W3C home > Mailing lists > Public > public-ws-chor@w3.org > May 2004

Re: Abstract, portable and concrete choreographies ... a proposed solution??

From: Monica J. Martin <Monica.Martin@Sun.COM>
Date: Sun, 02 May 2004 09:02:24 -0600
To: david.burdett@commerceone.com
Cc: public-ws-chor@w3.org
Message-id: <40950D80.9060003@sun.com>

>mm1: David, I am concerned you are adding complexity here.  Comments:
>    * Adding specialization of PackageBinding. This assumes that the
>      business semantics are a part of the WS-CDL (and in import). They
>      are not.
><DB>I know we have discussed this before, but I don't understand your point. How could a responsible designer define an "interaction", i.e. the exchange of information usually in the form of a message, in a WS-CDL definition without explaining what that interaction **means**, i.e without explaining its **semantics**.
mm1: WS-CDL lacks the business semantical definitions required and I 
stand my ground that any attempts to include them will limit the 
language. Previously, I provided you a detailed list of business 
semantical constructs: business transactional patterns, 
signals/messages, partner roles that are not service-based, business 
dialog and contract obligation, to name only a few. As I have indicated 
before, WS-CDL could look to existing languages to provide this boundary 
(constraints, priority, preference and commitment basis).

> If the semantics are missing from the WS-CDL definition, then how would an organization that wanted to use that definition as part of their implementation know that they were using it correctly?</DB>
mm1: They look to the existing language that provides that boundary for 
WS-CDL. This doesn't limit the WS-CDL language. It allows WS-CDL to do 
well what it is built to do while allowing it to look to existing 
languages (ebBP) to provide the business semantical boundary.

>    * Note 5: What impacts are realized if you change the original WSDL
>      definitions on the fly? How do you insure conformance when you
>      begin to change the underlying semantics that are expected? Aren't
>      these actually different WSDL definitions rather than an
>      augmentation of an existing one?
><DB>Firstly, I don't think that WSDL definitions would or should be changed on the fly - it's too risky. Secondly, the main assumption is that you can **only** do an alternative package binding if the underlying semantics are the same - if they are not then package binding won't work. Thirdly the approach assumes that the semantics of the WSDL definitions at each end are "equivalent". For example I would say that a UBL Order definition and a RosettaNet Order definition are semantically "equivalent", they just have diferent XML representations. The same goes for WSDL defintions, for example the actual names used for a port, message, document, operation etc are irrelevant as long as the behavior of the service "behind" the definition is the same. A good test for equivalence is if you can easily map between one definition and another. </DB>
mm1: So now you are saying that WS-CDL will do business process pattern 
matching to understand if the choreographies are semantically the same 
although syntactically different. When was this role (and duty) 
established for WS-CDL? How can you guarantee that the WSDL definitions 
at each end are 'semantically' equivalent? This clearly has not been 
established as within the WS-CDL scope.

>    * It appears that you are adding more semantics that already occur
>      in existing open standards. What is the provocation to create anew
>      that could be used in existing technologies? Isn't a mapping a
>      easier path?
><DB>I don't think I am adding more semantics. All the package binding idea provides is a method of either changing the values of elements/attributes in an existing package definition or adding in the values in the original package definition that were missing. The package binding does not introduce **any** new concepts or structures to the package element as currently defined in the latest spec. Can you give an example of where you think I am adding in more semantics as that was not the intention?</DB>
mm1: When you make changes to the underlying attributes of the package, 
you change the context of the interactions that depend on it David. See 
comment above about what capability WS-CDL has to ensure semantic 
equivalency.  In addition, I point to Daniel Austin's comments about the 
limitations of WSDL.

>    * What is the benefit of a fully abstract choreography? This relates
>      to previous question about recreating the wheel of other technologies.
><DB>The short answer is reuse and lower maintenance costs. A more detailed answer follows:
>1. Before two (or more) independent businesses can start exchanging messages as part of some shared business process, e.g. a buyer and a seller buying goods, they have to agree on two things: i) the WSDL interfaces they will expose that will accecpt messages, and ii) the sequence in which they exchange messages, i.e. the choreography definition.
>2. Many of those business' WSDL definitions will be different but semantically "equivalent" as described earlier
>3. A "concrete" choreography definition is, by definition, "tied" to specific WSDL definitions. This means that if the WSDL definitions change for some reason, then the choreography definition **must** change even if the sequence of exchanging messages has not.
>4. If you have an "abstract" choreography that is independent of the WSDL then you can change the WSDL definitions independently of the choreography definition therefore reducing the maintenance effort and enabling the choreography to be reused.
>5. Finally, **if** standards groups create "standard" abstract choreography definitions, then businesses can just agree to use them and then they only have to focus on how they align their WSDL definitions.
mm1: See previous comments regarding semantic 'equivalence'.  On: '4. If 
you have an "abstract" choreography that is independent of the WSDL then 
you can change the WSDL definitions independently of the choreography 
definition therefore reducing the maintenance effort and enabling the 
choreography to be reused':
This group's scope necessitates its use and dependence on WSDL v2.0 
(which is forthcoming).  This clearly points out a concern I have voiced 
before that combining the choreography description and the underlying 
interactions could create a limitation on the language because those 
concepts may not always complement one another. Don't get me wrong, I am 
not saying that we shouldn't define a choreography description, but 
suggest we recognize our scope boundaries and the capabilities that WSDL 
can support/understands. Your premise assumes that the underlying WSDL 
definitions will be capable of handling the variability of the 
choreography descriptions regardless if the latter may reference back to 
business semantics and contractual constraints outside of WS-CDL.

>    * You are adding yet another layer of abstraction in your
>      definitions - this is complexity may not be prudent and may
>      actually serve as an impediment to adoption by industry (abstract,
>      concrete-based on abstract, concrete with fillings, portable with
>      partial, etc and more....)
><DB>I don't see how I am adding another layer of abstraction. As I said earlier, the basic mechanism of a package binding is one of replacing values in a choreography definition by alternatives that are semantically equivalent. Where is the complexity in this approach? I don't see it. Can you provide an example? I do agree though, that if complexity exists, then it should be avoided.</DB>
mm1: I stand my ground that changing the values of the choreography 
definition based on the premise that WS-CDL understands semantic 
equivalence is fraught with risk (and ill advised).
Received on Sunday, 2 May 2004 11:04:25 UTC

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