移动实时嵌套事务的并发控制

在移动计算环境中，事务移动性和无线网络固有的缺陷使得传统分布式实时事务管理机制不足以支持移动实时事务的执行，故有必要为移动实时事务研究新的事务处理机制以提高其成功率．该文着重研究移动实时事务的并发控制机制．首先，该文给出了一个考虑事务定时限制以及移动性的嵌套事务模型．然后，为减少移动分布式环境中解决数据冲突的开销，该文研究了一种结合优先级夭折和优先级继承的基于封锁的并发控制协议PAI-2PL．当高优先级事务被低优先级事务阻塞时，对于相同家族事务，采用优先级继承方法解决冲突；而对于不同家族事务，则夭折重启低优先级事务．另外，为减少由于断接所引起的无效阻塞，PAI-2PL允许低优先级事务夭折处于断接状态的高优先级事务．通过性能测试，表明所提出的事务模型及并发控制机制能提高实时事务的成功率．

A Concurrency Control Mechanism for Mobile Real-Time Nested Transactions

For the mobility of transactions and the inherence limitations of wireless network, the transaction management mechanisms for traditional distributed real-time transactions are incompetent to support the real-time transactions in mobile distributed environment to meet their deadlines. In the paper, a nested transaction model for mobile real-time transactions is presented, which takes the mobility and timing constraints into consideration. In order to reduce the overhead to resolve data conflicts in mobile computing environments, a concurrency control protocol PAI-2PL based on two-phase locking which integrates high priority abort (HPA) and priority inheritance (PI) schemes is proposed. As the high priority transactions are blocked by the low priority transactions, if the transactions belong to the same family, the PI scheme is used; otherwise the low priority transactions are selected to abort. In addition, in order to decrease futile blocking due to the disconnection of wireless network, it is permitted in PAI-2PL that the low priority transactions abort the disconnected high priority transactions. The performance of PAI-2PL is measured and analyzed in the end of the paper. It is shown that the suggested transaction model and concurrency control mechanism can increase the successful ratios of mobile real-time transactions.