Nowadays, information systems are getting larger and
richer in content (environmental information systems,
multimedia databases, satellite data, the world wide web)
and at the same time they also grow in number and diversity.
These systems are usually dynamic, in the sense that
information is added, removed and altered over time. In
certain domains, knowledge itself changes over time. This
phenomenon known as concept drift appears often in
medical, environmental, financial and other information
systems that are concerned with complex and evolving
physical systems. Furthermore, advances in data communication
have allowed the interconnection of modern information
systems through the Internet or private net works. Some systems are composed of physically distributed
smaller units and use networks for communication
and cooperation.