Introduction to WSMO - Enabling Semantic Web Services: The Web Service Modeling Ontology - page 55

• Ontology-based. Ontologies are used as data models throughout WSMO,

meaning that all resource descriptions and all data interchanged during
service usage are based on ontologies. Ontologies are a widely accepted
state-of-the-art knowledge representation, and have thus been identified
as the central enabling technology for the Semantic Web. The extensive
usage of ontologies allows semantically enhanced information processing
and support for interoperability; WSMO also supports the ontology lan-
guages defined for the Semantic Web.

• Strict decoupling. Decoupling denotes that WSMO resources are defined

in isolation, meaning that each resource is specified independently without
regard to possible usage or interactions with other resources. This complies
with the open and distributed nature of the Web.

• Centrality of mediation. As a complementary design principle to strict

decoupling, mediation addresses the handling of the heterogeneities that
naturally arise in open environments. Heterogeneity can occur in terms of
data, the underlying ontology, the protocol, or the process. WSMO recog-
nizes the importance of mediation for the successful deployment of Web
services by making mediation a first-class component of the framework.

• Ontological role separation. Users or more generally, clients exist in specific

contexts which will not be the same as those of the available Web services.
For example, a user may wish to book a holiday according to preferences
about weather, culture, and childcare, whereas Web services will typically
cover airline travel and hotel availability. The underlying epistemology
of WSMO differentiates between the desires of users or clients and the
available services.

• Description versus implementation. WSMO differentiates between the de-

scriptions of elements of Semantic Web services (description) and exe-
cutable technologies (implementation). While the former requires a con-
cise, sound description framework based on appropriate formalisms in or-
der to provide concise semantic descriptions, the latter is concerned with
the support of existing and emerging execution technologies for the Seman-
tic Web and Web services. WSMO aims to provide an appropriate onto-
logical description model, and to be compliant with existing and emerging
technologies.

• Execution semantics. In order to verify the WSMO specification, the for-

mal execution semantics of reference implementations such as WSMX, as
well as other WSMO-enabled systems, provide the technical realization of
WSMO.

• Services versus Web services. A Web service is a computational entity

which is able to achieve a user's goal by invocation. A service, in contrast,
is the actual value provided by this invocation [6, 111]. 1 WSMO provides

1 Note that [111] also distinguishes between a computational entity in general and
a Web service, where the former does not necessarily have a Web-accessible in-
terface. WSMO does not make this distinction.