A system and method for controlling the execution of notifications in a computer system with multiple notification contexts. A RunOn operator enables context hopping between notification contexts. Push-based stream operators optionally perform error checking to determine if notifications combined into a push-based stream share a common notification context. Context boxes group together notification creators and associate their notifications with a common scheduler and notification context. Operators employ a composition architecture, in which they receive one or more push-based streams and produce a transformed push-based stream that may be further operated upon. Components may be used in combinations to implement various policies, including a strict policy in which all notifications are scheduled in a common execution context, a permissive policy that provides programming flexibility, and a hybrid policy that combines flexibility with error checking.