Transaction Scheduling in Distributed Real-Time Systems

Inthis paper, we study the performance of using optimistic approachto concurrency control in distributed real-time database systems(RTDBS). The traditional optimistic approach suffers from theproblem of unnecessary restarts. Transaction restarts can significantlyincrease the system workload and intensify resource and datacontention. In distributed environments, the complexity of thesystem and the high communication overhead exacerbate the problem.Therefore, the number of unnecessary restarts is the determinantfactor that affects the performance of optimistic approach indistributed RTDBS. When optimistic approach is extended to distributedenvironments, a number of issues resulting from the increasedcomplexity and communication overhead have to be resolved. Inthis paper, a new real-time distributed optimistic concurrencycontrol (DOCC) protocol with dynamic adjustment of serializationorder (DASO), called DOCC-DA is proposed. This protocol can avoidunnecessary transaction restarts by dynamically adjusting theserialization 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 validationscheme is included to facilitate transaction validation in distributedenvironments. The performance of the DOCC-DA protocol has beenexamined in detail by simulation. The results showed that theperformance of the DOCC-DA protocol is consistently better thanthat of other protocols.