Portals represent the most recent development of multi-layered Web applications. Portals try to make contents, which are distributed over several nodes of different providers, available at one single node providing a consistent look and feel. Portal servers are based on so-called portlets, which arrange contents and application logics in a navigation structure and layout appropriate for the portal. An independent aggregation component is used to integrate the set of different portlets into a uniform Web site. This aggregation can be specified by the portal provider, or manipulated by the user, e.g., through customization. The schematic architecture could be seen in the following figure (according to Kappel et al. 2006):
