数据库系统中动态调整串行次序的乐观并发控制算法

尽管已证明在RTDBSs中乐观并发控制方法的性能要优于锁式并发控制,但乐观并发控制存在不必要的重新启动以及较高的重启动代价的问题.该文介绍一种新的乐观并发控制算法,它通过结合动态调整串行次序和使用时标间隔来达到减少重新启动事务的目的,因而其性能高于传统的乐观并发控制方法.     

实时数据库系统中动态调整串行次序的乐观交发控制算法

尽管已证明在RTDBSs中乐观并发控制方法的性能要优于锁式并发控制，但乐观并发控制存在不必要的重新启动以及较高的重启动代价的问题。该文介绍一种新的乐观并发控制算法，它通过结合动态调整串行次序和使用时标间隔来达到减少重新启动事务的目的，因而其性能高于传统的乐观并发控制方法。     

一种支持工程设计事务的乐观并发控制方法

传统乐观并发控制方法对工程设计事务支持较弱。该文以嵌套事务模型为基础并结合检入＝检出机制,提出一种改进的乐观并发控制方法,能够较好地解决传统乐观并发控制方法在工程设计务应用上存在的问题并提高了事务的并发度。     

一种用于分布式数据库系统的并发控制算法—半乐观算法

并发控制是分布式数据库系统设计中的一个重要问题。目前已有许多并发控制算法,例如两阶段加锁,时间戳和乐观算法等。本文提出一种新的并发控制算法—半乐观算法。该算法已在基于dBASEⅢ的分布式数据库系统DdBASEⅢ中实现。它是将加锁和许可证结合起来,能较好地支持查询频繁而修改较少的应用。本文还给出了该算法的性能分析。     

一种新的实时事务并发控制方法

本文提出一种新的实时事务并发控制方法，并对其性能进行了模拟评测；与锁式协议，乐观协议，基于时间戳的协议相比，它可以获得更新的并发度，与诸如ε－可串行比、△－可串行化等一致性的放松可串行化并发控制技术相比，它维护强一致性，保证严格可串行化。     

基于验证因子的实时多版本并发控制

为了解决乐观并发控制方法的不必要重启与即将超过截止期事务的重启引发的负载问题，提出了验证因子的概念及基于验证因子的实时多版本并发控制方法（MVOCC-VF）．通过对验证因子的检测，优先调度完成度较高的事务；同时结合多版本技术增强事务调度的并发度，避免了更多的不必要重启问题，尤其能有效保证即将超截止期事务的完成，并证明了协议的正确性．实验结果表明：与已有的实时并发控制方法相比，MVOCC—VF降低了事务重启个数及超截止期百分率，提高了系统的总体性能．     

一种用于分布式数据库系统的并发控制新算法

针对分布式数据库的并发控制提出了一种新算法。本算法对新事务与本地节点上已启动但还未完成事务的信息进行比较,动态地在乐观并发控制算法与悲观并发控制算法之中选择一种算法实现对该新事务的并发控制。     

移动事务并发控制技术的研究

为有效降低移动计算环境下事务处理的重启动率、提高事务的吞吐量,在研究传统并发控制协议的基础上,提出了一种数据锁乐观并发控制协议DLOCC,DLOCC协议结合了锁并发控制协议与乐观并发控制协议的优点,利用数据项的更新次数和访问概率将服务器上的海量数据进行分类,依据不同的数据类型采用相应的并发控制协议.模拟实验结果表明,DLOCC协议性能优于OCC-TI协议,有效地提高了系统资源利用率.     

数据库管理系统的并发控制

并发控制(Concurrency Contol)是数据库系统的核心问题之一.对于事务的并发控制多年来进行了广泛而深入的研究,各个典型的数据库系统,各种各样的DBMS产品都研制和开发了各自的一套方法和算法,各种具有其本身特色的机制.但总的说来,迄今所讨论和采用的方法中不外乎三大类,这就是封锁方法,时间戳方法和乐观方法.其中封锁方法研究得最早,可以上溯到操作系统和文件系统的时代.时间戳方法和乐观方法则提出较晚,实践的经验也要少得多. 这里对并发控制提供一个简单的综述,以期能使读者花较少时间而能有一全面而较深入的了解.     

乐观并发协议在移动计算环境中的应用

在移动计算环境中,基本乐观并发协议的事务处理存在并发度低和重启率高等问题。为此,提出一种适用于移动计算环境的乐观事务并发协议――基于数据读写时间戳的乐观并发控制协议。仿真实验表明,该协议可以提高事务处理的并发度,降低事务重启概率,与基本乐观事务并发控制协议和基于时间戳的乐观事务并发控制协议相比,该协议更适用于移动计算环境。     

实时主动数据库有效性检查并发控制协议

在实时主动数据库中,事务不仅有时间约束,而且事务执行可能触发其它事务执行。传统并发控制协议不适应于实时主动数据库系统。该文研究了实时主动数据库事务执行模式,提出了有效性检查并发控制协议。协议使用动态调整串行次序策略,避免不必要的事务重启动。通过仿真模拟与HP2PL协议和OCC-TI-WAIT-50协议进行了比较。结果表明,该协议能有效地降低事务延误截止时间率和事务重启动率,性能优于HP2PL和OCC-TI-WAIT-50协议。     

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.     

移动分布式数据库中的实时事务处理

提出了移动事务实时提交协议(MTRTC)和多版本乐观并发控制协议(MVOCC)处理移动分布式实时事务。MVOCC有效性检查分为局部有效性检查和全局有效性检查，采用动态调整串行次序，避免了不必要的事务重启动，改善了只读事务的响应时间。MTRTC是一个实时提交协议，减少了通信信息。实验结果表明结合MVOCC和MTRTC事务处理协议优于其它事务处理协议。     

并行数据库实时多版本并发控制协议性能研究

随着对实时数据库事务处理性能要求的不断提高 ,将并行数据库和实时数据库相结合的并行实时数据库将成为新的发展方向 .然而通常的实时多版本并发控制协议不适合并行数据库的无共享结构 .比较了几种并发控制协议在无共享结构下的性能 ,该文提出了一种并行实时多版本并发控制协议 .它能有效地减少事务的重启 ,降低事务的错过率 .在长短事务混合的情况下 ,它的性能比其它的并发控制协议更好 .同时 ,它具有更好的可扩展性 .作者利用仿真模型对该协议进行了性能测试 ,与其它协议进行了比较 ,并分析了实验结果 ,仿真实验结果表明该协议性能良好     

一种改进的动态调整串行化顺序算法

乐观并发控制协议比锁机制更适用于实时数据库,但其中存在着不必要的重启及重大重启。该文针对OCC-TI算法的不足,将动态调整串行化顺序方法从优先权和最终时间戳选择机制2方面进行了扩展,提出了新的基于时间戳的并发控制算法——OCC-CPTI。仿真实验证明,OCC-CPTI算法在性能上优于OCC-TI。     

多版本两阶段封锁并发控制协议性能研究

在并行实时数据库中，并发控制协议不仅要满足数据库一致性约束，而且要满足时间约束。本文提出了实时多版本两阶段封锁并发控制协议，它能有效地减少延误截止时间事务数量。作者通过仿真模拟，在共享磁盘体系结构下与HP2PL和OCC-TI-WAIT-50协议进行了比较。实验结果表明，该协议比HP2PL和OCC-TI-WAIT-50协议性能更好。     

A hybrid distributed optimistic concurrency control method for high-performance real-time transaction processing

The conventional lock scheme tends to suffer from a cascade of blockings,while the optimistic concurrency control(OCC) scheme may suffer from wasting resources.To overcome these problems,some researchers have proposed a combination of OCC and lock in transaction processing.Using this method,Thomasian proposed the hybrid method for conventional distributed transaction processing,and Lam proposed the DOCC-DA protocol for distributed realtime databased system based on forward validation.This paper proposes a new protocol,called Hybrid Distributed Optimistic Concurrency COntrol Embedded in two-Phase Commit,which is based on back validation.The new protocol makes use of access invariance and runtime inforbased on back validation.The new protocol makes use of access invariance and runtime infor mation which can guarantee a rerun transaction to meet its deadline and abort the fruitless run transactions as early as possible.A series of simulation experiments have been done to investigate the performance of the new protocol.The results show that its performance is consitently better than that of other protocols.     

Efficient validation of mobile transactions in wireless environments

In broadcast environments, the limited bandwidth of the upstream communication channel from the mobile clients to the server bars the application of conventional concurrency control protocols. In this paper, we propose a new variant of the optimistic concurrency control (OCC) protocol that is suitable for broadcast environments. At the server, forward validation of a transaction is done against currently running transactions, including mobile transactions and server transactions. At the mobile clients, partial backward validation of a transaction is done against committed transactions at the beginning of every broadcast cycle. Upon completion of execution, read-only mobile transactions can be validated and committed locally and update mobile transactions are sent to the server for final validation. These update transactions have a better chance of commitment because they have gone through the partial backward validation. In addition to the nice properties of conventional OCC protocols, this protocol provides autonomy between the mobile clients and the server with minimum upstream communication, which is a desirable feature to the scalability of applications running in broadcast environments. This protocol is able to process both update transactions and read-only transactions at the mobile clients at low space and processing overheads.     

Distributed optimistic concurrency control methods forhigh-performance transaction processing

There is an ever-increasing demand for more complex transactions and higher throughputs in transaction processing systems leading to higher degrees of transaction concurrency and, hence, higher data contention. The conventional two-phase locking (2PL) Concurrency Control (CC) method may, therefore, restrict system throughput to levels inconsistent with the available processing capacity. This is especially a concern in shared-nothing or data-partitioned systems due to the extra latencies for internode communication and a reliable commit protocol. The optimistic CC (OCC) is a possible solution, but currently proposed methods have the disadvantage of repeated transaction restarts. We present a distributed OCC method followed by locking, such that locking is an integral part of distributed validation and two-phase commit. This method ensures at most one re-execution, if the validation for the optimistic phase fails. Deadlocks, which are possible with 2PL, are prevented by preclaiming locks for the second execution phase. This is done in the same order at all nodes. We outline implementation details and compare the performance of the new OCC method with distributed 2PL through a detailed simulation that incorporates queueing effects at the devices of the computer systems, buffer management, concurrency control, and commit processing. It is shown that for higher data contention levels, the hybrid OCC method allows a much higher maximum transaction throughput than distributed 2PL in systems with high processing capacities. In addition to the comparison of CC methods, the simulation study is used to study the effect of varying the number of computer systems with a fixed total processing capacity and the effect of locality of access in each case. We also describe several interesting variants of the proposed OCC method, including methods for handling access variance, i.e., when rerunning a transaction results in accesses to a different set of objects     

移动计算环境中实时嵌套事务并发控制协议的研究

提出了移动计算环境中多版本乐观并发控制协议(MultiversionOptimisticConcurrencyControl,MVOCC)处理移动实时嵌套事务.协议消除了只读事务和更新事务之间冲突,通过动态调整事务串行次序,避免不必要的事务重启动.只读事务在移动主机处理,只读事务的响应时间大大改善.事务有效性检查分两级局部有效性检查和全局有效性检查.局部有效性检查在移动主机进行.通过局部有效性检查的事务提交到服务器进行全局有效性检查.如此早地检测数据冲突,节省了处理和通信资源.通过模拟仿真,对协议进行了性能测试,并与OCC-TI-WAIT50和HP2PL协议进行了比较.实验结果表明该协议要优于其它协议.协议不但能有效地降低事务的重启动率和延误截止时间率,而且改善只读事物的响应时间.