Semantic Web Service Usage Tasks in WSMO - Enabling Semantic Web Services: The Web Service Modeling Ontology - page 110

does not stay the same, but changes over time. For example, a hotel will not
be able to book a room with a single bed on a specific date if all such rooms
in the hotel are already booked on that date.
Since clients are basically interested in finding abstract services which can
actually solve their problem at hand (as specified in a WSMO goal), discovery
in general needs to consider these dynamics in order to create accurate results.
This basically means that purely static descriptions of Web services are not
su cient in the general case. In applications where highly accurate results are
requested, Web service descriptions will have to contain a dynamic component
as well.
On a description level, there are two main options to achieve a proper
representation of a dynamically changing abstract service from a provider:
(1) use a purely static description of the abstract service and change the
description in its entirety every time the abstract service changes; (2) use
a static description where some parts refer to a well-defined resource that
reflects currently valid information about the dynamic part (for instance, a
database table containing all available rooms at a hotel at a certain point in
time). Clearly, the first approach is not really scalable, since constant changes
of a stored abstract-service description are needed. The second approach is
a viable one, as we can see with the dynamically generated Web pages of
online stores such as Amazon.com that provide an up-to-date perspective of
the prices of individual topics. In the latter case, we simply externalize the
details of the changing aspects of the abstract service and provide a reference
in the remaining part of the Web service description. Hence, the abstract-
service description, including the reference, does not need to change over time
when the abstract service of the provider changes, and hence the Web service
descriptions become more compact.
Nonetheless, the latter approach requires communication with the provider
of an abstract service (e.g. via a Web service that accesses a database table)
and thus can potentially be a costly step in terms of computation time. Hence,
there is a clear trade-off between the accuracy of discovery results and the
e ciency of the discovery process, which must be considered for any specific
application that leverages Web service discovery.
A Conceptual Model for Service Discovery
Approaches to automating discovery of suitable Web services must analyze
precisely what kinds of descriptions can be used for capturing the static and
dynamic aspects of a given Web service, and how such descriptions can be
exploited for e ciently and accurately locating a requested service. While
a number of proposals are available in our area of interest, (e.g. [11, 48, 84,
106, 126]), none of them has specifically addressed these aspects. Rather, they
have focused mainly on specific description languages and frameworks, partly
neglecting the overall needs. Therefore, we shall first define a model that
takes into account pragmatic considerations and defines borderlines between