In terms of interconnection topology, there are essentially two broad classes from which to choose: a hierarchy and a general graph. Existing distributed event notification services, such as JEDI [Cugola et al. 1998] and TIBCO’s product TIB/Rendezvous™, adopt a hierarchical topology. However, our analy- sis (presented in Section 6) shows that such a topology can exhibit significant performance problems. In SIENA we have adopted the general graph, which in common terms means that the servers are organized in a peer-to-peer rela- tionship, as we detail in Section 4. A hybrid of the two structures—whether a hierarchy of peers, or peers of hierarchies—is also a topology to consider, but requires.
