Extending SWRL to Express Fully-Quantified Constraints

Drawing on experience gained over a series of distributed knowledge base and database projects, we argue for the utility of an expressive quantified constraint language for the Semantic Web logic layer. Our Constraint Interchange Format (CIF) is based on classical range-restricted FOL. CIF allows the expression of invariant conditions in Semantic Web data models, but the choice of how to implement the constraints is left to local reasoners. We develop the quantified constraint representation as an extension of the current proposal for a Semantic Web Rule Language (SWRL). An RDF syntax for our extended CIF/SWRL is given in this paper. While our approach differs from SWRL in that existential quantifiers are handled explicitly rather than using OWL-DL constructs, we believe our proposal is still fully compatible with the use of the various OWL species as well as RDFS. We demonstrate the use of the CIF/SWRL representation in the context of a practical Semantic Web reasoning application, based on the CS AKTive Space demonstrator (the 2003 Semantic Web Challenge winner). We indicate where in our application it makes sense to use the existing SWRL directly, and where our CIF/SWRL allows more complex constraints to be expressed in a natural manner.