Apps Messaging - Semantics of a Message

[Doug Tody]

On Tue, 10 Apr 2007, Mark Taylor wrote:

> It is becoming clear that there are two quite different types of job that
> we want a messaging system to do.  As Doug said:
>
> Doug Tody wrote:
>
>> Most of the PLASTIC type scenarios seem to involve visualization tools
>> which operate upon high level data objects such as images, tables, and
>> so forth.  In general for this sort of thing I think a high level
>> object-oriented approach is most promising (as with PLASTIC I think).
>> That is, one merely sends an entire data object of some type to an
>> app, either as a reference (e.g., URL) or in-line, with some general
>> suggestion for what to do with it ("display", "load", etc.), and it
>> is up to the app from there.  This makes the messaging very simple,
>> and allows the full capabilities of the application to be utilized.
>> 
>> Another important case though, is where we have a powerful tool such
>> as an image display service (DS9, Aladin, iraf ximtool, etc.) which
>> has a more complex model defining things like image planes, associated
>> world coordinate systems, display modes, etc.  In this case the client
>> might want to command the tool, rather than merely send it high level
>> data objects.  This is more the sort of thing that XPA/DS9 and XImtool
>> do (also others such as the image viewer in AIPS++/CASA, and the
>> IDL visualization tools).  This is also an important case, and can
>> be very important in some cases, for example to drive visualization
>> tools from a Python, IDL, IRAF, etc. script application.
>
> For scenarios of the first type it makes perfect sense to say simply
> "here's a table".  For scenarios of the second type, it does not.
>
> It's not totally obvious that a single messaging system will serve
> both purposes, and I think we should keep an eye on whether we're trying to 
> produce one standard where two would be more appropriate;

In my terminology these two cases are served by different message
content models (message "payloads").  The basic messaging system can
otherwise be the same in both cases however.  It isn't necessary to
try to come up one scheme that does it all.

Personally, I think the high level object-oriented approach is the most
interesting one to explore initially.

 	- Doug