Information Technology Reference
In-Depth Information
process model based on PSL [54]. In the SWSF approach, there are two in-
dependent formalizations of the conceptual model, both expressed in variants
of the underlying SWSL.
In the following we briefly review the conceptual model as it is formalized
by the FLOWS (First-order Logic Ontology for Web Services) ontology , based
on the SWSL-FOL language variant. The second axiomatization, ROWS (Rule
Ontology for Web Services), shares the conceptual model but provides a dif-
ferent set of concrete semantics, which rely on the semantics of SWSL-Rules,
this semantics being closer to that of logic programming.
The process model is the part of the FLOWS ontology which offers con-
structs to describe the behavior of the service based on the PSL approach. In
order to do so, two fundamental elements are added: (1) the structured notion
of atomic processes as found in OWL-S, and (2) the infrastructure for specify-
ing various forms of data flow. By the use of PSL, each of the constructs used
inherits the semantics defined in PSL, which overcomes some initial problems
with the underdefined workflow semantics of the OWL-S process constructs.
The core part of this PSL extended by FLOWS is called PSL Outer Core,
and the resulting FLOWS subontology is called FLOWS-Core. The process
ontology is made up of six parts, which are divided according to their expres-
sivity such as ordering constraints and occurrence constraints.
8.2.2 Language
SWSL is a language for describing the concept of Web services and for de-
scribing individual services. As previously mentioned, SWSL comes in two
variants: SWSL-FOL and SWSL-Rules. The design of both languages was
driven by compliance with Web principles such as the use of URIs, integra-
tion with XML built-in types, and XML-compatible namespaces and import
mechanisms. Both languages are layered languages where every layer includes
a number of new concepts that enhance the modeling power of the language.
This means that they are richer and more expressive than OWL-S, which is
based on OWL DL. Being based on first-order logic, FLOWS makes use of
logic predicates and terms to model the state of the world.
Features from situation calculus (see for instance [113]), such as the use of
fluents (i.e., predicates, and terms which vary over time), have been introduced
to model changes in the world, in a way compatible with PSL. However, PSL
has a more elaborate ontology of time and the occurrences of activities than
has pure situation calculus. For details, we refer to [9, 54].
SWSL-Rules is a logic programming language, which includes features
from courteous-logic programs [51], HiLog [26] and F-Logic [72] and can be
seen as both a specification and an implementation language. SWSL-Rules
provides support for service-related tasks such as discovery, contacting, and
policy specification. The language is layered as illustrated in Fig. 8.2.
The core of the SWSL-Rules language is represented by its pure Horn
subset. This subset is extended by adding features such as disjunction in the
Search WWH ::
Custom Search