K锁算法—一种数据库上的可适应并发控制策略

本文提出了一种基于悲观和乐观思想的并发控制算法——K锁算法.K锁算法利用K锁机制,在一定程度上允许冲突执程的并发执行.和乐观算法比较,它减少了在系统的执程冲突概率较大时执程的重执次数,和悲观算法比较,它提高了冲突执程的执行并发度.     

An intelligent hybrid optimistic/pessimistic concurrency control algorithm for centralized database systems using modified GSA-optimized ART neural model

Concurrency control is the activity of synchronizing operations issued by concurrent executing transactions on a shared database. The aim of this control is to provide an execution that has the same effect as a serial (non-interleaved) one. The optimistic concurrency control technique allows the transactions to execute without synchronization, relying on commit-time validation to ensure serializability. Effectiveness of the optimistic techniques depends on the conflict rate of transactions. Since different systems have various patterns of conflict and the patterns may also change over time, so applying the optimistic scheme to the entire system results in degradation of performance. In this paper, a novel algorithm is proposed that dynamically selects the optimistic or pessimistic approach based on the value of conflict rate. The proposed algorithm uses an adaptive resonance theory–based neural network in making decision for granting a lock or detection of the winner transaction. In addition, the parameters of this neural network are optimized by a modified gravitational search algorithm. On the other hand, in the real operational environments we know the writeset (WS) and readset (RS) only for a fraction of transactions set before execution. So, the proposed algorithm is designed based on optional knowledge about WS and RS of transactions. Experimental results show that the proposed hybrid concurrency control algorithm results in more than 35 % reduction in the number of aborts in high-transaction rates as compared to strict two-phase locking algorithm that is used in many commercial database systems. This improvement is 13 % as compared to pure-pessimistic approach and is more than 31 % as compared to pure-optimistic approach.     

基于冲突率预测的自适应并发控制算法

并发控制算法能够保证数据库事务集并发执行的正确性和一致性。为了提高并发事务的执行效率,提出了一种基于冲突率预测的自适应并发控制算法(ACC-PRC)。该算法将并发控制过程分为信息收集和策略选择两个阶段。信息收集阶段利用先验事务队列保证事务执行的可串行化,并且利用循环冲突队列收集系统的事务执行状态。策略选择阶段在循环冲突队列上运用改进的加权移动平均法预测下一阶段冲突率,并根据双向阈值决策下一阶段的并发策略。所提算法在事务到达率较高时能保持良好的事务执行效率,同时能够准确及时地感知冲突率的变化。对比实验表明ACC-PRC算法的综合性能优于HCC算法和ADCC算法。     

一种分布式实时事务调度算法

传统的乐观并发控制策略利用了一些不必要的事务重启来保证数据的一致性，事务重启能够极大的增加系统载荷以及加强资源和数据的竞争，在分布式环境下，由于系统的复杂性和较高的通讯开销加剧了该问题．针对该问题本文提出了一种靳的乐观并发控制策略，通过动态调整事务串行化执行顺序来避免不必要的事务重启．当把这种新的并发控制策略在分布式实时环境中实现时，考虑到分布式事务的实时性要求，本文提出把写阶段从临界区中分离出来的方法，并用顺序加锁的策略来保证分布式事务执行的正确性，最后给出了该实现方法的正确性证明．     

运行时间自适应的事务调度算法

实时数据库中是基于事务的不同优先级进行事务的调度,所以事务优先级的确定对实时数据库的效率有着至关重要的作用。本文讨论了一个运行时间自适应的事务调度算法,对算法思想和调度策略进行了深入的研究。该算法事务的估计运行时间与CPU时钟无关,具有自适应的能力,并且可以得到周期性事务的准确运行时间,对于周期性事务比例比较高的应用系统非常适合。     

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。     

Read-down conflict-preserving serializability as a correctness criterion for multilevel-secure optimistic concurrency control: CSR/RD

It is important to note that conflict-preserving serializability and related theory are artifacts of an era of database development where correctness alone was the overriding concern. With the advent of multilevel-secure databases, there is clearly a need to reexamine such theories. Any correctness criterion to govern transaction processing in the multilevel security context has to incorporate both secureness and correctness in a unified manner. This paper makes original contributions in two different but closely related areas to the optimistic concurrency control in multilevel-secure, single-version databases. First, read-down conflict-preserving serializability (CSR/RD) captures multilevel-secure database consistency requirements and secure transaction correctness properties via a single notion. Second, it presents a multilevel-secure optimistic concurrency control (MLS/OCC) scheme that has several desirable properties: If lower-level transactions were somehow allowed to continue with their executions in spite of the conflict with high-level transactions, covert timing-channel freeness would be satisfied. This sort of optimistic approach for conflict insensitiveness and the properties of non-blocking and deadlock freedom make the optimistic concurrency control scheme especially attractive to multilevel-secure transaction processing.     

软件事务存储自适应竞争管理策略研究

在并行程序设计中,软件事务存储(STM)是对传统基于锁的同步机制的一种替代方式。其中竞争管理器用于消解事务冲突,决定哪个事务继续执行,哪个事务放弃,对于STM系统的性能有非常重要的影响。但是当前已有的竞争管理策略因受限于环境负载和STM系统的参数配置,性能表现往往是不稳定的。因此,提出了一种自适应的竞争管理策略(ACM),该策略通过构建竞争管理器池(Contention Manger Pool)和自适应竞争管理算法动态地为STM系统选择当前最优的竞争管理策略。实验结果表明ACM不仅可以通用于不同环境而且其平均性能也优于现有的静态竞争管理策略。     

计算机辅助协同设计系统并发控制机制的研究

介绍了采用图形数据库组织与管理数据的方法，阐述了基于语义的并发控制机制中事务结构、事务提交、锁机制和冲突协调等关键问题．最后提出了事务管理器的结构模型，并对事务的处理流程进行了剖析。     

Optimistic voting for managing replicated data

An epidemic model gives and efficient approach for transaction processing of replication systems in weakly connected environments.The approach has the advantages of high adaptation,support for low-handwidth network,and committing updates in an entirely decentralized control fachion.But the previous impolementing protocols,like ROWA protocol,quorum protocol,and voting protocol,have a common shortcoming that they are pessimistic in conflict reconciliation,therefore bring high transaction abort rate and reduce system performance dramatically when the workload scales up.In this paper,an optimistic voting protocol,which introduces condition vote and order vote in the voting process of transactions,is proposed.The condition vote and order vote postpone the final decision on conflicting transactions and avoid transaction aborts that are incurred by read-wr5ite and write-write conflicts.Experimental results indicate that the optimistic voting protocol decreases abort rate and improves average response time of transactions markedly when compared to other protocols.     

Divergence control for distributed database systems

This paper presents distributed divergence control algorithms for epsilon serializability for both homogeneous and heterogeneous distributed databases. Epsilon serializability allows for more concurrency by permitting non-serializable interleavings of database operations among epsilon transactions. We first present a strict 2-phase locking divergence control algorithm and an optimistic divergence control algorithm for a homogeneous distributed database system, where the local orderings of all the sub-transactions of a distributed epsilon transaction are the same. In such an environment, the total inconsistency of a distributed epsilon transaction is simply the sum of those of all its sub-transactions. We then describe a divergence control algorithm for a heterogeneous distributed database system, where the local orderings of all the sub-transactions of a distributed epsilon transaction may not be the same and the total inconsistency of a distributed epsilon transaction may be greater than the sum of those of all its sub-transactions. As a result, in addition to executing a local divergence control algorithm in each site to maintain the local inconsistency, a global mechanism is needed to take into account the additional inconsistency Recommended by: Meichum Hsu     

A comparative study of some concurrency control algorithms for cluster-based communication networks

This paper presents a comparative study of some concurrency control algorithms for distributed databases of computer clusters which emphasize high availability and high performance requirements. For this purpose, we have analyzed some concurrency control algorithms which are used in commercial DBMSs, such as the pessimistic locking algorithm as it verifies transaction conflicts early in their execution phase, and the optimistic algorithm which investigates the presence of conflicts after the execution phase. A new algorithm is proposed and implemented by a simulation program. The three algorithms were tested using different configurations. Simulation results showed that the locking algorithm performed better than the optimistic method in presence of conflicts between transactions, while the optimistic algorithm provided better results in the absence of conflicts. Furthermore, in a distributed database with a certain probability of conflicts, the locking algorithm can be used to guarantee strong consistency and an acceptable level of performance. However, if this probability is negligible, the system performance can be improved by using the optimistic algorithm. The proposed algorithm offers improved performance in numerous cases. As a result, it can be used in a distributed database to guarantee a satisfactory level of performance in the presence of conflicts.     

一种乐观嵌套工作流事务模型

为解决目前工作流事务管理的不足和存在的问题,本文基于多数据版本、三阶段执行的并发控制理论提出了一种乐观嵌套工作流事务模型,该事务模型借用时间戳的概念,通过对不同事务中活动类型的分类,较好地解决了长执行事务和协同事务的可靠性和正确性问题,提高了工作流处理的效率。乐观嵌套事务模型把嵌套事务、工作流模型和并发控制协议有机地结合在一起,定义了较为完整的事务操作原语及其语义。本文还给出了乐观事务模型到工作流模型的映射,使得事务工作流执行过程中,其操作原语和乐观事务模型的操作原语是一致的,工作流活动的转移控制与乐观嵌套事务模型的子嵌套事务的生成过程及其表示方式也是一致的。     

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.     

并行实时数据库系统中的一种自适应并发控制模式

根据事务的实时性及关键性分类，事务分为硬实时、固实时、软实时和非实时事务。传统的并发控制仅考虑其中一到两种事务，且将固／软实时事务统一为软实时事务考虑，带有很大的不精确性，这将降低系统吞吐率。为此，本文提出了一种自适应并发控制模式。该模式采用两级并发控制体系，一级是主并发控制器，探测不同类事务间可能存在
在的类间数据冲突；二级是从并发控制器，探测同类事务间可能存在的类内数据冲突，由四类从并发控制器实现。根据各类事务的不同特征，各控制器采用的策略不同，因而具有极大的自适应性并能最终提高系统的吞吐率，但需要增加额外的硬件开销。     

Bio-inspired adaptive control strategy for the highly efficient speed regulation of the DC motor under parametric uncertainty

The presence of parametric uncertainties decreases the performance in controlling dynamic systems such as the DC motor. In this work, an adaptive control strategy is proposed to deal with parametric uncertainties in the speed regulation task of the DC motor. This adaptive strategy is based on a bio-inspired optimization approach, where an optimization problem is stated and solved online by using a modification of the differential evolution optimizer. This modification includes a mechanism that promotes the exploration in the early generations and takes advantage of the exploitation power of the DE/best class in the last generations of the algorithm to find suitable optimal control parameters to control the DC motor speed efficiently. Comparative statistical analysis with other bio-inspired adaptive strategies and with linear, adaptive and robust controllers shows the effectiveness of the proposed bio-inspired adaptive control approach both in simulation and experimentation.     

移动环境下事务一致性的动态实现

移动分布式数据库中节点间的连接是一种弱连接，为了提高移动计算机的效率，事务执行通常采有乐观复制的方法，它的不足之处是要依赖用户对应用数据和节点行为的知识，难以适应移动环境下网络连接和配置的动态变化，首次提出了移动环境下事务处理的基于概率方法，采用服务质量定义动态实现移动环境下的事务一致性，是一种用户与系统协作来完成事务处理的方法，实验结果表明，基于概率方法在性能上优于简单的乐观复制方法。     

Formal aspects of optimistic concurrency control in a multiple version database system

Formal aspects of optimistic concurrency control in a multiple version database system are discussed. It is distinguished between element-optimistic and set-optimistic methods. In an element-optimistic method the unit of scheduling for the concurrency control is one action/transaction, while in a set-optimistic method a set of transactions is scheduled. The underlying decision problems of serializability are defined and shown to be NP-complete in a model, which is typical for most modern transaction oriented database management systems. Therefore it is most probable, that both optimistic concurrency control types cannot be implemented efficiently in the general case.     

一种基于移动环境的混合乐观实时事务处理策略

随着移动计算技术的快速发展，移动环境下涉及到实时事务处理的应用需求正逐渐增长．由于移动环境下固有特性：高的网络延迟、频繁的断接性和移动性等，采用传统的事务处理技术，在移动环境下很难满足事务的截止期要求．提出了一种基于高优先级两段锁的混合乐观实时事务并发控制协议(HORTCC—SHP2PL)．该协议在移动实时事务处理中将乐观并发控制和高优先级两段锁结舍起来，利用两阶段提交协议实现移动实时事务的全局提交．为了进一步减少移动实时事务重启的数目．在并发控制协议中引入了相似性的概念．仿真实验显示．与分布式高优先级两段锁(HP2PL)比较，HORTCC—SHP2PL明显地减少了实时事务错过截止期的比率，提高了事务的并发度，能更好的满足移动实时事务截止期的要求．     

线性电机伺服系统的自适应鲁棒控制

通过对一类音圈电机直驱式精密伺服系统运动控制问题的研究,在只考虑系统标称模型的情况下,采用分数阶形式的趋近律,得到了一种有限时间滑模控制算法;与此同时,当系统存在参数不确定和外界干扰的情况下,将有限时间滑模控制方法和自适应控制理论相结合,构造了一种自适应滑模控制律.另一方面,本文还考虑了系统具有输入饱和约束的情况,通过引入一个辅助系统,进而设计了一种自适应抗饱和控制律.而且,文中对闭环系统的稳定性进行了证明.最后,仿真和实验对所提出控制策略的有效性和伺服精度进行了对比、分析和验证.