Snoop: An expressive event specification language for active databases

摘要

Making a database system active to meet the requirements of a wide range of applications entails developing an expressive event specification language and its implementation. Extant systems support mostly database events and in some cases a few predefined events.This paper discusses an event specification language (termed Snoop) for active databases. We define an event, distinguish between events and conditions, classify events into a class hierarchy, identify primitive events, and introduce a small number of event operators for constructing composite (or complex) events. Snoop supports temporal, explicit, and composite events in addition to the traditional database events. The novel aspect of our work lies not only in supporting a rich set of events and event expressions, but also in the notion of parameter contexts. Essentially, parameter contexts augment the semantics of composite events for computing their parameters. For concreteness, we present parameter computation for the relational model. Finally, we show how a contingency plan that includes time constraints can be supported without stepping outside of the framework proposed in this paper.