数据广播环境下移动实时事务的有效并发控制

在无线数据广播环境下,从移动客户机到数据库服务器的上行带宽非常有限.传统的并发控制协议不适合这种不对称通讯环境.本文结合乐观并发控制协议和时标排序协议提出一种并发控制协议.只读事务满足弱一致性,包含所有更新事务和一个移动只读事务的可串行化图是非循环的.使用本文提出的并发控制协议,移动只读事务能够自主执行,移动更新事务能够较早地检测到数据冲突.模拟试验显示,我们提出的协议相对其它已经存在的并发控制协议来说,能够更好地满足事务截止期.     

Real-time transaction scheduling in database systems

A database system supporting a real-time application, which can be called “a real-time database system (RTDBS)”, has to provide real-time information to the executing transactions. Each RTDB transaction is associated with a timing constraint, usually in the form of a deadline. Efficient resource scheduling algorithms and concurrency control protocols are required to schedule the transactions so as to satisfy both timing constraints and data consistency requirements. In this paper,^† we concentrate on the concurrency control problem in RTDBSs. Our work has two basic goals: real-time performance evaluation of existing concurrency control approaches in RTDBSs, and proposing new concurrency control protocols with improved performance. One of the new protocols is locking-based, and it prevents the priority inversion problem^{, †} by scheduling the data lock requests based on prioritizing data items. The second new protocol extends the basic timestamp-ordering method by involving real-time priorities of transactions in the timestamp assignment procedure. Performance of the protocols is evaluated through simulations by using a detailed model of a single-site RTDBS. The relative performance of the protocols is examined as a function of transaction load, data contention (which is determined by a number of system parameters) and resource contention. The protocols are also tested under various real-time transaction processing environments. The performance of the proposed protocols appears to be good, especially under conditions of high transaction load and high data contention.     

Concurrency control in distributed databases through time intervalsand short-term locks

A method for concurrency control in distributed database management systems that increases the level of concurrent execution of transactions, called ordering by serialization numbers (OSN), is proposed. The OSN method works in the certifier model and uses time-interval techniques in conjunction with short-term locks to provide serializability and prevent deadlocks. The scheduler is distributed, and the standard transaction execution policy is assumed, that is, the read and write operations are issued continuously during transaction execution. However, the write operations are copied into the database only when the transaction commits. The amount of concurrency provided by the OSN method is demonstrated by log classification. It is shown that the OSN method provides more concurrency than basic timestamp ordering and two-phase locking methods and handles successfully some logs which cannot be handled by any of the past methods. The complexity analysis of the algorithm indicates that the method works in a reasonable amount of time     

一种实时数据库系统的基于时间戳的多版本并发控制协议

实时数据库系统的定时限制包括数据的定时限制和事务的定时限制,一个好的并发控制协议必须要较好地满足这些定时限制。文章详细讨论了与并发控制有关的实时数据和实时事务的各种特征以及分类。并根据这些特点对传统数据库系统的多版本并发控制机制进行了扩展,提出了一种基于时间戳的多版本实时数据库系统的并发控制协议。该协议对硬实时事务不产生任何延迟,能够很好地保证实时事务和实时数据的定时限制,缺点是该并发控制协议仅是一个准一致性的协议。     

时标排序并发控制算法在移动分布式实时数据库中的应用分析

随着移动计算技术的发展,人们要求能够随时随地实时存取数据库的数据,这就导致了移动分布式实时数据库(MDRTDBS)的实现.移动实时应用的根本特性是其事务具有移动性和实时特性,数据具有定时特性.这对移动分布式实时数据库并发控制协议提出了新的挑战,目前这方面的研究还很少.对实时数据库的时标排序并发控制协议进行了分析,针对移动实时应用的特点对将它们应用到MDRTDBS的优缺点进行了阐述,提出了改进的方法.最后提出了新的适用于MDRTDBS的时标排序协议,并指出了未来的研究方向.     

一种面向混合实时事务调度的并发控制协议

首先给出了一个两层结构的混合实时数据库系统模型,其中支持采用非定期任务调度算法来改进系统的性能．进一步,针对这种模型下混合事务的数据一致性问题,提出了一种新的并发控制协议——MCC-DATI．该协议采用动态优先级驱动的调度算法,通过限制非定期的软实时事务对硬实时事务的阻塞时间,保证硬实时事务的可调度性;同时,采用非定期任务调度算法以及基于时间戳间隔的动态串行化顺序调整机制来减少软实时事务的截止期错失率．仿真实验表明,相对于先前的混合事务的并发控制协议,该协议在不同的系统负载与截止期约束下都能够改进系统的性能。     

基于级联深度的主动实时乐观并发控制协议

主动实时数据库因结合了时间限制与主动机制而使系统事务的并发控制变得更为复杂。主动规则的引入使事务触发新的事务且在执行上具有多种耦合方式,传统的实时并发控制策略无法对具有复杂执行模式的事务进行有效调度,而基于主动数据库的并发控制机制也没有考虑事务的实时性问题。通过对事务不同耦合方式的实时要求及事务间冲突关系进行分析,提出了新的主动实时数据库乐观并发控制方法,对不同事务级联深度进行评估,结合事务执行的时间信息对冲突事务进行动态调整串行化顺序。理论分析与实验证明,能在保证事务可串行性的同时降低了不必要事务重启个数,更好地满足系统的实时性。     

Prescheduling policy for real-time concurrency control: A performance evaluation

A new priority management policy, aprescheduling policy, is proposed. This policy can be applied on any conventional concurrency control protocol to schedule a real-time transaction. Costly preemption is avoided by the prescheduling policy, and parsing dataset of a transaction is not needed. Three widely used conventional concurrency control protocols (dynamic two-phase locking, basic timestamp ordering, and optimistic) are incorporated with the prescheduling policy to form three real-time concurrency control protocols. Performance of the three protocols is evaluated from three different viewpoints: database management systems, protocols, and transaction. From a database management system viewpoint, we show the prescheduling policy can improve the performance of protocols by raising thevalid ratio and reducingrestart counts. In general, two-phase locking with the prescheduling policy performs the best in most cases and yields the best choice for concurrency control in a real-time application. Deciding factors that affect performance of each protocol are identified from protocol viewpoint. Some suggestions are given for writing a timely transaction from the aspect of transaction viewpoint.     

基于耦合模式的主动实时并发控制算法

主动实时数据库中触发事务与被触发事务在执行上具有多种耦合模式，传统的并发控制无法对具有复杂耦合模式的事务进行有效调度。通过对不同耦合模式实时要求及事务间冲突关系的分析，提出了新的主动实时并发控制算法(ARTCC-CM)，采用时戳区间策略，在验证阶段检测冲突事务触发度及执行时间，动态调整串行化顺序。理论分析与实验证明，在保证可串行性同时减少了不必要的事务重启，提高了系统性能。     

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     

数据库系统的优化并行控制算法

The concurrency control problem in database systems has been examined by many people and several concurrency control algorithms have been proposed. The most popular algorithms are two-phase locking and timestamp ordering. Due to its inefficiency in some situations, we propose the optimized method which is combined of timestamp ordering and optimistic method. With this method, the efficiency can be improved in those systems where the conflicts between transactions rarely happen.     

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。     

一种新的实时数据库乐观并发控制方法

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。     

Towards a General Concurrency Control Algorithm for Database Systems

The concurrency control problem in database systems has been examined by many people and several concurrency control algorithms have been proposed. The most popular algorithms are two-phase locking and timestamp ordering. This paper shows that two-phase locking and timestamp ordering are special cases of a more general concurrency control algorithm. This general algorithm is described in detail and is proven to work correctly. We show that two-phase locking and timestamp ordering represent the two end points of a series of concurrency control algorithms. Each of them is a special case of the general algorithm proposed in this paper. Moreover, each of these special cases can be selected in advance, and can even be changed dynamically during execution.     

Real-time concurrency control in a multiprocessor environment

Although many high-performance computer systems are now multiprocessor-based, little work has been done in real-time concurrency control of transaction executions in a multiprocessor environment. Real-time concurrency control protocols designed for uniprocessor or distributed environments may not fit the needs of multiprocessor-based real-time database systems because of a lower concurrency degree of transaction executions and a larger number of priority inversions. This paper proposes the concept of a priority cap to bound the maximum number of priority inversions in multiprocessor-based real-time database systems to meet transaction deadlines. We also explore the concept of two-version data to increase the system concurrency level and to explore the abundant computing resources of multiprocessor computer systems. The capability of the proposed methodology is evaluated in a multiprocessor real-time database system under different workloads, database sizes and processor configurations. It is shown that the benefits of the priority cap in reducing the blocking time of urgent transactions are far greater than the losses involved in committing less urgent transactions. The idea of two-version data also greatly improves the system performance because of a much higher concurrency degree in the system     

嵌入式实时数据库中基于价值函数的并发研究

提出一种新的适用于嵌入实时数据库系统（ERTDBs）的并发控制协议VB-SCC,该协议将事务的价值函数和SCC协议相结合。其目标是提高并发事务带给系统的价值收益。另外,该协议建立在基于功能替代的实时事务模型上,适用于嵌入式实时数据库系统。最后通过模拟试验证实VB-SCC比现有的VOSP、VPSP协议具有更好的稳定性。     

Semantic Based Concurrency Control of Open Nested Transactions in Active Object Oriented Database Management Systems

Active object oriented database management systems (AODBMS) are finding increasing application in different application domains and especially for cooperative and long duration activity management. In this paper, we propose a concurrency control mechanism for open nested transactions in an AODBMS. It exploits the semantics of the transactions to achieve controlled cooperation and concurrency among the transactions. Atomic AODBMS transactions are treated as base transactions. A complex transaction type is formed from a collection of base and complex transactions, a set of detached mode ECA rules and a state transition model. The cooperation semantics of a complex transaction type with other complex transaction types is specified by associating with each state of a complex transaction, a set of cooperating complex transaction types. The abort semantics of a complex transaction type is specified by means of compensating base transactions. We define a correct class of schedule called CoopComp-schedule which satisfies the state transition semantics of the individual complex transactions and also maintains the cooperation and compensation semantics of the individual complex transactions within the generated schedule. A concurrency control scheme called NP-QuadLock for generating only CoopComp-schedules has been provided. The proof of correctness of the proposed concurrency control mechanism is also given.     

实时Client/Server数据库并发控制和恢复机制研究

该文提出了实时Client／Server数据库系统多版本两阶段封锁并发控制协议和有效的恢复机制。协议区分只读事务和更新事务。只读事务在执行读操作时遵从多版本时间排序协议,更新事务执行强两阶段封锁协议,即持有全部锁直到事务结束。只读事务读请求从不失败,不必等待等特性。在典型数据库系统中,读操作比写操作频繁。这个特性对于实践来说至关重要。为了提高只读事务的响应时间,协议让每个客户端与一个一致数据库影子相联,只读事务在客户端处理。更新事务提交到服务端运行。服务端每个事务Ti在提交时系统必须向所有客户端广播信息。客户端根据得到的广播信息自动构造一致数据库影子。一致数据库影子还将用于系统恢复。通过仿真模拟。与2V2PL和OCC-TI-WAIT-50协议进行比较,结果表明：该并发控制协议不仅能有效降低事务延误截止时间率和重起动率,而且能改善只读事务的响应时间,减少优先级高事务的锁等待时间。协议性能优于2V2PL协议和OCC-TI-WAIT-50协议。     

Alternative correctness criteria for concurrent execution oftransactions in multilevel secure databases

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     

移动实时事务预留广播空间的验证协议

在无线数据广播环境中,从移动客户机到服务器的上行带宽非常有限,传统的并发控制协议不适合这种不对称通信环境.本文提出一种可变乐观并发控制协议,服务器周期性向移动客户机广播数据对象,同时把一个广播周期分成若干个子周期,在两个连续的子周期间,一个保留空间被预留来存放服务器更新事务在第一个子周期开始后修改的所有数据对象,移动只读事务通过比较服务器更新事务提交的写集合和自己读集合来自主进行一致性验证,如果只读事务不能通过部分验证,它不会被草率地夭折并重新启动,相反,应用一个改进的向前验证策略,帮助移动只读事务争取更多提交机会.最后,本文进行充分的实验对提出的算法性能进行了评价.