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。     

面向对象工程数据库管理系统O2S并发事务模型

Ｏ２Ｓ是我们在ＳＵＮ工作站上开发的一面向对象工程数据库管理。本文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显著特点，阐述了Ｓ２Ｓ的事务管理模型。对工程域事务的管理，数据库的恢复和封锁技术进行了讨论。     

基于补偿操作的嵌套事务死锁处理方法

补偿操作是实现复杂的数据库应用环境中嵌套事务回滚的有效方法，但是基于封锁机制的补偿操作在执行时有可能导致不可补偿死锁．文中介绍了基于补偿操作的嵌套事务死锁处理方法，讨论了两种可避免不可补偿死锁产生的安全补偿方法，并比较了它们的特点．     

工程数据库中一种支持合作设计事务的并发控制机制

长期性，合作性与用户控制是工程设计环境对数据库事务的新要求，传统的并发控制机制缺乏支持，文中提出了嵌套二阶段有序相容性锁协议以嵌套事务模型为基础，以有序相容性封锁为并控制手段，能够较好地支持上述要求，此外由于该协议是基于二阶段封锁协议，因此易于在现有ＤＢＭＳ中通过扩充来实现，是一个实用的并发控制机制。     

基于事务截止期的动态多粒度封锁机制

多粒度封锁机制提高了数据库系统的并发性,但在实时数据库中,由于事务处理的限时特性,对传统的多粒度封锁机制提出了挑战。综合考虑多粒度封锁机制与事务的截止期,提出了一种基于事务截止期的动态多粒度封锁机制,由事务截止期确定事务冲突级别,当事务冲突到达或降到一定级别后,根据当前加锁粒度决定是否对其进行调整。通过仿真实验证明本机制可减少事务冲突,降低事务错失率及事务重启率,提高了事务并发性和实时性。     

WHYMX单级事务的并发控制

本文论述了面向对象数据库系统WHYMX单级事务的并发控制机制,阐明了一种适用于面向对象数据库的并发控制策略和封锁协议,有效地解决了单级事务的并发控制问题。     

实时Client/Server数据库多版本两个阶段封锁并发控制协议

提出了实时Client／Server数据库多版本两阶段封锁并发控制协议．该协议具有多版本并发控制机制与两阶段封锁机制的优点，使用如下策略以减少延误截止时间事务数量：若冲突集中有比持锁事务Ti优先级高的事务，且Ti重启动不会延误截止时间，则Ti重启动，冲突集中优先级最高的事务获得锁；否则，冲突集中其它事务等待．为了提高只读事务的响应时间，客户端设有一致数据库影子，只读事务在客户端处理．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，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．     

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…     

NXD数据库事务机制的研究

原生XML数据库(NXD)的事务处理机制是保障数据库正常运行的核心机制,是当前研究的一个重点.在分析了现事务处理机制的基础上,结合关系数据库中成熟的封锁理论,提出基于XPath的XPL四种锁封锁机制,对数据库的操作和事务做出了明确的定义,并给出了实例进行验证说明.     

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。     

Global lock escalation in database management systems

Since database management systems (DBMSs) have limited lock resources, transactions requesting locks beyond the limit must be aborted, degrading the performance abruptly. Lock escalation can be effectively used in such circumstances to alleviate the problem. Many lock escalation methods have been proposed and implemented in commercial DBMSs. However, they have certain problems due to the local nature of their decisions on when to execute lock escalation. In this paper, we propose a new lock escalation method, global lock escalation, that makes such decision globally based on the total number of locks. Through extensive simulation, we show that the global lock escalation method outperforms the existing ones significantly. Especially, we show that the number of concurrent transactions allowable increases by 2-16 times. We believe our method can be easily implemented in the commercial DBMSs enhancing the performance significantly under excessive lock requests.     

Implementation and modeling of two-phase locking concurrency control—a performance study

Two-phase locking (2PL) is the concurrency control mechanism that is used in most commercial database systems. In 2PL, for a transaction to access a data item, it has to hold the appropriate lock (read or write) on the data item by issuing a lock request. While the way transactions set their lock requests and the way the requests are granted would certainly affect a system's performance, such aspects have not received much attention in the literature. In this paper, a general transaction-processing model is proposed. In this model, a transaction is comprised of a number of stages, and in each stage the transaction can request to lock one or more data items. Methods for granting transaction requests and scheduling policies for granting blocked transactions are also proposed. A comprehensive simulation model is developed from which the performance of 2PL with our proposals is evaluated. Results indicate that performance models in which transactions request locks on an item-by-item basis and use first-come-first-served (FCFS) scheduling in granting blocked transactions underestimate the performance of 2PL. The performance of 2PL can be greatly improved if locks are requested in stages as dictated by the application. A scheduling policy that uses global information and/or schedules blocked transactions dynamically shows a better performance than the default FCFS.     

A formal approach to lock escalation

Since database management systems(DBMSs) have limited lock resources, transactions requesting locks beyond the limit must be aborted, degrading the performance abruptly. Lock escalation is considered a solution to this problem. However, existing lock escalation methods have been designed in an ad hoc manner. So, they do not provide a complete solution. In this paper, we propose a formal model of lock escalation. Using the model, we analyze the roles of lock escalation formally and solve the problems of the existing methods systematically. In particular, we introduce the concept of the unescalatable lock that cannot be escalated due to conflicts. We identify that the unescalatable lock is the major cause of exhausting lock resources. We then analyze the reasons why unescalatable locks are generated and propose a new lock escalation method, adaptive lock escalation, which controls lock escalation based on the number of unescalatable locks. Through extensive simulation, we show that adaptive lock escalation significantly outperforms existing methods reducing the number of aborts and the average response time and increasing the throughput to a great extent. Adaptive lock escalation drastically reduces (more than 10 fold) the number of lock resources required to maintain the same level of throughput and average response time. At the same time, the throughput and average response time when using adaptive lock escalation are rather insensitive to the number of lock resources. Existing methods rely on users to estimate this number accurately at system initialization time. Adaptive lock escalation greatly alleviates this burden.     

On Using Real-Time Static Locking Protocols for Distributed Real-Time Databases

The use of Static Two Phase Locking Protocols (S2PL) for concurrency control in real-time database systems (RTDBS) has received little attention in the past. Actually, real-time S2PL (RT-S2PL) protocols do possess some desirable features making them suitable for RTDBS, especially for distributed real-time database systems (DRTDBS) in which remote locking is required and distributed deadlock is possible. In this paper, different RT-S2PL protocols are proposed. They differ in their methods of reducing the blocking time of higher priority transactions. Their performance is studied and compared with a real-time dynamic two phase locking protocol (RT-D2PL), called Hybrid Two Phase Locking (Hb2PL). The impact of different system and workload parameters, such as mean inter-arrival time of transactions, number of remote lock requests of a transaction, communication overhead for sending messages, and database size on their performance have been examined. The performance results indicate that the RT-S2PL protocols are suitable for DRTDBS in which the proportion of local locks of a transaction is small and the communication overhead for locking is high.     

Graph directed locking

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     

Analysis of locking behavior in three real database systems

Concurrency control is essential to the correct functioning of a database due to the need for correct, reproducible results. For this reason, and because concurrency control is a well-formulated problem, there has developed an enormous body of literature studying the performance of concurrency control algorithms. Most of this literature uses either analytic modeling or random number-driven simulation, and explicitly or implicitly makes certain assumptions about the behavior of transactions and the patterns by which they set and unset locks. Because of the difficulty of collecting suitable measurements, there have been only a few studies which use trace-driven simulation, and still less study directed toward the characterization of concurrency control behavior of real workloads. In this paper, we present a study of three database workloads, all taken from IBM DB2 relational database systems running commercial applications in a production environment. This study considers topics such as frequency of locking and unlocking, deadlock and blocking, duration of locks, types of locks, correlations between applications of lock types, two-phase versus non-two-phase locking, when locks are held and released, etc. In each case, we evaluate the behavior of the workload relative to the assumptions commonly made in the research literature and discuss the extent to which those assumptions may or may not lead to erroneous conclusions. Edited by H. Garcia-Molina. Received April 5, 1994 / Accepted November 1, 1995     

Performance Evaluation of Centralized Databases with Static Locking

The performance of transaction processing systems is determined by the contention for hardware as well as software resources (database locks), due to the concurrency control mechanism of the database being accessed by transactions. We consider a transaction processing system with a set of dominant transcation classes. Each class needs to acquire a certain subset of the locks in the database before it can be processed, i.e., predeclared lock requests with static locking. Straightforward application of the decomposition method requires the numerical solution of a two-dimensional Markov chain. Equivalently, a hierarchical simulation method, where the computer system is represented by a composite queue with exponential service rates, can be used to analyze the system. We propose an inexpensive analytic solution method, also based on hierarchical decomposition, such that the throughput of the computer system ic characterized by the number of active transactions (regardless of class). Numerical results are provided to show that the new method is adequately accurate compared to the other two rather costly methods. It can be used to determine the effect of granularity of locking on system performance. The solution method is also applicable to multiresource queueing systems with multiple contention points.     

以B链树为索引的动态多版本数据库的并发控制

在对B链树极高同步性能研究的基础上,提出了一种将B链树作为数据库索引并和多版本技术相接合的一种新颖方案。该方案将事务分为只读事务或更新事务,只读事务不需要获取锁,而更新事务也只需要少量的锁,不会形成死锁。实验表明,在并发环境下这种方案能较大的提高数据库性能和事务的吞吐量。     

Performance analysis of concurrency control using locking withdeferred blocking

The concurrency control (CC) method employed can be critical to the performance of transaction processing systems. Conventional locking suffers from the blocking phenomenon, where waiting transactions continue to hold locks and block other transactions from progressing. In a high data contention environment, as an increasing number of transactions wait, a larger number of lock requests get blocked and fewer lock requests can get through. The proposed scheme reduces the blocking probability by deferring the blocking behavior of transactions to the later stages of their execution. By properly balancing the blocking and abort effects, the proposed scheme can lead to better performance than either the conventional locking or the optimistic concurrency control (OCC) schemes at all data and resource contention levels. We consider both static and dynamic approaches to determine when to switch from the nonblocking phase to the blocking phase. An analytical model is developed to estimate the performance of this scheme and determine the optimal operating or switching point. The accuracy of the analytic model is validated through a detailed simulation     

A Priority Based Distributed Deadlock Detection Algorithm

Deadlock handling is an important component of transaction management in a database system. In this paper, we contribute to the development of techniques for transaction management by presenting an algorithm for detecting deadlocks in a distributed database system. The algorithm uses priorities for transactions to minimize the number of messages initiated for detecting deadlocks. It does not construct any wait-for graph but detects cycles by an edge-chasing method. It does not detect any phantom deadlock (in the absence of failures), and for the resolution of deadlocks it does not need any extra computation. The algorithm also incorporates a post-resolution computation that leaves information characterizing dependence relations of remaining transactions of the deadlock cycle in the system, and this will help in detecting and resolving deadlocks which may arise in the future. An interesting aspect of this algorithm is that it is possible to compute the exact number of messages generated for a given deadlock configuration. The complexity is comparable to the best algorithm reported. We first present a basic algorithm and then extend it to take into account shared and exclusive lock modes, simultaneous acquisition of multiple locks, and nested transactions.