Composite Event Specification in Active Databases: Model & Implementation

Active database systems require facilities to specify triggers that fire when specified events occur. Wepropose a language for specifying composite events as event expressions, formed using event operators andevents (primitive or composite). An event expression maps an event history to another event history thatcontains only the events at which the event expression is ‘‘satisfied’’ and at which the trigger should fire.We present several examples illustrating how quite complex event specifications are possible using eventexpressions. In addition to the basic event operators, we also provide facilities that make it easier to specify compositeevents. ‘‘Pipes’’ allow users to isolate sub-histories of interest. ‘‘Correlation variables’’ allow users toensure that different parts of an event expression are satisfied by the same event, thereby facilitating thecoordination of sub-events within a composite event. We show how to efficiently implement event expressions using finite automata. Each event causes anautomaton to change state. When an automaton reaches an accepting state, a composite event of interest isrecognized, and the corresponding trigger fired. Events have attributes. For primitive events, these could be parameters of the activity that caused the event,selected parts of the database state, or functions computed therefrom. For composite events, attributes arederived from the attributes of the constituent primitive events. These attributes can be used in checkingconditions, and in any actions triggered. Event expressions can specify values (or sets or ranges of values)for particular attributes, and can even require that some attributes be equal. The composite event specifiedby the expression does not occur unless the specified condition on attributes is satisfied.