Structural Issues in Active Rule Systems

Active database systems enhance the functionality of traditional databases through the use of active rules or`triggers'. There is little consensus, though, on what components should be included in a rule system. In this paper, the expressive power of some simple active database rule systems is examined and the eeect of choosing diierent features studied. Four important parameters of variation are presented, namely the rule language, the external query language, the meta rule language and the pending rule structure. We show that each of these is highly innuential in determining the expressiveness of the rule system as a whole, and that an appreciation of them can serve as a basis for understanding the broader picture of system behaviour.