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

Data replication is one of the main techniques by which database systems can hope to meet the stringent temporal constraints of current time-critical applications, especially Web based directory and electronic commerce services. A pre-requisite for realizing the benefits of replication, however is the development of high performance concurrency control mechanisms. We present MIRROR (Managing Isolation in Replicated Real-time Object Repositories), a concurrency control protocol specifically designed for firm deadline applications operating on replicated real time databases. MIRROR augments the optimistic two-phase locking (O2PL) algorithm developed for non real time databases with a novel and simple to implement state based conflict resolution mechanism to fine tune real time performance. Using a detailed simulation model, we compare MIRROR's performance against the real time versions of a representative set of classical protocols for a range of transaction workloads and system configurations. Our performance studies show that: (a) the relative performance characteristics of replica concurrency control algorithms in the real time environment could 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 conflict resolution mechanism works almost as well as more sophisticated (and difficult to implement) strategies.