Transaction scheduling in distributed real-time systems

In this paper, we study the performance of using optimistic approach to concurrency control in distributed real-time database systems (RTDBS). The traditional optimistic approach suffers from the problem of unnecessary restarts. Transaction restarts can significantly increase the system workload and intensify resource and data contention. In distributed environments, the complexity of the system and the high communication overhead exacerbate the problem. Therefore, the number of unnecessary restarts is the determinant factor that affects the performance of optimistic approach in distributed RTDBS. When optimistic approach is extended to distributed environments, a number of issues resulting from the increased complexity and communication overhead have to be resolved. In this paper, a new real-time distributed optimistic concurrency control (DOCC) protocol with dynamic adjustment of serialization order (DASO), called DOCC-DA is proposed. This protocol can avoid unnecessary transaction restarts by dynamically adjusting the serialization order of the conflicting transactions. Therefore, resources can be saved and more transactions can meet their deadlines. In the DOCC-DA protocol, a new distributed circular validation scheme is included to facilitate transaction validation in distributed environments. The performance of the DOCC-DA protocol has been examined in detail by simulation. The results showed that the performance of the DOCC-DA protocol is consistently better than that of other protocols.