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Re: MEP proposal

From: Amelia A. Lewis <alewis@tibco.com>
Date: Fri, 21 Feb 2003 14:09:17 -0500
To: Jacek Kopecky <jacek@systinet.com>
Cc: mgudgin@microsoft.com, jeffsch@windows.microsoft.com, www-ws-desc@w3.org
Message-Id: <20030221140917.2039921f.alewis@tibco.com>

On 21 Feb 2003 19:33:57 +0100
Jacek Kopecky <jacek@systinet.com> wrote:
> Amy, please see below. 8-)
> 
> On Fri, 2003-02-21 at 18:44, Amelia A. Lewis wrote:
> > On 21 Feb 2003 18:20:05 +0100
> > Jacek Kopecky <jacek@systinet.com> wrote:
> > > SOAP MEPs (similar to what I would call abstract MEPs) are describing
> > > the message flow among a given set of nodes (two or more, usually). They
> > 
> > Precisely two, according to the SOAP spec.
> 
> AFAICS, the definition of SOAP MEPs doesn't say anything about the
> number of nodes: http://www.w3.org/TR/soap12-part1/#soapmep

SOAP 1.2 part 1, section 3.1, in the discussion of features, restricts everything to between two adjacent soap nodes (a problem that I thought I had pointed out to the working group; the word "two" is unnecessarily limiting; "adjacent nodes" is adequate).  Perhaps, since it bugs me so much, I'm reading too much into it.

> The two MEPs provided by SOAP spec do have precisely two nodes, but that
> is not a general constraint on SOAP MEPs.

Okay.

> > > my point), but the WSDL MEP can be understood to have the service and
> > > "the world" (blue/middle box) or only two communicating nodes
> > > (pink/small box). Now which ones were you doing? Which ones do we want
> > > to do? Which ones do we want abstract?
> > 
> > I'm sorry.  I realize that for most people, a picture is worth a
> > thousand words.  To me, alas, a picture is just visual noise.  In
> > other words, I can't make head or tails of a picture.  Can you explain
> > it in words?
> 

<snip>

I really apologize, but a description of the picture doesn't really help me at all.  And yes, I realize it's odd to encounter someone so particularly non-visual, when pictures are a common aid in these matters, but it's a picture of an abstract thing that looks rather venn diagrammish only not really.

Let me see if I can describe the three "levels" of MEPs in words (and without reference to the picture, if you don't mind; I have a little trouble with pink MEPs versus green MEPs, and still really meaning no offence, okay?).

At the smallest level, a MEP may represent the interaction between two conceptual processes (although there may be more actual processes involved; there are only two roles), which will do all the processing internally, on one machine, or in a fashion that is otherwise completely opaque to the mechanism of WSDL for description.

At the largest level, a MEP may represent the interaction of multiple processes, which may expose service interfaces that are in turn dependent upon other service interfaces, and are susceptible to description in WSDL.  For instance, the mep8 earlier mentioned might be an example of this level.

I don't understand where the middle level fits, either.  Sorry.

The two that I outline above are, in my opinion, a WSDL MEP/operation (the small level) and a choreography "process".  That's how I think things ought to be targeted, at least.

> I've heard in this WG before statements that seemed to say: why break up
> the middle MEP into two small MEPs if it's really just one MEP logically
> and since it's still from the point of view of the service?

As you can see, I don't quite understand how the middle one is described.  I can't find its boundaries, in verbal description.

> I think my picture contains no more control flow than request/response
> contains. Is there anything else choreographish you'd ascribe to any of
> the "WSDL? MEP"s?

I apologize, but I don't know what the picture represents.

Of the MEPs defined, I think number one is the simplest, with the content model:

IN

mep7 is probably the most complex, and can be characterized with the content model:

OUT, ( (IN, oFault?) | iFault )*

Perhaps you could provide a content model description for the MEP in the picture?

> I think visible intermediaries should be describable with WSDL. A

I disagree.

> visible intermediary is a service, too, so you can't dismiss this again
> saying we decided WSDL is from the point of view of the service. 8-)

Not about to.  I think it should be described as a service.  The client is going to contact the intermediary, though, not the ultimate service; it doesn't really know it's talking to that other service.

> But a visible intermediary does have the other side that logically
> belongs in the same operation but contains a third role.

I disagree; I think it simply plays the client role to the ultimate server.

> Again, I'm not saying WSDL must contain an intermediary MEP, but we must
> allow it.

I think we do already.

Amy!
-- 
Amelia A. Lewis
Architect, TIBCO/Extensibility, Inc.
alewis@tibco.com
Received on Friday, 21 February 2003 14:09:08 GMT

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