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Comments on WSDL-S abstract

From: David Martin <martin@AI.SRI.COM>
Date: Fri, 11 Nov 2005 06:59:21 -0800
Message-ID: <4374B1C9.2050700@ai.sri.com>
To: public-sws-ig@w3c.org

Hi Rama, Joel, John, Meenakshi, Marc-Thomas, Amit, and Kunal -

Congratulations on the acknowledgement of the WSDL-S submission.

Since the submission abstract mentions some points of comparison with
OWL-S, I would like to offer some clarification and response regarding
these points.  I have copied the abstract below (quoted) and I respond
inline.  My primary objective here is to avoid any misperceptions that
might create unnecessary barriers to collaboration and synthesis of
WSDL-S with OWL-S.

> Abstract:
> The current WSDL standard operates at the syntactic level and lacks
> the semantic expressivity needed to represent the requirements and
> capabilities of Web Services. Semantics can improve software reuse and
> discovery, significantly facilitate composition of Web services and
> enable integration of legacy applications as part of business process
> integration. The Web Service Semantics document defines a mechanism to
> associate semantic annotations with Web services that are described
> using Web Service Description Language (WSDL). It is conceptually
> based
> on, but a significant refinement in details of, the original WSDL-S
> proposal [WSDL-S] from the LSDIS laboratory at the University of
> Georgia. In this proposal, we assume that formal semantic models
> relevant to the services already exist. In our approach, these models
> are maintained outside of WSDL documents and are referenced from the
> WSDL document via WSDL extensibility elements. The type of semantic
> information that would be useful in describing a Web Service encompass
> the concepts defined by the semantic Web community in OWL-S and
> other efforts [METEOR-S, WSMO]. The semantic information specified in
> this document includes definitions of the precondition, input, output
> and effects of Web service operations. This approach offers multiple
> advantages over OWL-S.

First, the wording just above seems to imply (or could imply to some
readers) that WSDL-S is a competing approach with OWL-S, in the sense
that it could replace OWL-S.  Since WSDL-S does not define a semantic
model, it is potentially confusing to suggest this.  I suggest that you
might say something like "offers advantages over the exclusive use of
OWL-S", but also further emphasize that OWL-S offers one particular
semantic model that could be referenced using WSDL-S elements.  (Below I
suggest some specific, straightforward examples of how this could work.)

Second, it seems odd to single out OWL-S in this regard.  As I
understand your points below, to the extent they are valid, they apply
equally to WSMO and other approaches that propose a particular semantic
model.  Thus, I suggest it would be more helpful to the reader to say
something like "offers advantages over the exclusive use of OWL-S, WSMO,
or other single-semantic-model approaches".

> First, users can describe, in an upwardly compatible way, both the
> semantics and operation level details in WSDL-
> a language that the developer community is familiar with.

As expressed above, this claim (that semantics can be described "in
WSDL") is misleading.  As your abstract states above, the "formal
semantic models ... already exist ... and are maintained outside of WSDL
documents."  As the first sentence of your abstract states, WSDL "lacks
the semantic expressivity needed to represent the requirements and
capabilities of Web Services".  Just below, you claim that
"externalizing the semantic domain models" is an advantage.  Claiming
that semantics can be described "in WSDL" is clearly inconsistent with
these other claims.  I suggest that you shouldn't (and needn't) claim to
have it both ways.

> Second, by externalizing the semantic domain models, we take an
> agnostic approach to ontology representation languages. This allows
> Web service developers to annotate their Web services with their
> choice of ontology language (such as UML or OWL) unlike in OWL-S.

There is indeed a substantive point here, and I agree it can be an
advantage, for some purposes, to have the flexibility to refer to a
variety of semantic specifications.  For other purposes, however, this
could be viewed as a distinct *disadvantage*.  For example, for Web
service tool vendors who need to know what semantic models they need to
support, it is distinctly unhelpful to stipulate that "anything goes".
However, there is much to be said on both sides of that particular issue
and I won't dwell on it here.

Again, it's a bit puzzling as to why OWL-S is singled out in this
regard.  In fact, it is somewhat misleading to suggest that OWL-S
somehow precludes the use of other semantic models.  If you disregard
the WSDL extension tags proposed by OWL-S (which I discuss at the end of
this message, and which can be regarded as optional), there is nothing
in OWL-S that precludes the use of other semantic models.  That is, an
OWL-S service description exists externally of WSDL, and is grounded to
a particular WSDL service by specifying a mapping of certain OWL-S 
constructs to certain WSDL constructs.  There's no reason in principle 
why the same WSDL service, or other WSDL services used with it, cannot 
be described using other semantic models.

In any case, I suggest that it would be beneficial to the community to
emphasize that OWL-S (or, more likely, some derivative "lite" version of
OWL-S) and WSDL-S can potentially be used together.  For example, a
WSDL-S modelReference element attached to a WSDL operation could refer
to an OWL-S atomic process (and this would be perfectly consistent with
existing practice in OWL-S, where the grounding ontology also specifies
such mappings).  A WSDL-S modelReference attached to a WSDL input or
output element could refer to an input or output of an OWL-S atomic
process.  A WSDL-S category element could reference a subclass of
Profile in a class hierarchy of OWL-S profiles.  A WSDL-S schemaMapping
element could refer to an XSLT script that OWL-S already uses to define
a mapping between OWL-S input/output and WSDL I/O types defined in XML
schema.

> This is significant
> because the ability to reuse existing domain models expressed in
> modeling languages like UML can greatly alleviate the need to
> separately model semantics. Finally, it is relatively easy to update
> the existing tooling around the WSDL specification to accommodate our
> incremental approach.

This final claim also is misleading.  For one thing, as I explain above,
OWL-S actually requires *less* change to tools that handle WSDL
specifications, because we have proposed only a few WSDL extensions that
may be considered optional.  For another thing, it's pretty much a moot
point.  What you are suggesting, I think, is that a WSDL tool can easily
be updated not to choke on any of the proposed WSDL-S extension
elements.  This is certainly true of WSDL-S proposed extension elements,
as it is of OWL-S extension elements.  But of course it must be noted
that just by accepting the syntax of a proposed extension element, you
aren't getting any new functionality.  To update a tool to actually *do
something useful* with the external semantic models, of course, likely
involves very substantial changes to the tool.

I grant, however, that a WSDL vendor could say, for example, "in our
next version we will support the use of the WSDL-S modelReference
element attached to operations and referring to a UML diagram", and yes,
that could be very nice for WSDL vendors.  But note that, by the same
token, the vendor could offer to support the use of modelReference
referring to an OWL-S atomic process, or, for that matter, they could
offer to support the use of "owl-s-process" (an extension element
proposed by OWL-S) referring to an atomic process.

Finally, I would like to point out that OWL-S introduced the idea of
WSDL extension elements to refer to external semantic models with our
0.7 release in Oct. 2002.  This is a public release and these proposed
extension elements are also discussed in at least one conference
publication.  To my knowledge, this fact has never been mentioned in any
publications associated with WSDL-S.  Granted, they are not emphasized
very much in OWL-S work, and are not developed very extensively, but,
nevertheless, they are there.  See, for example Section 6.2 in the
Technical Overview at http://www.daml.org/services/daml-s/0.7/daml-s.html.

In summary, my main point here is that by and large WSDL-S and OWL-S are
compatible, and most aspects of WSDL-S are already consistent with the
approach supported by the OWL-S grounding.  No doubt some details need
to be worked out, but I think we can benefit the community of interest
by working together on these details, and hopefully there will be
opportunities to do so :-).

Best regards,
David Martin
Received on Friday, 11 November 2005 15:07:05 GMT

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