The asynchrony, heterogeneity, and inherent loose coupling that characterize applications in a wide-area network promote event interaction as a natural design abstraction for a growing class of software systems. An emerging build- ing block for such systems is an infrastructure called an event notification ser- vice [Rosenblum and Wolf 1997]. We envision a ubiquitous event notification service accessible fromevery site on a wide-area network and suitable for supporting highly distributed appli- cations requiring component interactions ranging in granularity from fine to coarse. Conceptually, the service is implemented as a network of servers that provide access points to clients. Clients use the access points to advertise infor- mation about events and.
