QSM is a multicast engine designed to support a style of distributed programming in which application objects are replicated among clients and updated via multicast. The model requires platforms that scale in dimensions previously unexplored; in particular, to large numbers of multicast groups. Prior systems werenpsilat optimized for such scenarios and canpsilat take advantage of regular group overlap patterns, a key feature of our application domain. Furthermore, little is known about performance and scalability of such systems in modern managed environments. We shed light on these issues and offer architectural insights based on our experience building QSM.