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The development of complex distributed systems demands for the creation of suitable architectural styles (or paradigms) and related run-time infrastructures. An emerging style that is receiving increasing attention is based on the notion of event. In an event-based architecture, distributed software components interact by generating and consuming events. An event is the occurrence of some state change in a component of a software system, made visible to the external world. The occurrence of an event in a component is asynchronously notified to any other component that has declared some interest in it. This paradigm (usually called "publish/subscribe" from the names of the two basic operations that regulate the communication) holds the promise of supporting a flexible and effective interaction among highly reconfigurable, distributed software components.In the past two years, we have developed an object-oriented infrastructure called JEDI (Java Event-based Distributed Infrastructure). JEDI supports the development and operation of event-based systems and has been used to implement a significant example of distributed system, namely, the OPSS workflow management system (WFMS). The paper illustrates JEDI main features and how we have used them to implement OPSS. Moreover, the paper provides an initial evaluation of our experiences in using the event-based architectural style and a classification of some of the event-based infrastructures presented in the literature. KeywordsEvent-based systems, distributed systems, software architectures, workflow, business processes, object-orientation, publish/subscribe middleware.Convergence between telecommunication, broadcasting, and computing is opening new opportunities and challenges for a potentially large market of innovative network-wide services. The class of users interested by this revolution is significantly large: families, professionals, large organizations, government agencies, and administrations. The services range from home banking and electronic commerce, to coordination and workflow support for large dispersed teams, within the same company or even across multiple companies. Many research and industrial activities are currently being carried out to identify feasible strategies to develop and operate these services in an effective and economically viable way. The requirements and technical problems that have to be addressed are complex and critical:• Services must be able to operate on a wide area network with acceptable performance.• The software technology used to implement these services must be "light", i.e., it should be scalable in terms of the number of both components and users involved and of their distribution.• The technology must enable a "plug and play" approach to support dynamic reconfiguration and introduction of new service components.• Finally, it is essential to support openness and interoperability between different platforms since the services are usually implemented in a heterogeneous hardware infrastructure.To foster the diffusion of netw...

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