This paper proposes a conceptual model for modeling static and dynamic event sources by reusing the information in WSDL. A set-theoretic semantics for event subscription is introduced, based on which two metrics, recall and precision, are proposed to measure the accuracy of event subscriptions. We discuss the accuracy of several event subscription strategies under the framework of current web service event subscription standard proposals (WS-Eventing and WS-Notification). Four major types of event broker design patterns are discussed based on two visibility/reachability factors: if sink knows the source and if source can deliver events directly to the sink. The implication on the broker state and message routing is studied in this analysis. A prototype implementation indicated that these design patterns are feasible.