论文部分内容阅读
Most of the proposed concurrency control protocols for real-time database systems are based on serializability theorem. Owing to the unique characteristics of real-time database applications and the importance of satisfying the timing constraints of transactions, serializability is too strong as a correctness criterion and not suitable for real-time databases in most cases. On the other hand,relaxed serializability including epsilon-serializability and similarity-serializability can allow more real-time transactions to satisfy their timing constraints, but database consistency may be sacrificed to some extent. We thus propose the use of weak serializability(WSR)that is more relaxed than conflicting serializability while database consistency is maintained. In this paper, we first formally define the new notion of correctness called weak serializability. After the necessary and sufficient conditions for weak serializability are shown,corresponding concurrency control protocol WDHP(weak serializable distributed high priority protocol) is outlined for distributed realtime databases, where a new lock mode called mask lock mode is proposed for simplifying the condition of global consistency. Finally,through a series of simulation studies, it is shown that using the new concurrency control protocol the performance of distributed realtime databases can be greatly improved.