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within the workflow). Results and outputs may depend on a condition, again
expressed in a third-party logical language (SWRL, KIF, or DRS).
Again, a possible problem, besides that of conditions, is that the semantics
of the workflow constructs is not expressible in the description logics under-
lying OWL, for which reason this semantics has been externally defined [98].
8.1.3 Service Grounding
In order to map to the Web service world, an OWL service can
support
a
grounding which maps the constructs of the process model to detailed speci-
fications of message formats, protocols, and so forth. In detail, OWL-S allows
one to map atomic processes to WSDL operations and their inputs and out-
puts to WSDL messages. These message mappings might have XSLT trans-
formations attached, in order to solve the problem of lifting/lowering from the
ontological representation in OWL to XML Schema in WSDL.
1
.Notethat,
although WSDL is the only completely defined grounding for OWL, OWL-S
does not restrict itself to WSDL as the only underlying service technology.
Rather, OWL-S should be understood as a general service description ontol-
ogy, and is extensible to other grounding mechanisms.
8.1.4 Relationship Between OWL-S and WSMO
The most obvious difference between OWL-S and WSMO is probably that
the former is specified using the Web Ontology Language, whereas WSMO
uses an abstract MOF model (as introduced in Section 5.3). By reusing OWL
as a recommended standard, OWL-S gained considerable momentum. On the
other hand, OWL-S defines its meta model in the same language that it uses
for concrete service descriptions, which may be viewed as a disadvantage.
WSMO's basis in the abstract MOF model successfully avoids this problem.
Moreover, OWL-S has the disadvantage of the need to “retrofit” more ex-
pressive languages into the OWL framework which then opens up new research
questions on how they should interact. WSMO aims to avoid this problem
upfront by providing a single language framework for expressing ontologies,
conditions, and also dynamic aspects of WSMO within a single language,
WSML.
When the language suggestions for WSML and OWL-S are compared, it
turns out that OWL-S aims at combining various notations and semantics with
OWL for the description of service conditions and effects, while WSML takes a
more cautious approach. WSML does not distinguish between languages used
for inputs/outputs and other description elements of the Web service, but
rather provides one uniform language for capability descriptions. Additionally,
1
For a more in-depth discussion of what is meant by lifting/lowering, we refer to
Section 9.5 later in this topic, where this problem is discussed in the context of
WSMO.
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