Asmentioned in the introduction, the information associatedwith an event is represented by a data structure called a refer to the datamodel or encoding schema of notifications as the event notification model or simply event model. Most existing event notification services adopt a simple record- like structure for notifications, while some more recent frameworks define an object-oriented model (., the Java™ Distributed Event Specification [Sun Microsystems 1998] and the CORBA Notification Service [Object Management Group 1998b]). Closely related to the eventmodel is the subscription language,which defines the form of the expressions associated with subscriptions. Two aspects of the subscription language are crucial to the issue of expressiveness
