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User Modeling for Personalized Web-Based Systems 217
is not necessarily the best option for representing user model in a web-based system. User
model must be able to store semi-structured data representing various domain-related user
characteristics. Moreover, relational databases currently lack the direct support for reasoning
mechanisms.
8.1.3 XML
Another approach to user model representation is to employ XML technologies (e.g., in
AHA! system [22]). XML provides good enough expressivity allowing, similarly to relational
databases, to store attribute-value pairs as values and attributes of XML tags.
Because building and maintaining user model is a non-trivial task, there is an effort to
alleviate the design of adaptive web-based systems and pull the user modeling part out of the
system into the form which would allow for its sharing across multiple adaptive applications.
Both relational databases and XML-based approaches fail to provide a good-enough support
for shareability. Relational databases are platform-dependent and an application needs
to know the exact used relational model (database schema) to be able to work with it.
While XML is a platform-independent and “web-ready” technology, it requires the definition
of a common vocabulary and agreement on syntax to serve for sharing purposes among
different adaptive applications.
8.1.4 Ontology
Ontology (in computer science) is defined as an “explicit formal specification of shared
conceptualization” [58]. By a term conceptualization we mean a formally represented, abstract
and simplified view of the world. Because term ontology encompasses a whole set of different
models with different degrees of semantic richness and complexity [52], we specify that by
using a term ontology, we mean a model expressed in OWL language 1 , based on RDF 2 .
Ontologies expressed in OWL language form ontological layer of the Semantic Web [54] and
OWL has a key role in realizing the Semantic Web vision [11].
OWL language can be expressed using XML and thus fulfills the basic preconditions
for shareability of models expressed with it. The language defines, along with RDF (more
precisely RDF Schema), a way how to describe resources using common dictionaries. The
basic concept of the language is a class, which has properties, instances and prescribes
relations to another classes (and their instances).
As adaptive web-based systems start to use ontology-based domain and user models,
we look for such model expression which would allow for the greatest possible reusability
and shareability. Directly shared models facilitates reuse of knowledge about domain and
user by multiple applications, but we can not assume that, in such a dynamic environment
as the Web is, one common universal model acceptable by all adaptive web-based systems
would emerge. That is the reason why OWL language defines means for ontology mapping
(owl:equivalentClass, owl:equivalentProperty).
1 OWL, Web Ontology Language, http://www.w3.org/2004/OWL/
2 RDF, Resource Description Framework, http://www.w3.org/RDF/