多数据库事务并发调度算法优化技术研究

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.     

多数据库事务处理模型中局部代理的设计与实现

在多数据库事务处理模型中，局部代理提供了全局事务管理层与局部数据库系统的接口，它使多数据库系统能够对分布在不同站点的局部数据库进行透明访问。文中提出了基于两阶段代理（２ＰＣＡ）的局部代理模型，它集成了支持两阶段提交或单阶段提并的数据库系统、以及并不支持事务提交协议的文件系统。文中还介绍了采用专用接口和ＯＤＢＣ两种实现局部代理的方法。文中着重介绍了使用ＯＤＢＣ方法的具体实现，并对两种方法实现的局部代     

一个数据库互操作事务调度模型

全局事务调度是分布异构数据库互操作系统研究的重要方面，调度的目的是保证全局事务执行的原子性、顺序化及与本地事务的互斥。本文介绍一个基于两阶段提交的事件计数法的互操作事务调度模型，包括全局事务调度、远程事务调度及互操作事务与本地事务的互斥。     

网格数据库事务管理策略

网格技术的本质就是提高资源的共享能力和实现计算资源的协同处理能力.网格数据库就是使传统数据库能够充分利用网格平台的这种优势,实现传统数据库的存储、管理、处理、分析等能力的共享和协同.网格数据库中的事务也存在全局事务和局部事务之分,对全局事务要保证得到正确的执行结果,其串行化是关键.给出了网格数据库全局事务的形式化定义,以及全局事务实现串行化的条件,并进行了相关证明.     

基于多代理的数据库并行事务处理模型

为了优化一般数据库管理系统的事务处理能力,提出了一种数据库事务并行处理算法。基于多代理技术,建立了一种数据库并行事务处理模型,该模型可以自动识别数据库事务的类别并分配相应的事务代理。通过多代理工具Jade实现了该模型中如事务类型的判别、事务代理的分配、代理的事务处理、结果的返回等基本功能。仿真实验结果表明,对比于传统的单任务事务处理方式,该模型是一种提高数据库事务处理能力的有效方案。     

移动分布式实时数据库中并发控制框架

在移动分布式数据库系统中采用三层结构，提出了DMVOCC-DA-2PLV（Distributed Multiversion Optimistic Concurrency Control—Dynamic Adjustment of Serialization Order-Two-Phase Local Validation）协议处理移动分布式实时事务。移动实时事务处理分两阶段进行。第一阶段在移动主机（MHs）上处理，并进行局部部分有效性检查性确认，使用向后有效性确认机制，与在服务器提交事务进行有效性确认。及早地检测数据冲突，节省了处理和通信资源。第二阶段在服务器处理，通过局部部分有效性确认的事务，提交到服务器进行局部最终有效性确认。协议消除了移动只读事务和移动更新事务的冲突，使用多版本动态调整串行次序技术，避免了不必要的事务重启动。如果移动只读事务所有读数据项通过局部部分向后有效性确认，则可提交，大大降低了移动只读事务的响应时间。在全局有效性确认中对分布更新事务进行检查，以保证分布串行性。通过模拟仿真，对DMVOCC-DA-2PV协议进行了性能测试，并与DTO-2PC和DHP-2PL进行了比较。实验结果表明DMVOCC-DA-2PV并发控制协议要优于其它协议。     

一个工程数据库管理系统并发控制模型

针对工程应用域事务的嵌套性、长时性、数据库逻辑分区性和涉及数据量大的四个特点，本文提出了一种工程数据库管理系统的并发控制模型，对工程应用域事务的管理、恢复和二段式检出协议、谓词二段式封锁作了阐述。     

基于代理的3PC分布式事务提交协议

原有3PC分布式事务提交协议能克服协同者发生故障而有可能产生事务阻塞问题，但其开销大。该文提出了一种基于代理的3PC事务提交协议，该协议通过增加协同者的代理节点，使得参与者相信协同者是正常的，从而不必关心新协同者的选举问题，能降低3PC无故障时的额外开销。     

基于多代理的数据库多副本事务处理模型研究

针对数据库多副本提出了一种数据库事务并行处理算法。基于多代理技术,建立一种数据库多副本并行事务处理模型,该模型可以自动识别数据库事务的类别并分配相应的事务代理并对超过瞬时处理能力的事物进行拥塞控制,能提高数据库事务处理能力。     

分布式实时事务提交协议

在分布式实时数据库系统中，保证事务原子性的唯一途径是研究和开发出一个实时的原子提交协议．首先详细分析了事务因数据访问冲突而形成的各种依赖关系，在此基础上提出了实时的原子乐观提交协议——2SC协议，该协议减少了事务的等待时间，提高了事务的并发度，且能无缝地和现有的并发控制协议集成在一起，保证事务的可串行化和原子性．通过模拟实验研究表明，采用该协议能够减少超过截止期的事务数目。     

An extended transaction to maintain consistency and recovery in multidatabase systems

A complete practical solution to transaction management preserving multidatabase consistency in the presence of multidatabase updates and failures is presented. The approach developed does not require support for the two-phase commit (2PC) protocol in the participating local database management systems (LDBMSs). Furthermore, it does not violate local autonomy; the source code of the LDBMSs is not modified in any way and the multidatabase system (MDBS) does not access or modify any control information of the LDBMS. The principles of the 2PC protocol in the process of global transaction commitment are adopted. The presented method does not rely on any specific concurrency control mechanism for LDBMSs. Consideration is given to global transaction failures due to subtransaction aborts by the LDBMSs and local site crashes. The recovery process is based on undo operations. While a global transaction is in progress, the tables accessed by subtransactions of this transaction at each local site are locked using specially initiated table locks. These locks are stored and maintained in the local database itself as control tables. The approach taken is easy to implement, and its limitations are discussed.     

On Concurrency Control in Multidatabase Systems with an Extended Transaction Model

A major concurrency control problem that we have to cope in multidatabase systems is the global deadlock detection and resolution problem. This detection must take into account the autonomy of local systems, which make impossible the visibility of the state of local transactions. A well-known approach to detect such deadlocks, called potential global deadlocks, is one based on the potential conflict graph (PCG) appropriate for the multidatabase transaction model with a global commit protocol. This classical transaction model is very constraining for applications manipulating great volumes of information, and where subtransaction terminations (commit or abort) of global transactions are not totally dependant. In this paper we present an effective potential global deadlock characterization, and an efficient potential global deadlock detection algorithm, in multidatabase systems with an extended transaction model more suited for such applications.     

Transaction management issues in a failure-prone multidatabase system environment

This paper is concerned with the problem of integrating a number of existing off-the-shelf local database systems into a multidatabase system that maintains consistency in the face of concurrency and failures.The major difficulties in designing such systems stem from the requirements that local transactions be allowed to execute outside the multidatabase system control, and that the various local database systems cannot participate in the execution of a global commit protocol. A scheme based on the assumption that the component local database systems use the strict two-phase locking protocol is developed. Two major problems are addressed: How to ensure global transaction atomicity without the provision of a commit protocol, and how to ensure freedom from global deadlocks.     

Using tickets to enforce the serializability of multidatabasetransactions

To enforce global serializability in a multidatabase environment the multidatabase transaction manager must take into account the indirect (transitive) conflicts between multidatabase transactions caused by local transactions. Such conflicts are difficult to resolve because the behavior or even the existence of local transactions is not known to the multidatabase system. To overcome these difficulties, we propose to incorporate additional data manipulation operations in the subtransactions of each multidatabase transaction. We show that if these operations create direct conflicts between subtransactions at each participating local database system, indirect conflicts can be resolved even if the multidatabase system is not aware of their existence. Based on this approach, we introduce optimistic and conservative multidatabase transaction management methods that require the local database systems to ensure only local serializability. The proposed methods do not violate the autonomy of the local database systems and guarantee global serializability by preventing multidatabase transactions from being serialized in different ways at the participating database systems. Refinements of these methods are also proposed for multidatabase environments where the participating database systems allow schedules that are cascadeless or transactions have analogous execution and serialization orders. In particular, we show that forced local conflicts can be eliminated in rigorous local systems, local cascadelessness simplifies the design of a global scheduler, and that local strictness offers no significant advantages over cascadelessness     

P2P多数据库系统中的事务并发控制研究

P2P多数据库系统是一种分布式的数据库系统,它是P2P计算系统和传统数据库系统的结合体.P2P计算系统的动态性使得P2P多数据库系统的事务管理缺少全局控制机制.在事务传播过程不存在循环的前提下,文章提出了一种保证P2P多数据库系统中事务全局串行化的并发控制协议,它通过控制一系列全局事务传播过程中每个熟识关系的两级串行化来实现.     

Revisiting Transaction Management in Multidatabase Systems

A lot of research efforts have focused on global serializability, global atomicity, and global deadlocks in multidatabase systems. Surprisingly, however, very few transaction processing model exists that ensures global serializability, global atomicity, and freedom from global deadlocks in a uniform manner. In this paper, we examine previous transaction processing models and propose a new transaction processing model that generates globally serializable and deadlock-free schedules in failure-prone multidatabase systems. A new transaction processing model adopts rigid conflict serializability as a correctness criterion on global serializability, and follows an emulated 2PC, criteria for global commitment, and an abort-based multidatabase recovery scheme for global serializability in failure-prone multidatabase systems. In addition, a deadlock-free policy is suggested where rigid conflict serializability is enforced when each subtransaction, including redo transactions, begins its execution. To practically support a new transaction processing model, Rigid Ticket Ordering (RTO) methods are designed. The proposed transaction processing model has the following improvements: (a) it resolves abnormal direct conflicts identified in this paper, (b) it imposes no restrictions on the execution of local transactions, and (c) it relaxes the restrictions on the execution of global transactions.     

Overview of multidatabase transaction management

A multidatabase system (MDBS) is a facility that allows users access to data located in multiple autonomous database management systems (DBMSs). In such a system,global transactions are executed under the control of the MDBS. Independently,local transactions are executed under the control of the local DBMSs. Each local DBMS integrated by the MDBS may employ a different transaction management scheme. In addition, each local DBMS has complete control over all transactions (global and local) executing at its site, including the ability to abort at any point any of the transactions executing at its site. Typically, no design or internal DBMS structure changes are allowed in order to accommodate the MDBS. Furthermore, the local DBMSs may not be aware of each other and, as a consequence, cannot coordinate their actions. Thus, traditional techniques for ensuring transaction atomicity and consistency in homogeneous distributed database systems may not be appropriate for an MDBS environment. The objective of this article is to provide a brief review of the most current work in the area of multidatabase transaction management. We first define the problem and argue that the multidatabase research will become increasingly important in the coming years. We then outline basic research issues in multidatabase transaction management and review recent results in the area. We conclude with a discussion of open problems and practical implications of this research.     

Concurrency control in hierarchical multidatabase systems

Over the past decade, significant research has been done towards developing transaction management algorithms for multidatabase systems. Most of this work assumes a monolithic architecture of the multidatabase system with a single software module that follows a single transaction management algorithm to ensure the consistency of data stored in the local databases. This monolithic architecture is not appropriate in a multidatabase environment where the system spans multiple different organizations that are distributed over various geographically distant locations. In this paper, we propose an alternative multidatabase transaction management architecture, where the system is hierarchical in nature. Hierarchical architecture has consequences on the design of transaction management algorithms. An implication of the architecture is that the transaction management algorithms followed by a multidatabase system must be composable– that is, it must be possible to incorporate individual multidatabase systems as elements in a larger multidatabase system. We present a hierarchical architecture for a multidatabase environment and develop techniques for concurrency control in such systems. Edited by R. Sacks-Davis. Received June 27, 1994 / Accepted September 26, 1995