets called when a new msg
arrives:

  saga::endpoint ep;
  ep.add_callback ("Message", my_cb);
  ep.serve ();

my_cb is a user defined class which implements
saga::callback, and whose cb() method gets then called on
incoming messages.


Sorry if that was somewhat lengthy.  Anyway, point is: async
ops and notification are covered, by means of the SAGA Core
Look&Feel, which is inherited by this API.

Cheers, Andre.
  

> 
> 	Werner
> 
> 
> On Thu, 18 Jan 2007 14:18:52 -0600, Andre Merzky <andre at merzky.net> wrote:
> 
> > Hi John, Andrei,
> >
> > you are right: getting some feedback from the transport
> > level folx is certainly a good idea.  The API draft won't go
> > into public comment for another month or so (at least), and
> > then it will stay in public comment for another 2 months or
> > longer - that should give us enough time to contact them.
> >
> > About ordering: the text Andrei cited is in the spec because
> > ordering is, as of now, not an attribute of the connection
> > or endpoint - so the spec tries to nail it down.  It says
> > "MUST be ordered, but no global ordering is required"
> > because I thought that this covers the majority of use
> > cases.
> >
> > I don't think there are use cases which require global
> > ordering - or at least not enough to justify a requirement
> > for global ordering.  What is your opinion?  Also, thats
> > really difficult to implement in Grids IMHO.
> >
> > Use cases which do not require ordering should be happy with
> > order preserving connections, too.  Question now is: does
> > the benefit of un-ordered implementations (simplier, smaller
> > footprint) justify an attribute on API level?  Or are there
> > use cases which require non-ordered delivery for other
> > reasons?
> >
> > Cheers, Andre.
> >
> >
> > Quoting [Andrei Hutanu] (Jan 18 2007):
> >>
> >> Hi,
> >>
> >> >>
> >> >>2) I see ordering is enforced, could that be an option?
> >> >
> >> >
> >> >I think ordering is *not* enforced, but I do wonder if it should be
> >> >an option or a channel property (certainly semireliable will likely
> >> >result in some reording whereas a TCP channel would enforce ordering
> >> >of the messages for instance).
> >> >
> >> >This is a controversial topic in the HPC message passing community
> >> >(whether msg. ordering is a good or bad-thing to enforce in at the
> >> >hardware level).
> >> >
> >> I was thinking the same (no strong feelings for either option or
> >> property) but the text tells otherwise :
> >> In 2.1 introduction :
> >> In contrast, this message API extension guarantees that message blocks
> >> of arbitrary size are delivered in order, and intact, without the need
> >> for additional application level coordination or synchronization.
> >> and
> >>
> >> then in 2.1.7 reliability corectness and ordering
> >> The order of sent messages MUST be preserved by the implementation.
> >> Global ordering is, however, not guaranteed to be preserved:
> >>
> >> Assume three endpoints A, B and C, all connected to each other. If A
> >> sends two messages [a1, a2], in this order, it is guaranteed that both B
> >> and C receive the messages in this order [a1, a2]. If, however, A sends
> >> a message [a1] and then B sends a message [b1], C may receive the
> >> messages in either order, [a1, b1] or [b1, a1].
> >>
> >> Andrei
-- 
"So much time, so little to do..."  -- Garfield