background image
100
S y s t e m O ve r v i e w
The architecture of the system we designed to federate the previously described Digital
Libraries is based on the use of XML files, named Concept Trees and Mapping Trees, that store
key information that controls how all modules of the system work. Concept Trees representing
all relevant "searchable" concepts of the component databases are used to help the user build the
query. Each underlying database has a Mapping Tree that is used to translate the query to the
appropriate database query language.
Architecture of the System
The architecture of the system described in this paper is composed of four separate layers.The
communication among them is carried out by using two exchange, intermediate languages, exclu-
sively defined for this purpose.This architecture, shown in Illustration 1, is fully described else-
where.
11
What follows is only a brief overview:
1. Layer 1. User Interface:The user interface is generated every time a user accesses the
system, so it is not a real layer of the architecture. However, we represent it as a con-
ceptual layer in order to describe properly the interaction of the users with our sys-
tem, because all interactions (queries and answers) are made through the user interface.
2. Layer 2. Mediator:The Mediator generates the user interface following the Concept Tree,
as we shall see in the next section. After a user expresses his query, the Mediator analyses
and redirects it to the Wrappers of the databases involved in that query. After the query is
executed in the pertinent databases, the Mediator shows the user an answer Web page
with a summary of the results obtained from the databases as well as the appropriate links
to the answer interfaces of these digital libraries, so the user can navigate through the
obtained documents.
11
See N. R. Brisaboa, M. R. Pen-
abad, A. S. Places, and F. J. Rodríguez,
"Ontologías en Federación de Bases
de Datos," Novática 158 (2002):
45-54, http://www.ati.es/novatica/
2002/158/nv158sum.html.
Illustration 1: Architecture of the System.
DC_Emblemsbook_180204 19.02.2004 11:26 Uhr Seite 100