Book excerpt: Doctoral Thesis / Dissertation from the year 2006 in the subject Computer Science - Applied, grade: 1.0, University of Dusseldorf "Heinrich Heine", language: English, abstract: Federated information systems provide access to interrelated data that is distributed over multiple autonomous and heterogeneous data sources. The integration of these sources demands for flexible and extensible architectures that balance both, the highest possible autonomy and a reasonable degree of information sharing. In current federated information systems, the integrated data sources do only have passive functionality with regard to the federation. However, continuous improvements take the functionality of modern databases beyond former limits. The significant improvement, on which this work is based on, is the ability of modern active database systems to execute programs written in a standalone programming language as user-defined functions or stored procedures from within their database management systems. We introduce Enhanced Active Database Systems as a new subclass of active databases that are able to interact with other components of a federation using external program calls from within triggers. We present several concepts and architectures that are specifically developed for Enhanced Active Databases to improve interoperability and consistency in federated information systems. As the basic concept we describe Active Event Notifications to provide an information system with synchronous and asynchronous update notifications in real-time. Based on this functionality, Enhanced Active Databases are able to actively participate in global integrity maintenance executing partial constraint checks on interrelated remote data. Furthermore, we present an architecture for a universal wrapper component that especially supports Active Event Notifications, which makes it perfectly suitable for event-based federated systems with real-time data processing. This tightly coupled wrapper architecture is used to build up the Dígame architecture for a peer data management system with push-based data and schema replication. Finally, we propose a Link Pattern Catalog as a guideline to model and analyze P2P-based information systems.