Software systems rely on events for logging, coordination, handling unusual situations, and more. Monitoring events from systems that provide services can ensure that the service complies with policies, regulations, and other business rules. Notably, detecting violations of rules as early as possible is much desired as, for example, the service may reclaim resources from erring enactments. The primary goal of this paper is to develop techniques for detecting rule violations as early as possible. We formalize a model for events and a language to specify constraints on event timing and data. We develop algorithms to detect violations of individual rules at the earliest possible time, then use a chase process to detect violations of acyclic sets of rules. We also present optimization techniques to reduce monitoring overhead. Finally, we implement and evaluate our algorithms through experiments to demonstrate our approach is feasible and beneficial.