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support these users to find services for their needs, ideally by reasoning about
the services in terms of the user's domain language.
Therefore, as pointed out in [127], components should not only be defined
via technical, machine-readable low-level descriptions of their program inter-
faces (the “how”), but also by “the higher-level semantics of the underlying
tools, allowing each user to specify his or her requirements in terms of their
own, problem-specific terminology” (the “what”). The common way of pro-
viding problem-specific or domain-specific terminology is the definition of a
controlled vocabulary by means of an ontology, and to use the ontology as
classification scheme for the objects of the application domain. In the case
of services in workflow management systems, an ontology would provide a
service classification scheme, into which all available services are sorted.
Fig. 1.6
Exemplary semantic service classification
Revisiting the image processing workflow example from Section 1.1.1, a
simple hierarchical service classification scheme may for instance look like
that depicted in Figure 1.6: the various available
image processing
services are
classified further into
data input
,
image conversion
and
data output
services.
While
data input
comprises different services for reading pictures from a
file
,
camera
or
database
,the
data output
group contains different services
for
saving
,
printing
and
e-mailing
images. The
image conversion
services
are then further distinguished into the categories
image improvement
,
image
adaptation
and
image extension
, into which then, for instance, the different
ImageMagick services (cf. Section 1.1.1) can be sorted. This way, both human
users and software applications can easily identify services for a particular
purpose by using the terminology of the domain-specific service vocabulary.
A thorough semantic handling of services would at the same time address
the issue of service transparency: For instance, the provider-oriented classifi-
cation of services that is present in many systems leads to the situation that
the principally same functionality occurs multiple times in the service list,
forcing the workflow developer to decide for one specific server. This may be
necessary and useful in some cases, but “it contradicts the idea of preventing
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