Introduction to WSMO - Enabling Semantic Web Services: The Web Service Modeling Ontology - page 57

5.3 The Language for Defining WSMO

WSMO is meant to be a metamodel for aspects of Semantic Web services. To
introduce this model, we make use of the Meta-Object Facility (MOF) [103]
specification, which defines an abstract language and framework for specifying,
constructing, and managing technology-neutral metamodels. MOF defines a
metadata architecture consisting of four layers, namely:

• The information layer comprises the data we want to describe.

• The model layer comprises the metadata that describe the data in the

information layer.

• The metamodel layer comprises the descriptions that define the structure

and semantics of the metadata.

• The metametamodel layer comprises the description of the structure and

semantics of the metametadata.

In terms of the four MOF layers, the language in which WSMO is defined
corresponds to the metametamodel layer; WSMO itself constitutes the meta-
model layer; the specifications of the ontologies, services, goals, and mediators
constitute the model layer; and the actual data described by the ontologies and
exchanged between Web services constitute the information layer. Figure 5.2
shows the relation between WSMO and the MOF layered architecture.

MOF

metaŦmetaŦmodel M3 layer

metaŦmodel M2 layer WSMO

model M1 layer WSMO Descriptions

information M0 layer Concrete Web Services, Domains and Data to be described

Fig. 5.2. The relation between WSMO and MOF

The MOF metamodeling construct used most in the definition of WSMO
is the Class construct (and, implicitly, its class generalization ( sub-Class )
construct), together with its Attributes ,the type of the Attributes ,andtheir
multiplicity specifications. In defining WSMO, the following assumptions are
made:

• Every Attribute has its multiplicity set to “multi-valued” by default; when

an Attribute requires its multiplicity to be set to “single-valued”, this is
explicitly stated in the listings where WSMO elements are defined.

• Some WSMO elements define Attributes that take values from a union

of several types, a feature that is not directly supported by the MOF
metamodeling constructs; this can be simulated in MOF by defining a
new Class as a super-Class of all the types required in the definition of