MIRROR: a state-conscious concurrency control protocol for replicated real-time databases

Data replication can help database systems meet the stringent temporal constraints of current real-time applications, especially Web-based directory and electronic commerce services. A prerequisite for realizing the benefits of replication, however, is the development of high-performance concurrency control mechanisms. In this paper, we present managing isolation in replicated real-time object repositories (MIRROR), a concurrency control protocol specifically designed for firm-deadline applications operating on replicated real-time databases. MIRROR augments the classical O2PL concurrency control protocol with a novel state-based real-time conflict resolution mechanism. In this scheme, the choice of conflict resolution method is a dynamic function of the states of the distributed transactions involved in the conflict. A feature of the design is that acquiring the state knowledge does not require inter-site communication or synchronization, nor does it require modifications to the two-phase commit protocol. Using a detailed simulation model, we compare MIRROR's performance against the real-time versions of a representative set of classical replica concurrency control protocols for a range of transaction workloads and system configurations. Our performance studies show that (a) the relative performance characteristics of these protocols in the real-time environment can be significantly different from their performance in a traditional (non- real-time) database system, (b) MIRROR provides the best performance in both fully and partially replicated environments for real-time applications with low to moderate update frequencies, and (c) MIRROR's simple to implement conflict resolution mechanism works almost as well as more sophisticated strategies