“…A number of designs and implementations of active databases now exist, and proposals have been made which build upon relational 33,18], extended relational 31], deductive 34,19] and object-oriented 10, 13, 17] data models. Active databases o er a number of advantages over their passive predecessors for a range of di erent categories of application, involving extensions to database systems 5,12,24], data intensive domains 8, 4] and control of external activities 3].…”