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176 Selected Studies on Software and Information Systems
Physical server 1
Cocoon
SOAP
Physical server 2
WS: Factic
Browser
HTTP
HTTP
JOP
Business
logic
&
presentation
SOAP
SOAP
Physical server 3
WS: JDBSearch
Physical server 4
WS: SemanticLog
Figure 6-7. SOA based job offer portal.
SOA based JOP has also weaknesses. One of them may be the transformation of the Factic tool
into web service. This causes problems from the point of view that in Factic the definition
of the facets is realized by a user in the graphical user interface and there is intensive
interaction with him. Other cases are components which are intensively called, for example
SemanticLog. It is overextended if several thousands of users work with the system. The
realization of the logging as web service is convenient but it may slow down the application.
However, this depends on the concrete implementation and service distribution over physical
servers.
SOA is not universally the best way how any system can be built. We presented JOP,
a Semantic web portal for job offer domain to discuss different aspects of SOA. From this
it arises that SOA brings benefits but its usage has also disadvantages in some situation.
These positive and negative aspects must be taken into account before we decide to develop
application based on SOA.
6.2 Semantic Web Services Description
The vision of Semantic web services brought a need of additional meta-data to the description
of WSDL based services. This meta-data must contain enough information for understanding
what the service does and how it should be used. All of this must be described in such a way
that machines can process it.
There exist several formalisms for Semantic web service description [77]. They annotate
elements of services with terms from domain models, including industry standards, vocabularies,
taxonomies, and ontologies. The most know approaches are WSDL-S [1], OWL-S
[63] and WSMO [70]. Each of these allows adding semantic information to services to be
able to perform effective discovery, composition, execution, and monitoring. They differ in
the complexity and expressivity of their construction elements. Nevertheless, there are some
similarities [65]. Even though existing approaches are a step forward in semantic annotation
of web services, [47] argues that they are insufficient for the efficient usage in SOA. The
problems occur mainly when composition of web service is required.
Web service description is performed manually or using (semi)automatic approaches.
In [37] a tool for annotating Semantic web service is presented. This tool provides a user
interface for manual annotation, as well as machine-learning assistance for semi-automatic