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 we-ren’t optimized for such scenarios and can’t take ad-vantage of regular group overlap patterns, a key fea-ture of our application domain. Furthermore, little is known about performance and scalability of such sys-tems in modern managed environments. We shed light on these issues and offer architectural insights based on our experience building QSM.