Composite Events in Chimera

In this paper, we extend event types supported by Chimera, an active object-oriented database system. Chimera rules currently support disjunctive expressions of set-oriented, elementary event types; our proposal introduces instance-oriented event types, arbitrary boolean expressions (including negation), and precedence operators. Thus, we introduce a new event calculus, whose distinguishing feature is to support a minimal set of orthogonal operators which can be arbitrarily composed. We use event calculus to determine when rules are triggered; this is a change of each rule's internal status which makes it suitable for being considered by the rule selection mechanism. The proposed extensions do not affect the way in which rules are processed after their triggering; therefore, this proposal is continuously evolving the syntax and semantics of Chimera in the dimension of event composition, without compromising its other semantic features. For this reason, we believe that the proposed approach can be similarly applied for extending the event language of other active database systems currently supporting simple events or their disjunction.