Information Technology Reference
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there are small but important differences, such as the use of ontologies as
shared vocabularies, which promises a higher degree of automation. In partic-
ular, Semantic Web technologies are expected to have the most potential in
the following service-related tasks:
•
Discovery
. Before a service published on the Web or within an enterprise
can be used in a distributed application, it must first be located. Current
technologies such as UDDI support this step only with keyword search,
although limited forms of standardized vocabularies such as UNSPC
12
are
starting to be more widely used. As opposed to such standards,
semantic
annotations of service capabilities
via decentralized ontologies, intercon-
nected via logical axioms, by which inference can determine which ser-
vices match a goal, can be expressed equally well in ontological terms.
These descriptions could also involve more fine-grained notions such
for-
mal descriptions of preconditions and postconditions
,andofthe
inputs and
outputs
of the service using terms specified in an ontology.
•
Negotiation
. Whenever a suitable provider has been determined to serve a
certain goal, it is necessary to negotiate a service instance from the possibly
many services that a particular provider can offer. This includes establish-
ment of trust policies, determination of payment modalities, selection of
offers, etc., where corresponding semantic annotations are required. For
the purpose of automating this task, it is important that a semantic ser-
vice description not only specifies the functionality of a service, but also
includes
nonfunctional aspects
, such as supported policies, and security
protocols.
•
Composition
. In cases where a particular goal cannot be achieved by means
of a single Web service, semantic descriptions can at least help to deter-
mine a promising combination of multiple Web services to achieve the re-
quested functionality. Having learned the lessons of 20 years of AI research,
we would be naive to expect promises to be fulfilled that efforts towards
automated programming have not yet fulfilled. Nevertheless, clever selec-
tion of services and lightweight planning or scheduling can, hopefully, ease
the pain of manual integration. Composition requires not only the seman-
tic annotation of the overall capabilities of a service, but also a
behavioral
description
of how to interact with the Web service, in order to achieve a
certain functionality.
•
Invocation
. After a (combination of) services has been selected, the final
step is execution. To this end,
possible input and output values
need to be
extracted from the semantic goal description and
adapted
to the negotiated
message formats and communication protocols. Ideally,
monitoring and
transaction control
should be supported during invocation.
12
United Nations Standard Products and Services Code; see
http://www.unspsc.
org
.
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