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RE: Message Exchange Patterns -- p2c and/or p2e

From: David Booth <dbooth@hp.com>
Date: Fri, 20 May 2005 22:02:25 -0400
To: Jonathan Marsh <jmarsh@microsoft.com>
Cc: public-ws-desc-comments@w3.org
Message-Id: <1116640945.4250.164.camel@nc6000.w3.org>

I am satisfied with the resolution.


On Thu, 2005-05-12 at 13:51 -0700, Jonathan Marsh wrote:
> Thank you for your comment, tracked as LC50 [1].  The WG agreed to a
> definition of "node" [2] which clarifies somewhat which node would be
> appropriate for a given scenario.  You can see the resolution in the
> latest Working Draft [3].
> 
> If we don't hear from you within two weeks, we'll assume this resolution
> is satisfactory.
> 
> [1] http://www.w3.org/2002/ws/desc/4/lc-issues/issues.html#LC50
> [2] http://lists.w3.org/Archives/Public/www-ws-desc/2004Nov/0070.html
> [3] http://www.w3.org/TR/2005/WD-wsdl20-adjuncts-20050510
> 
> 
> > -----Original Message-----
> > From: public-ws-desc-comments-request@w3.org [mailto:public-ws-desc-
> > comments-request@w3.org] On Behalf Of David Booth
> > Sent: Tuesday, September 28, 2004 3:21 PM
> > To: public-ws-desc-comments@w3.org
> > Subject: Message Exchange Patterns -- p2c and/or p2e
> > 
> > 
> > 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].
> > 
> > RATIONALE
> > 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
> > decision:
> > 
> > 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,
> > Out-Optional-In).
> > 
> > 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
> > TF.
> > 
> > 
> > References
> > 1. WSDL 2.0 in-out MEP:
> > http://www.w3.org/TR/2004/WD-wsdl20-extensions-20040803/#in-out
> > or: http://tinyurl.com/4rsvz
> > 2. p2e:
> > http://dev.w3.org/cvsweb/~checkout~/2002/ws/desc/wsdl12/meps-vs-
> > iops/meps-vs-iops_clean.htm#p2e
> > or: http://tinyurl.com/53hqp
> > 3. p2c:
> > http://dev.w3.org/cvsweb/~checkout~/2002/ws/desc/wsdl12/meps-vs-
> > iops/meps-vs-iops_clean.htm#p2c
> > or: http://tinyurl.com/4pjo4
> > 
> > 
> > --
> > 
> > David Booth
> > W3C Fellow / Hewlett-Packard
> > 

-- 

David Booth, Ph.D.
HP Software / Boston
Hewlett-Packard, Inc.
Received on Saturday, 21 May 2005 02:04:24 GMT

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