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

From: Steve Ross-Talbot <steve@enigmatec.net>
Date: Sun, 2 May 2004 18:59:03 +0100
Message-Id: <65E92DB6-9C62-11D8-970D-000393D13C9A@enigmatec.net>
Cc: <david.burdett@commerceone.com>, "Monica J. Martin" <Monica.Martin@Sun.COM>, <public-ws-chor@w3.org>
To: "Ugo Corda" <UCorda@SeeBeyond.com>

Equivalence is a tricky subject. The best treatment I have seen is that 
two systems are bi-similar. What this means is that wrt their 
observable behaviour they cannot be distinguished. There are two ways 
to do bi-simulation. The static method is to apply reductions to the 
process algebraic definitions of the two systems to see if they remain 
the same through the reduction. If they do then they are said to be the 
same. Problem with the approach is you end up with state explosion as 
you are effectively looking at their labelled state transitions. Some 
techniques for managing state explosion have been applied in different 
contexts and so it might be possible to do something akin to 
bi-simulation. The only other way I can think of is continual runtime 
monitoring which leads to runtime bi-simulation and does much the same 


Steve T

On 2 May 2004, at 18:14, Ugo Corda wrote:

>> How can you guarantee that the WSDL definitions at each end are
> 'semantically' equivalent?
> Yes, the issue of semantic equivalence of Web services and their WSDL
> interfaces has been raised a few times in the last couple of years in
> various committees (WS-I Basic Profile, W3C WS Architecture and, I
> think, WSDL WG). All the times the conclusion has been that such a
> subject is out of scope and should be best left alone for now.
> Right now we don't have any precise way of defining the semantics of a
> Web service, let alone establishing that two Web services are
> semantically equivalent.
> Ugo
>> -----Original Message-----
>> From: public-ws-chor-request@w3.org
>> [mailto:public-ws-chor-request@w3.org] On Behalf Of Monica J. Martin
>> Sent: Sunday, May 02, 2004 8:02 AM
>> To: david.burdett@commerceone.com
>> Cc: public-ws-chor@w3.org
>> Subject: Re: Abstract, portable and concrete choreographies
>> ... a proposed solution??
>>> 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. </DB>
>> 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 13:57:34 UTC

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