Reactivity on the Web: Event Queries and Composite Event Detection in XChange

Reactivity, the ability to detect events and respond automatically in a timely manner, is an essential requirement in many present-day information systems. The reactive, rule-based language XChange aims at filling the gap between the current, largely passive Web and the need for reactivity. XChange programs consist of rules, and each rule specifies events — happenings at (possibly remote) Web sites — it reacts upon using an event query. Event queries in XChange not only specify situations in which a rule reacts, but also extract and make available data from events that is relevant for the particular reaction. Oftentimes, the situations are not given by a single (atomic) event, but a series of events, leading to the notion of composite events and composite event queries. This work is dedicated to the design of XChange’s event query language, with an emphasis on the ability to query composite events. In this work, a number of language constructs are provided that allow to combine atomic event queries into composite event queries. Their intuitive meaning is underpinned with formal, declarative semantics for both atomic and composite event queries. By design, composite queries need only events with a bounded lifespan to be evaluated, and a proof of this based on the declarative semantics is offered in this work. Algorithms for the evaluation of event queries, in particular the detection of composite events, are provided and accompanied by a prototype implementation.