移动实时事务的相对时间正确性及其实现

Relative Time Correctness for Mobile Real-Time Transactions and Its Implementation

随着移动通信技术的迅速发展 ,人们提出了新的应用要求 :在移动环境下处理实时事务 .而移动通信的带宽有限性引起较大的数据访问延迟 ,有时甚至由于网络传输的断接而使得事务得不到所需访问的数据 ,这些严重影响了移动环境下实时事务截止期的满足 .实时事务的时间要求 (典型地为截止性 )的满足通常比事务执行的逻辑结果的完全正确更为重要 .该文提出了移动环境下实时事务的“相对”时间正确性标准 ,只要事务访问的数据的陈旧度 (staleness)在控制的范围内 ,就不一定要求实时事务访问的数据在事务提交时是有效的 .文中提出了移动实时事务的相对时间正确性标准、保证事务相对时间正确性标准的算法及移动实时事务执行模型 .所有这些策略一方面降低了移动实时事务的并发控制代价 ,另一方面以牺牲事务的“绝对”时间正确换取事务截止期的满足 ,这与实时数据库“宁愿要部分正确但及时的结果而不愿要绝对正确但过时的结果”的系统目标是相一致的 .实验表明 :相对时间正确性标准的提出及相应的事务执行模型能提高系统中满足截止期事务的比率 ,从而提高了系统性能 .

With the rapid advances in mobile computing technology, there is an increasing demand for processing real-time transactions in a mobile environment. Due to the intrinsic limitation of a mobile environment, there can be a large time latency for a data accessing. Sometimes the data can even be unavailable due to the network disconnection. All these have a negative impact on the timing constraint satisfaction for a mobile real-time transaction. For a mobile real-time transaction, the deadline satisfaction is more important than the absolute correctness for the logical result. In this paper, a relative time correctness criterion for mobile real-time transactions is proposed. As long as the staleness for the accessed data of a transaction is in a controlled scope, it is unnecessary to require that all the accessed data be valid when a mobile real-time transaction commits. Then a transaction execution model is introduced together with the policy to implement the transaction relative time correctness. All these strategies reduce the concurrency control costs. Extensive experiments show that the proposed relative time correctness criterion can increase the ratio for transactions that meet their deadlines and thus the system performance is improved greatly.