In this paper, we investigate the benefits and challenges of integrating history into a near-real-time monitoring system; and present a general purpose continuous monitoring engine, called Moirae, that supports this integration. Moirae is designed to enable different types of queries over live and historical data. In particular, Moirae supports (1) queries that look up specific historical information for each newly detected event and (2) queries that complement new events with information about similar past events. Moirae focuses on applications where querying a historical log in its entirety would be too slow to meet application needs, and could potentially yield an overwhelming number of results. The goal of the system is to produce the most relevant approximate results quickly and, when necessary, additional more precise results incrementally. In this paper, we discuss the challenges of integrating history into a continuous monitoring engine, present the design of Moirae, and sho...