Reactivity, the ability to detect events and respond to them
automatically through reactive programs, is a key requirement in many
present-day information systems. Work on Web Services re
ects the need
for support of reactivity on a higher abstraction level than just message
exchange by HTTP. This article presents the composite event query facilities
of the reactive rule-based programming language XChange. Composite
events are important in the dynamic world of the Web where
applications, or Web Services, that have not been engineered together
are composed and have to cooperate by exchanging event messages