一种实时事务并发控制的新方法:SMVCC

提出了新颖的实时数据库事务并发控制方法：语义多版本并发控制协议（SMVCC）。利用实时数据库中存在多版本数据的特点，考虑实时数据和实时事务语义，结合实时数据库相似性概念，对传统多版本并发控制方法进行扩展，对不同事务采用不同并发控制策略，提高了事务并发度。     

嵌入式数据库并发控制研究

嵌入式数据库中多个并发执行的事务需进行控制来保证数据库的完整性和一致性。介绍几种常见的数据库并发控制策略,提出事务并发控制管理器体系结构和封锁机制,给出基于锁的嵌入式数据库并发控制实现策略,及并发过程中出现死锁时的处理机制,为嵌入式数据库并发控制的研究提供了理论依据。     

如何清除ORACLE服务器中非正常的会话

１．引言大型数据库是一个共享资源,多个用户程序可并行地存取数据库,如果不对并发操作进行控制,会存取不正确的数据或破坏数据库数据的一致性。ＯＲＡＣＬＥ利用事务和封锁机制提供数据并发存取和数据完整性,在一事务内由语句获取的全部封锁在事务期间被保持,直至该事务提交或回滚时被释放。对于用ＯＲＡＣＬＥ７数据库服务器构造的较大规模的企业级客户／服务器应用系统,众多的客户端由于各种各样的原因（硬件故障、意外掉电等等）难免造成用户会话的意外中止,而ＯＲＡＣＬＥ本身不主动清除这些垃圾会话（ｓｅｓｓｉｏｎ）,如果这…     

传感器网络中协作实时数据库事务的提交控制

传统的事务模型由于其严格的ACID特性无法适应传感器网络中协作事务的实时性要求.因此着重研究传感器网络中协作实时数据库事务的提交机制.首先,分析传感器网络中实时数据库应用的协作特性,放松事务的严格ACID要求,运用控制区域的概念提出一种基于事务的行为语义控制区域的事务模型.然后,给出该模型的提交、回滚、可见规则及能保证数据库一致性和协作事务实时性的三阶段提交协议,并通过补偿事务保证外化信息准确性.性能测试表明,该模型及提交机制提高了采集数据的外化率和协作实时事务的成功率.     

基于截止期的可推测实时数据库事务并发控制

在实时数据库应用环境中,最大的并发数或者吞吐量不再作为性能的重要衡量尺度,而把在截止期之前完成的事务数量作为决定性的性能指标。因此,综合乐观并发控制（OCC）和悲观并发控制（PCC）的方法,提出了基于截止期的两阶段可推测并发控制技术（PSCC）以提高事务满足时间限制的可能性。首先,基于事务的截止期对潜在的可能带来数据不一致性的威胁进行推测,尽早地重启那些变为现实的威胁可能性较大的冲突事务,允许变为现实可能性较小的冲突事务继续执行。其次,在校验阶段采用广播式的提交技术控制未违背提交一致性的事务的提交,保证了数据库一致性,同时暂时阻塞违背提交一致性的事务并利用优先级升级技术减少被阻塞事务的不必要的重启。最后,给出了PSCC的行为规则及算法描述。性能测试结果分析表明,PSCC比OCC-BC和HPA有更低的失败率的重启率。     

分布式数据库多级一致性统一建模理论研究

分布式数据库系统出现了支持多协调器和多副本存储的新架构,这给事务调度的正确性带来了新的挑战,包括缺少中心协调器带来的新数据异常以及多副本机制带来的读取数据一致性等问题.基于事务隔离级别和分布式系统一致性协议的定义,为多协调器多副本分布式数据库的事务多级一致性构建了一个混合依赖图模型.该形式化模型为事务的正确调度提供具有鲁棒性的评价标准,可以方便地对数据库事务调度情况进行动态或静态分析检验.     

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。     

实时数据库并发控制

可串行化的并发控制对传统应用是适合的,它旨在确保数据库的一致性,提高系统的吞吐量及减小事务平均响应时间,而在实时数据库中,一方面系统追求的主要目标是单个事务定时限制（典型地为截止期）的满足,另一方面,系统中的数据库是外部环境在逻辑世界中的映射,局部的数据库不一致能够随下一次数据采样恢复而不会大规模扩散,这些都为有利于传统严格一致的可串化并发控制的并发控制协议的采用提供了必要性和可能性,本文首先给出了数据库“相似性”的形式化定义,接着提出了“视图一致性”的概念,然后提出了一种新的并发控制协议,它考虑了数据的相似性,并使只读事之间并不满足传统的严格可串行化的标准,但并没有破坏数据库的一致性,事务执行的并发度有了提高,有利于实时事务定时限制的满足,最后我们将这种并发控制协议的性能与有名的H2PL协议的性能进行了比较。     

分布式事务处理的若干关键技术

保证本地/异地访问控制和同构/异构数据库管理的数据一致性和事务完整性是分布式联机事务处理的关键因素之一。在数据库系统中,事务是维护数据一致性的单位,它使数据库从一种旧的一致状态转变为一种新的一致状态。在每个事务结束时,数据库系统总是保持一致性。事务分为单节点数据库事务、多节点同构数据库事务和多节点异构数据库事务。     

Delphi程序中的事务处理

SQL数据库服务器处理本地任务叫事务.事务是一组SQL语句,它们必须在服务器对数据库更新前成功完成.所有的语句要么都成功,要么都失败.数据库事务保证了数据库的一致性,即使有硬件故障时也如此.当有多个用户并行使用数据库时保证了数据的完整性.例如:更新订单表ORDERS的应用程序表明接受一个项目的订单,同时程序也应更新投资表INVERTORY以反映投资方的现有资金的减少.如果在第一个更新之后而在第二个更新之前硬件发生故障,数据库就出现不一致,因为投资库中没有反映出输入的订单.在事务控制下,两个语句同时提交就可防止这种情况.事务控制在多用户应用中尤其重要.     

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.     

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     

基于扩展自然序树的概化关联规则增量挖掘方法

概化关联规则挖掘作为数据挖掘领域一个重要的拓展性研究课题,首先提出了一种概化扩展自然序树(generalized extended canonical-order tree,GECT)结构及其增量挖掘算法GECT-IM.该算法对原始分类事务数据库只扫描一次,就可以将所有交易信息映射至一棵压缩格式的GECT,然后通过对更新交易数据集扫描得到更新数据集中各项集的计数,结合相关性质及运算就可以发现大部分更新后的概化频繁项集;其次,针对GECT规模较大以及GECT-IM 算法仍然可能需要遍历初始GECT树的局限,在界定数据库更新和重构概念的基础上,基于一种可量化度量的准最小支持度阈值,提出了一种改进的准频繁概化扩展自然序树(pre-large generalized extended canonical-order tree,PGECT)结构及其增量挖掘算法PGECT-IM.由于有效避免了对初始GECT进行遍历的情形,从而进一步提升了概化关联规则增量挖掘效率.实验证明,提出的概化关联规则增量挖掘算法 GECT-IM 及其优化算法PGECT-IM,比现有增量挖掘算法具有更高的挖掘效率和更好的扩展性.     

A non-two-phase locking protocol for global concurrency control indistributed heterogeneous database systems

A concurrency control method is proposed for global transactions in a distributed heterogeneous database system. This method is applicable when the database sites are interconnected in a rooted tree fashion. It guarantees deadlock freedom in addition to serializability. A general architecture of a heterogeneous system is given. The global transaction manager (GTM) decomposes the global transactions initiated at a site and the subtransactions received from other sites into smaller subtransactions, some of which are sent to the GTMs of the other sites, and those remaining, called g-local transactions, are to be executed by the local transaction manager (LTM) at that site. A concurrency control mechanism ensures serializability among: the local transactions (including the g-local transactions of the global ones) at each site, the global transactions and the global and local transactions together     

Global scheduling for flexible transactions in heterogeneousdistributed database systems

A heterogeneous distributed database environment integrates a set of autonomous database systems to provide global database functions. A flexible transaction approach has been proposed for the heterogeneous distributed database environments. In such an environment, flexible transactions can increase the failure resilience of global transactions by allowing alternate (but in some sense equivalent) executions to be attempted when a local database system fails or some subtransactions of the global transaction abort. We study the impact of compensation, retry, and switching to alternative executions on global concurrency control for the execution of flexible transactions. We propose a new concurrency control criterion for the execution of flexible and local transactions, termed F-serializability, in the error-prone heterogeneous distributed database environments. We then present a scheduling protocol that ensures F-serializability on global schedules. We also demonstrate that this scheduler avoids unnecessary aborts and compensation     

Application Specific Transaction Management in Multidatabase Systems

We present a transaction model for multidatabase systems with autonomous component systems, coined heterogeneous 3-level transactions. It has become evident that in such a system the requirements of guaranteeing full ACID properties and full local autonomy can not be reconciled. In the heterogeneous 3-level transaction model, semantics of actions and specific properties of the architecture of multidatabase systems and of applications are utilized to define application specific compromises between these competing goals. We consider different definitions of conflicts between actions together with application specific sets of allowed (autonomous) local transactions and global subtransactions. A formal model for serializability is given. It is proven that heterogeneous 3-level transactions guarantee all consistency constraints that are ensured in serial executions. In contrast to most other models, we do not require data to be partitioned into global and local data. In particular, heterogeneous 3-level transactions even allow the same data to be updated by local and global transactions in parallel, if these updates are found to be semantically non-conflicting. Recovery is handled by inverse actions. We present a formal framework for this approach which is fully integrated into serializability theory by considering inverse actions as ordinary actions. This has an important impact on the relationship of concurrency control and recovery in our model: By changing the definition of conflicts between actions in order to tailor the model according to application needs, recovery is implicitly affected and no further explicit adaptation of recovery algorithms is necessary. The heterogeneous 3-level transaction model assumes component database systems to support ACID transaction properties. Further properties of local transactions or interfaces of the component database systems are not required. We describe a prototype implementation of heterogeneous 3-level transactions in the object-oriented database system VODAK.     

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     

Supporting consistent updates in replicated multidatabase systems

Replication is useful in multidatabase systems (MDBSs) because, as in traditional distributed database systems, it increases data availability in the presence of failures and decreases data retrieval costs by reading local or close copies of data. Concurrency control, however, is more difficult in replicated MDBSs than in ordinary distributed database systems. This is the case not only because local concurrency controllers may schedule global transactions inconsistently, but also because local transactions (at different sites) may access the same replicated data. In this article, we propose a decentralized concurrency control protocol for a replicated MDBS. The proposed strategy supports prompt and consistent updates of replicated data by both local and global applications without a central coordinator.     

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.