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Message Exchange Patterns -- p2c and/or p2e

From: David Booth <dbooth@w3.org>
Date: Tue, 28 Sep 2004 18:20:51 -0400
To: public-ws-desc-comments@w3.org
Message-Id: <1096410050.3212.260.camel@nc6000.w3.org>

As currently defined, the WSDL 2.0 in-out MEP[1] requires the service to
send the response message back to the original requester agent, as
illustrated by pattern p2e[2] in "MEPs versus IOPs".  (This is in
contrast with pattern p2c[3], which permits the response to go to a
third party.)  

I think the WG should either:
	a. Add a pattern like p2c[3] to the MEPs in part 2,
	   in addition to the existing in-out MEP; 
	b. Replace the existing in-out MEP with a pattern like p2c[3].

Suppose a service wishes to permit the response to be sent to a third
party.  For example: requester agent A sends a request to service S,
which sends the response to agent B (by use of an addressing extension,
for example).  What should service S do, given that the semantics of the
in-out MEP require the response to be sent back to A?  How should
service S be described in WSDL?  It could:
	a. modify the semantics of WSDL 2.0 by use of a required
	   extension (wsdl:required = "true");
	b. use an MEP that is not pre-defined in WSDL 2.0, such as
	   p2c[3]; or
	c. violate the WSDL 2.0 specification.

Pattern p2c[3] was considered earlier by the WG, when the WG discussed
MEPs, and I think the main reasons for adopting p2e[2] instead were:
	- It captures the intent of many common interactions.
	- It permits a requester toolkit to generate code that 
	supports a very simple, function-like usage style, 
	independent of what binding is used:
	    Reply message = service.send(requestMessage);
	while not precluding an event-driven
	implementation style that would be needed by p2c[3].

However, I believe two things have changed since the WG made that

1. The WG became more permissive about the set of MEPs that it defines
in Part 2, adding MEPS for additional fault treatments (Robust-In-Only,
Robust-Out-Only) and optional responses (In-Optional-Out,

2. The use of Web service addressing extensions is now receiving more
attention.  For example, the W3C is now considering creating a Web
services addressing WG.

3. A requester toolkit could have an option (for each operation) to
generate code in the function-like usage style shown above, even if the
service specifies pattern p2c[3], and it could even be the default
option.  (In this case, the requester toolkit would hide the fact that A
and B need not be the same in p2c[3].)  Others may correct my memory,
but I don't think this possibility was considered at the time when the
MEP TF was discussing the pros and cons of p2c versus p2e.  

Incidentally, one potential argument for adopting BOTH p2e[2] and p2c[3]
is that p2e[2] also allows the *service* toolkit to optimize
interactions because it knows that the reply always goes back to the
same agent that sent the request.  For example, it permits a single HTTP
interaction to transmit both the request and the response.  This is a
further consideration that I don't remember discussing during the MEP

1. WSDL 2.0 in-out MEP:
or: http://tinyurl.com/4rsvz 
2. p2e:
or: http://tinyurl.com/53hqp 
3. p2c:
or: http://tinyurl.com/4pjo4 


David Booth
W3C Fellow / Hewlett-Packard
Received on Tuesday, 28 September 2004 22:21:10 UTC

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