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WG mediators connect Web services and goals when a Web service is not
usable for solving a goal a priori. The mediation techniques used are data-
level mediation via OO mediators, functional-level mediation to establish the
usability of a Web service for resolving a goal if not given a priori, and process-
level mediation for resolving potential mismatches on the communication level
between the source and target components.
WW mediators connect Web services that interact but are not compatible
a priori. Their source and target components are Web services. The related
mediation techniques are data-level via use of OO mediators, functional-level
mediation for handling functional heterogeneities, and process-level mediation
for resolving mismatches between the source and target service with respect
to communication and to coordination of interaction. Most commonly, the
source component of a WW mediator is a Web service
W
that aggregates
other Web services
W
1
,...,W
j
,...,W
n
in its orchestration, and the target
component is one of the aggregated Web services
W
j
.
In conclusion, the WSMO mediation model realizes Wiederhold's concep-
tion and can be considered su
cient for the Semantic Web. It defines architec-
tural components that apply mediation facilities that are appropriate for the
types of heterogeneity that can appear between the core elements of Semantic
Web service systems. The main merit of this model is that each mediator is
minimal
(i.e., it covers only a minimal aspect of heterogeneity handling), and
modular
(i.e., several mediators are combined for the purpose of specific appli-
cations). This enables reuse of mediation facilities and eases their maintenance
within dynamic and evolving environments such as the Internet.
9.4 Composition
Composition in the context of Semantic Web services refers to the automated,
online combination of existing services to meet some user goal or to the manual
or (semi-) automated, o
ine design of some composite service from existing
parts. In each case it is necessary to have some semantic description of the
composite service and how it is composed; this description is referred to as an
orchestration. An orchestration must necessarily define three aspects: (1) the
participants
in the composition; (2) the
control-flow
governing the order and
choice between execution of these participants (3) the
data-flow
governing
what is communicated between these participants.
9.4.1 Participants
It is usual within Semantic Web services that a composite service, on exe-
cution, will involve the execution of a number of participant services from
different providers. Furthermore, in the context of goal-driven execution de-
scribed in this section, it is possible that participants may be described as goals
rather than as concrete, grounded services. Therefore, even in the context of
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