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that the second and third outgoing messages contain instances of
station
that can be mediated to
route
by the mappings defined above. Hence, the
process mediator applies the order inversion pattern in order to hold the first
message from
R
, and then - after data level mediation has taken place -
uses the merging pattern. The information can now be submitted to
P
in the
expected order and using the expected terminology.
Fig. 9.7.
Example of process-level mediation
Functional-Level Mediation
Heterogeneities at the functional level arise when the functionality provided
by a Web service does not precisely match that one requested by a client. For
instance, in our VTA scenario, a requester may define a goal of purchasing
a ticket to travel from Innsbruck to Vienna without specifying the type of
ticket (i.e. for a bus, train, or plane). An available Web service offers train
tickets from Innsbruck to Vienna. Here, the Web service is only usable for the
request under the condition that the ticket is a train ticket.
We expect situations like this to be common when Web services are used.
In order to determine the usability of a Web service for a given request -
commonly referred to as functional discovery, a central reasoning task for
automated Web service usage - complex and thus expensive reasoning pro-
cedures are required [66]. As this hampers the e
ciency of Semantic Web
technologies with regard to Web scalability, we use “
∆
-relations” to denote
functional heterogeneities and allow us to eliminate or reduce the need for
such expensive operations.
Techniques for Functional-Level Mediation
The central technique for functional-level mediation is the use of
∆
-relations
that denote the explicit logical relationship between functional descriptions
of Web services and goals. Functional descriptions are a central pillar of com-
prehensive Web service description frameworks such as OWL-S and WSMO.
Defined by conditions on preconditions and effects in some ontology language,
they provide a black-box description of normal runs of a Web service, omitting
information about the technical details of the invocation of the service [67].
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