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@merzky.net> wrote:

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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):

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Hi,

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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