Combining Nonmonotonic Knowledge Bases with External Sources

The developments in information technology during the last decade have been rapidly changing the possibilities for data and knowledge access. To respect this, several declarative knowledge representation formalisms have been extended with the capability to access data and knowledge sources that are external to a knowledge base. This article reviews some of these formalisms that are centered around Answer Set Programming, viz. HEX-programs, modular logic programs, and multi-context systems, which were developed by the KBS group of the Vienna University of Technology in cooperation with external colleagues. These formalisms were designed with different principles and four different settings, and thus have different properties and features; however, as argued, they are not unrelated. Furthermore, they provide a basis for advanced knowledge-based information systems, which are targeted in ongoing research projects.