Re: section 2.2.22 Message Exchange Pattern (MEP)

Chris,

I see the MEP as a fundamental construct that relates an initial
message and its possible response message(s).  In the context of SOAP,
this is a pattern that gets supported by a binding which tends to view
things from the perspective of interacting nodes.  Protocol binding
specs "declare their support for one or more named MEPs".  In the
context of WSDL, an MEP is viewed from the perspective of the nodes
themselves.

The SOAP definition occurs in the context of the SOAP spec with its
binding framework.  Even in that context, it probably should have been
tightened up a bit, but it certainly needs some qualification in
our spec since we are in a more general context.

I think that David Booth was trying to capture a more WSDL-centric
view of an MEP with the phrase "a single use of the service".  For
example, the SOAP Req/Resp MEP looks like an in-out pattern at the
operation level from the perspective of the ultimateReceiver.

I personally find the SOAP view of an MEP more coherent.  The problem
with the WSDL operation perspective is that a single, simple SOAP
request/response MEP ends up having complementary WSDL operation
patterns for each endpoint in a peer-to-peer environment -- an out-in
at the initial sender and an in-out at the ultimateReceiver.  But
these operations are not formally related to each other in the WSDL
framework.  The SOAP view provides that coherence.


What about the following definition:

 A message exchange pattern is a template for the exchange of messages
 between agents that arise from a message and its responses, if any.

Is that any better?

By the way, I don't think we want to say that choreography isn't concerned with
patterns.  They can be MEPPs -- MEP Patterns!

Mark Jones
AT&T

	To: www-ws-arch@w3.org
	From: Christopher B Ferris <chrisfer@us.ibm.com>
	Date: Mon, 7 Jul 2003 16:36:25 -0400
	Subject: Re: section 2.2.22 Message Exchange Pattern (MEP)

	Mark,

	I am still uncertain as to what "a single use of the service" means in 
	this, or any
	context. A service may have many operations, and fulfilment of the service 
	may
	require more than one of the operations to be invoked by the client. Given 
	this,
	where does the SOAP Req/Resp MEP come into play?

	I believe that WSD WG is mapping MEP at the operation level and hence
	at the very least, we should be aligning our definition with that notion.

	I agree that it is important to distinguish between an MEP and a 
	choreographed
	exchange of messages, one is a pattern (the P in MEP) and the other is 
	not.

	SOAP1.2[1] defines an MEP as:

	        A Message Exchange Pattern (MEP) is a template that establishes a 
	pattern for the exchange of messages between SOAP nodes. 

	Why isn't that definition good enough for us?

	[1] http://www.w3.org/TR/soap12-part1/#soapmep

	Cheers,

	Christopher Ferris
	STSM, Emerging e-business Industry Architecture
	email: chrisfer@us.ibm.com
	phone: +1 508 234 3624

Received on Tuesday, 8 July 2003 11:32:25 UTC