分布式实时事务的原子提交

分布式实时数据库系统中，事务除具有传统数据库事务的ACID特性外，还具有定时限制（如截止期等）。传统提交协议及已研究出的实时提交协议都不保证分布式实时事务的原子性，结合实时事务截止期要求，描述了分布式实时事务的原子提前及其正确性准则，给出了适合于固（或软）实时事务的原子提前协议RTACP及其处理过程并证明了其正确性，还对RTACP进行了性能比较和评价，表明其分别在不同方面更优越。     

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     

基于预分析的实时事务处理

实时数据库系统中事务可能有实时限制（典型地为截止期），事务超过截止期可能给系统带来灾难性后果，这就要求系统能对事务的执行进行预分析与预安排，传统事务处理由于事务执行过程中的多种不确定因素（数据I／O，页面缺失，事务之间的各种相关性等），事务执行具有动态不可预报性，提出了一种事务预分析处理的三段法，数据说明阶段的静态预分析提取事务静态信息；初始动态预分析阶段构造事务的“影了事务”，并在适当的时机调度影子事务执行，以保证事务操作之前所需数据已在内存，事务执行时动态预分析阶段构造事务的相关性，为事务执行的结构，行为及时间正确性提供统一的实现机制，事务执行具有更好的可预报性，从而事务能更好地满足截止期。     

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。     

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．     

The PROMPT real-time commit protocol

We investigate the performance implications of providing transaction atomicity for firm-deadline real-time applications, operating on distributed data. Using a detailed simulation model, the real-time performance of a representative set of classical transaction commit protocols is evaluated. The experimental results show that data distribution has a significant influence on real-time performance and that the choice of commit protocol clearly affects the magnitude of this influence. We also propose and evaluate a new commit protocol, PROMPT (Permits Reading Of Modified Prepared-data for Timeliness), that is specifically designed for the real-time domain. PROMPT allows transactions to “optimistically” borrow, in a controlled manner, the updated data of transactions currently in their commit phase. This controlled borrowing reduces the data inaccessibility and the priority inversion that is inherent in distributed real-time commit processing. A simulation-based evaluation shows PROMPT to be highly successful, as compared to the classical commit protocols, in minimizing the number of missed transaction deadlines. In fact, its performance is close to the best on-line performance that could be achieved using the optimistic lending approach. Further, it is easy to implement and incorporate in current database system software. Finally, PROMPT is compared against an alternative priority inheritance-based approach to addressing priority inversion during commit processing. The results indicate that priority inheritance does not provide tangible performance benefits     

多版本两阶段封锁并发控制协议性能研究

在并行实时数据库中，并发控制协议不仅要满足数据库一致性约束，而且要满足时间约束。本文提出了实时多版本两阶段封锁并发控制协议，它能有效地减少延误截止时间事务数量。作者通过仿真模拟，在共享磁盘体系结构下与HP2PL和OCC-TI-WAIT-50协议进行了比较。实验结果表明，该协议比HP2PL和OCC-TI-WAIT-50协议性能更好。     

Data access scheduling in firm real-time database systems

A major challenge addressed by conventional database systems has been to efficiently implement the transaction model, which provides the properties of atomicity, serializability, and permanence. Real-time applications have added a complex new dimension to this challenge by placing deadlines on the response time of the database system. In this paper, we examine the problem of real-time data access scheduling, that is, the problem of scheduling the data accesses of real-time transactions in order to meet their deadlines. In particular, we focus on firm deadline real-time database applications, where transactions that miss their deadlines are discarded and the objective of the real-time database system is to minimize the number of missed deadlines. Within this framework, we use a detailed simulation model to compare the performance of several real-time locking protocols and optimistic concurrency control algorithms under a variety of real-time transaction workloads. The results of our study show that in moving from the conventional database system domain to the real-time domain, there are new performance-related forces that come into effect. Our experiments demonstrate that these factors can cause performance recommendations that were valid in a conventional database setting to be significantly altered in the corresponding real-time setting.This work was done while the author was with the Computer Sciences Department, University of Wisconsin-Madison. This research was partially supported by the National Science Foundation under grant IRI-8657323.     

Processing real-time transactions in a replicated database system

A database system supporting a real-time application has to provide real-time information to the executing transactions. Each real-time transaction is associated with a timing constraint, typically in the form of a deadline. It is difficult to satisfy all timing constraints due to the consistency requirements of the underlying database. In scheduling the transactions it is aimed to process as many transactions as possible within their deadlines. Replicated database systems possess desirable features for real-time applications, such as a high level of data availability, and potentially improved response time for queries. On the other hand, multiple copy updates lead to a considerable overhead due to the communication required among the data sites holding the copies. In this paper, we investigate the impact of storing multiple copies of data on satisfying the timing constraints of real-time transactions. A detailed performance model of a distributed database system is employed in evaluating the effects of various workload parameters and design alternatives on the system performance. The performance is expressed in terms of the fraction of satisfied transaction deadlines. A comparison of several real-time concurrency control protocols, which are based on different approaches in involving timing constraints of transactions in scheduling, is also provided in performance experiments. Recommended by: A. ElmagarmidThis work was initiated while the author was at the Computer Science Department, University of Illinois at Urbana-Champaign.     

Optimistic priority-based concurrency control protocol for firm real-time database systems

This paper presents an optimistic priority-based concurrency control protocol that schedules active transactions accessing firm deadline real-time database systems. This protocol combines the forward and backward validation processes in order to control concurrent transactions with different priorities more effectively. For a transaction in the validation phase, it can be committed successfully if the serialization order is adjusted in favour of the transactions with higher priority and aborted otherwise. Thus, this protocol establishes a priority ordering technique whereby a serialization order is selected and transaction execution is forced to obey this order. This priority-based protocol addresses the problem of satisfying data consistency, with the goal being to increase the number of transactions that commit by their deadlines. In addition, for desirable real-time conflict resolution, this protocol intends to meet more deadlines of higher priority transactions then lower priority transactions.     

Secure Concurrency Control in Firm Real-Time Database Systems

Many real-time database applications arise in electronic financial services, safety-critical installations and military systems where enforcing security is crucial to the success of the enterprise. For real-time database systems supporting applications with firm deadlines, we investigate here the performance implications, in terms of killed transactions, of guaranteeing multilevel secrecy. In particular, we focus on the concurrency control (CC) aspects of this issue.Our main contributions are the following: First, we identify which among the previously proposed real-time CC protocols are capable of providing covert-channel-free security. Second, using a detailed simulation model, we profile the real-time performance of a representative set of these secure CC protocols for a variety of security-classified workloads and system configurations. Our experiments show that a prioritized optimistic CC protocol, OPT-WAIT, provides the best overall performance. Third, we propose and evaluate a novel dual-CC approach that allows the real-time database system to simultaneously use different CC mechanisms for guaranteeing security and for improving real-time performance. By appropriately choosing these different mechanisms, concurrency control protocols that provide even better performance than OPT-WAIT are designed. Finally, we propose and evaluate GUARD, an adaptive admission-control policy designed to provide fairness with respect to the distribution of killed transactions across security levels. Our experiments show that GUARD efficiently provides close to ideal fairness for real-time applications that can tolerate covert channel bandwidths of upto one bit per second.     

Transaction Scheduling in Distributed Real-Time Systems

Inthis paper, we study the performance of using optimistic approachto concurrency control in distributed real-time database systems(RTDBS). The traditional optimistic approach suffers from theproblem of unnecessary restarts. Transaction restarts can significantlyincrease the system workload and intensify resource and datacontention. In distributed environments, the complexity of thesystem and the high communication overhead exacerbate the problem.Therefore, the number of unnecessary restarts is the determinantfactor that affects the performance of optimistic approach indistributed RTDBS. When optimistic approach is extended to distributedenvironments, a number of issues resulting from the increasedcomplexity and communication overhead have to be resolved. Inthis paper, a new real-time distributed optimistic concurrencycontrol (DOCC) protocol with dynamic adjustment of serializationorder (DASO), called DOCC-DA is proposed. This protocol can avoidunnecessary transaction restarts by dynamically adjusting theserialization order of the conflicting transactions. Therefore,resources can be saved and more transactions can meet their deadlines.In the DOCC-DA protocol, a new distributed circular validationscheme is included to facilitate transaction validation in distributedenvironments. The performance of the DOCC-DA protocol has beenexamined in detail by simulation. The results showed that theperformance of the DOCC-DA protocol is consistently better thanthat of other protocols.     

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     

分布式实时数据库并发控制

在集中式实时数据库中，事务并发控制一般采用基于优先级的事务重启或优先级继承．在分布式实时数据库中，由于一个主事务的各参与事务可能分布在多个节点上，事务重启一方面会造成系统资源的极大浪费，另一方面又会加大网络传输的负担．而在优先级继承策略中，被阻塞的高优先级事务的执行完全依赖于阻塞它的低优先级事务，在分布式环境中，事务的执行具有更多的不可确定的因素，这就很难保证被阻塞事务的截止期．本文提出了一种基于数据多映像的并发控制协议，事务因访问数据而形成不同的依赖关系，改变高优先级事务和低优先级事务的依赖关系从而更有利于高优先级事务而又不矢折或阻塞其他事务，从而大大提高事务执行的并发度，更好地满足实时事务的截止期．     

A Commit Strategy for Distributed Real-Time Transaction

Ramamritham gives three common types of constraints for the execution his-tory of concurrent transactions. This paper extends the constraints and gives the fourth type of constraint. Then the weak commit dependency and abort dependency between transactions, be-cause of data access conflicts, axe analyzed. Based on the analysis, an optimistic commit protocol 2LC (two-Level Commit) is proposed, which is specially designed for the distributed real-time do-main. It allows transactions to optimistically access the locked data in a controlled manner, which reduces the data inaccessibility and priority inversion inherent and undesirable in distributed real-time database systems. Furthermore, if the prepared transaction is aborted, the transactions in its weak commit dependency set will execute as normal according to 2LC. Extensive simulation ex-periments have been performed to compare the performance of 2LC with that of the base protocol,the permits reading of modified prepared-data for timeliness (PROMPT) and the deadline-driven conflict resolution (DDCR). The simulation results show that 2LC is effective in reducing the num-ber of missed transaction deadlines. Furthermore, it is easy to be incorporated with the existing concurrency control protocols.     

Databases with deadline and contingency constraints

Real-time database systems associate the concept of deadlines with transaction executions. Previous approaches use “best effort” techniques to schedule a given set of transactions to meet the deadlines as well as to ensure the consistency of the database. However, such approaches are inadequate for target applications which have “hard” real-time deadlines that need to be met in the event of crisis situations. In such cases, it is important to obtain contingency plans that may be invoked with guaranteed execution time characteristics. This paper presents an alternative model for real-time database systems in which deadlines are associated with “contingency” constraints rather than directly with transactions. Our approach leads to a predicate-based model that intrinsically incorporates both triggering and relative timing constraints regarding the transaction executions. We exhibit that selecting contingency plans with respect to various optimality criteria has inherent computational inefficiencies. We study the issues in scheduling of the selected plans with the focus on the contention among the transactions for data resources. Our results exhibit that the data contention, by itself, has a severe adverse impact on the schedulability of the deadline-constrained transactions. We discuss some of the practical implications of our results, and we suggest some counter-measures to handle the computational complexities     

分布式实时事务提交处理

由于提交处理的复杂性,分布式实时事务很难满足其截止期.提出了一种新的提交协议A2SC(主动的双空间提交),它适合于分布式实时事务提交处理的需要.分析了由于数据冲突访问而形成的各种依赖关系.当处于准备状态的事务和处于提交状态的事务发生数据冲突访问时,A2SC允许处于执行状态的事务在一种控制的方式下乐观地访问锁住的数据.当处于准备状态的事务夭折时,仅仅只有其夭折依赖集中的事务夭折.进一步提出了"没有结果的运行"的观念.当一个事务发现它是没有结果的允许时,它将主动夭折.进行了广泛的模拟实验比较A2SC和其它协议比如基准协议、PROMPT和DDCR的性能.模拟结果表明A2SC在最小化错过截止期的事务数方面较成功,因此A2SC适合于高性能分布式实时事务.     

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。     

一种改进的内存数据库封锁机制

事务之间的封锁协议一直是数据库领域中的研究重点。在典型的内存数据库系统中,由于事务通常都比较短小,执行速度快,因此事务之间通常采用类似单写多读这样较为简单的封锁控制协议,将不同事务近似串行地分别执行。在事务并发量较高但是相互之间数据交集不大的情况下,可以通过引入粗粒度意向锁和锁粒度变换的方式来进一步提高事务之间的并行程度,加速事务的整体执行,提高事务吞吐量和系统响应速度。     

SWIFT—A new real time commit protocol

Although there are several factors contributing to the difficulty in meeting distributed real time transaction deadlines, data conflicts among transactions, especially in commitment phase, are the prime factor resulting in system performance degradation. Therefore, design of an efficient commit protocol is of great significance for distributed real time database systems (DRTDBS). Most of the existing commit protocols try to improve system performance by allowing a committing cohort to lend its data to an executing cohort, thus reducing data inaccessibility. These protocols block the borrower when it tries to send WORKDONE/PREPARED message [1, 6, 8, 9], thus increasing the transactions commit time. This paper first analyzes all kind of dependencies that may arise due to data access conflicts among executing-committing transactions when a committing cohort is allowed to lend its data to an executing cohort. It then proposes a static two-phase locking and high priority based, write-update type, ideal for fast and timeliness commit protocol i.e. SWIFT. In SWIFT, the execution phase of a cohort is divided into two parts, locking phase and processing phase and then, in place of WORKDONE message, WORKSTARTED message is sent just before the start of processing phase of the cohort. Further, the borrower is allowed to send WORKSTARTED message, if it is only commit dependent on other cohorts instead of being blocked as opposed to [1, 6, 8, 9]. This reduces the time needed for commit processing and is free from cascaded aborts. To ensure non-violation of ACID properties, checking of completion of processing and the removal of dependency of cohort are required before sending the YES-VOTE message. Simulation results show that SWIFT improves the system performance in comparison to earlier protocol. The performance of SWIFT is also analyzed for partial read-only optimization, which minimizes intersite message traffic, execute-commit conflicts and log writes consequently resulting in a better response time. The impact of permitting the cohorts of the same transaction to communicate with each other [5] on SWIFT has also been analyzed. Recommended by: Ahmed Elmagarmid