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Fig. 9.5.
Dimensions of mediation
of heterogeneities that can arise within the Semantic Web, and (2)
logical com-
ponents
that connect resources and apply the required mediation techniques.
These are embedded in a software architecture for dynamic invocation and
execution. Figure 9.5 shows the structure of the mediation model, which we
explicate below.
9.3.1 Mediation Levels and Techniques
The first dimension of the model is concerned with the types of heterogeneities
that can occur within the Semantic Web. The various heterogeneity types re-
quire specific techniques for mismatch resolution, which we refer to as
levels
of mediation
. The development of Semantic Web technology has revealed four
types of heterogeneity. We shall explain this categorization and describe me-
diation techniques for each level developed around WSMO, referring to other
material for more detailed explanation where necessary. The types of hetero-
geneity are as follows:
1.
Terminology.
Web services or other Web resources use different terminolo-
gies. For example one entity understands
name
to be the full name of a
person, and another one defines
name
to denote only the family name.
This can hamper successful interoperation on the semantic level.
2.
Representation format and transfer protocol.
Interacting resources use dif-
ferent formats or languages for information representation (e.g. HTML,
XML, RDF, or OWL), or different protocols for information transfer (e.g.
HTTP or RPC); incompatibilities on this level can obviously hamper use-
ful information interchange.
3.
Functionality.
This is specific to Web services, and refers to functionali-
ties of a provider and a requester that do not match exactly. This enforces
complex and thus expensive reasoning procedures for detecting Web ser-
vices that are usable for a given request. The need for such expensive
operations can be reduced by gaining and utilizing knowledge about the
functional heterogeneities, as explained in more detail below.
4.
Business process.
Also specific to Web services, this denotes mismatches
in the supported interaction behavior of Web services and clients. This
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