Intuitively, a simple event notification service that provides no selection mechanism can be reduced to a multicast routing and transport mechanism for which there are numerous scalable implementations. However, once the service provides a selection mechanism, then the overall efficiency of the ser- vice and its routing of notifications are affected by the power of the language used to construct notifications and to express filters and patterns. As the power of the language increases, so does the complexity of the processing. Thus, in practice, scalability and expressiveness are two conflicting goals that must be traded off
