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1.
-可串行化、 可串行化、准一致性可串行化等现有面向事务的放松可串行化正确性准则虽能帮助更多无线通信事务满足其截止期要求,但会导致数据的受限不一致。该文针对移动实时环境和应用,形式化定义并分析了 可串行化和 可串行化2种新的面向用户的并发控制策略,它们都比冲突可串行化宽松,但都能始终保证移动用户读取数据的一致性。  相似文献   

2.
人们基于准一致性可串行化的正确性标准,提出了一种乐观并发控制协议,提高了事务执行的并发度,更有利于实时事务定时限制的满足。  相似文献   

3.
研究了实时数据广播环境中的一致性维护问题。由于广播环境的非对称通讯特性,传统可串行化并发控制方法过于严格而不必要;现有放松可串行化方法都会导致数据库受限不一致、使各个用户读取受限不一致数据。提出了使用т-可串行化并发控制,首先形式化定义了т-可串行化正确性准则,证明了它虽比传统可串行化宽松但能始终保证数据一致性,使各个用户始终读取一致性数据,进而给出了相应的并发控制协议。实验结果表明,新协议可显著降低平均响应时间,使更多移动实时事务满足截止期要求。  相似文献   

4.
研究了实时数据广播环境中的一致性维护问题。由于广播环境的非对称通讯特性,传统可串行化并发控制方法过于严格而不必要;现有放松可串行化方法都会导致数据库受限不一致、使各个用户读取受限不一致数据。提出了使用?子-可串行化并发控制,首先形式化定义了?子-可串行化正确性准则,证明了它虽比传统可串行化宽松但能始终保证数据一致性,使各个用户始终读取一致性数据,进而给出了相应的并发控制协议。实验结果表明,新协议可显著降低平均响应时间,使更多移动实时事务满足截止期要求。  相似文献   

5.
研究了实时广播中的数据一致性问题。传统冲突可串行化并发控制协议对非对称通信的广播环境来说过于严格且不必要。为此,提出使用Q可串行化:形式化定义了Q可串行化,证明了它虽比传统冲突可串行化宽松但能始终保证数据一致性,基于可动态调整串行化顺序的时标段排序方法给出了相应的并发控制协议。实验结果表明:新协议可显著降低平均响应时间,使更多移动实时事务满足截止期要求。  相似文献   

6.
一种嵌入式移动实时数据库的并发控制策略   总被引:3,自引:0,他引:3  
嵌入式移动实时事务的并发控制除了满足传统事务的基本特性外,还要着重考虑优先级颠倒、不必要的事务重启和全局数据一致性及混合事务系统的性能等问题.基于绝对串行化时序与选择重启的乐观并发控制算法(OCC-ASTOSR),利用绝对时标在广播循环的支持下调整移动客户端和中心数据库服务器上的事务串行化顺序,检测事务的数据访问冲突,并应用选择重启的方法解决冲突.事务随着运行过程更新读写数据集信息,每个数据对象都维护相关的时间信息,通过无线网络通信传递这些控制信息,交互地完成移动客户端和中心服务器中的本地事务验证和移动事务两阶段验证提交.  相似文献   

7.
一种新的实时事务并发控制方法   总被引:1,自引:0,他引:1  
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。  相似文献   

8.
研究优化调度数据库中事务问题,为保证多数据库中事务提交顺序,提高事务执行及提交的并发度,研究了事务提交图的调度算法TM2,针对算法TM2中事务管理器无法检测到全局事务与局部事务的间接冲突,提出了优化算法TMO,采用ticket算法在全局事务的每个子事务之间增加数据操作,以便在每个成员数据库的全局子事务之间创造直接冲突,并通过添加保存点恢复中止事务代替重做事务,保持了多数据库中事务的可串行化.通过仿真,对比了两种调度算法的性能.得出算法TMO解决了事务的可串行化问题,提高了事务的并发度,保证了事务的一致性.  相似文献   

9.
实时数据库嵌套事务的并发控制   总被引:4,自引:2,他引:4  
传统满足可串行化正确性标准的并发控制把数据库作为一个整体来考虑,事务的执行保证数据库的一致性。往往在应用中数据库的不同对象集合之间可以独立保证一致性而不必关心别的对象集合的一致性限制以提高事务执行的并发度,这就比较适合嵌套事务模型中父子事务及兄弟事务之间的应用。本文测对嵌套事务的并发控制进行了讨论。  相似文献   

10.
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.  相似文献   

11.
Investigates issues related to transaction concurrency control in multilevel secure databases. This paper demonstrates how the conflicts between the correctness requirements and the secrecy requirements can be reconciled by proposing two different solutions. It first explores the correctness criteria that are weaker than one-copy serializability. Each of these weaker criteria, though not as strict as one-copy serializability, is required to preserve database consistency in some meaningful way, and moreover, its implementation does not require the scheduler to be trusted. It proposes three different, increasingly stricter notions of serializability (level-wise serializability, one-item read serializability and pair-wise serializability) that can serve as substitutes for one-copy serializability. The paper then investigates secure concurrency control protocols that generate one-copy serializable histories and presents a multiversion timestamping protocol that has several very desirable properties: it is secure, produces multiversion histories that are equivalent to serial one-copy histories in which transactions are placed in a timestamp order, eliminates starvation and can be implemented using single-level untrusted schedulers  相似文献   

12.
安全数据库系统中的事务   总被引:1,自引:0,他引:1  
在多级安全数据库系统中经典的BLP模型的“向上写”违反了数据库的完整性,并产生隐通道和带来多实例问题,事务间的提交和回退依赖也会产生隐通道,在对事务安全性分析的基础上提出了安全事务模型和安全事务正确性标准一安全冲突可串行化(SCSR),最后给出了一个避免隐通道的安全并发控制算法.  相似文献   

13.
In the presence of semantic information, serializability is too strong a correctness criterion and unnecessarily restricts concurrency. Many researchers have investigated the use of semantic information to allow interleaving among transactions which are non-serializable, but which nonetheless preserves the consistency of the database and is acceptable to the users. In this paper we consider a class of schedules, calledconflict-correct schedules, first proposed by Farrag and Özsu, which enlarges upon the class of serializable schedules by taking semantic information of transactions into account. In this paper we show that the problem of recognizing schedules in this class is NP-complete. Thus it is unlikely that there exists an efficient scheduler which accepts the entire class of conflict-correct schedules.This research was partially supported by MICRO grants with IBM and XEROX Corporations.  相似文献   

14.
考虑到诸如带宽、移动性、频繁断接等限制,传统可串行化并发控制协议显得过于严格而无法满足移动实时数据库环境的要求。该文首先形式化定义并分析了弱可串行化正确性准则,弱可串行化比传统冲突可串行化宽松且能始终保证数据库的一致性。进而给出了一种适宜于移动实时数据库的弱可串行化并发控制协议WMHP-2PL。最后性能实验证实了新协议的有效性。  相似文献   

15.
A theory of global concurrency control in multidatabase systems   总被引:2,自引:0,他引:2  
This article presents a theoretical basis for global concurrency control to maintain global serializability in multidatabase systems. Three correctness criteria are formulated that utilize the intrinsic characteristics of global transactions to determine the serialization order of global subtransactions at each local site. In particular, two new types of serializability, chain-conflicting serializability and sharing serializability, are proposed and hybrid serializability, which combines these two basic criteria, is discussed. These criteria offer the advantage of imposing no restrictions on local sites other than local serializability while retaining global serializability. The graph testing techniques of the three criteria are provided as guidance for global transaction scheduling. In addition, an optimal property of global transactions for determinating the serialization order of global subtransactions at local sites is formulated. This property defines the upper limit on global serializability in multidatabase systems.  相似文献   

16.
A non-two-phase database concurrency control technique is introduced. The technique is deadlock-free, places no restrictions on the structure of the data, never requires data to be reread, never forces a transaction to be rolled back in order to achieve serializability, applies a type of lock conversion, and allows items to be released to subsequent transactions as soon as possible. The method introduced, database flow graph locking (FGL), uses a directed acyclic graph to direct the migration of locks between transactions. Unlike many previous non-two-phase methods, the database need not be structured in any specific fashion. The effect of these changes is that, with the same serializable schedule, FGL obtains a higher degree of concurrency than two-phase locking (2PL). Overhead requirements for database flow graph locking are comparable to those for two-phase locking, with 2PL being better in low conflict situations and FGL better in high conflict  相似文献   

17.
The authors propose a paradigm for developing, describing, and proving the correctness of concurrency control protocols for replicated databases in the presence of failures or communication restrictions. The approach used is to hierarchically divide the problem of achieving one-copy serializability by introducing the notion of a `group' that is a higher level of abstraction than transactions. Instead of dealing with the overall problem, the paradigm breaks it into two simpler ones: (1) a local policy for each group that ensures a total order of all transactions in that group; and (2) a global policy that ensures a correct serialization of all groups. The paradigm is used to demonstrate the similarities between several concurrency control protocols by comparing the way they achieve correctness  相似文献   

18.
可串行化的并发控制对传统应用是合适的。而在实时数据库中,为了满足事务定时限制(典型地为截止期),并且考虑到局部的数据库不一致能够随下一次数据采样恢复,人们提出了准一致可串行化标准。本文基于这一标准提出了一种新的乐观并发控制协议,它考虑了数据的相似性及事务特点,提高了事务执行的并发度,有利于实时事务定时限制的满足。  相似文献   

19.
广播环境中的移动实时事务并发控制   总被引:1,自引:0,他引:1  
研究了实时数据广播中的一致性问题,由于广播环境的通信不对称性,传统的可串行化并发控制协议显得过于严格,不必要,不可行,为此,提出使用伪可串行化并发控制;形式化定义了伪可串行化的概念,证明了它虽经传统可串行宽松但能保证数据一致性,给出相应的并发控制协议,实验结果表明,新协议可显著降低平均响应时间,使更多移动实时事务满足截止期要求。  相似文献   

20.
基于事务语义的多数据库系统并发存取控制方法   总被引:2,自引:0,他引:2  
庆成三 《计算机学报》1996,19(5):369-373
本文提出了一种多数据库系统(MDBS)的基于事务语义的并发存取控制方法。文中首先按对数据库一致性的影响将事务分类;接着提出了新的MDBS的并发调度的非可串行正确性准则;然后;叙述了称为“事务语义协议”的并发存取控制原理和方法,给出了全局调度和交付的算法和方法正确性证明。  相似文献   

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