基于数据时态特性的实时事务并发控制

通过对数据时态特性及其对事务调度的影响进行分析,提出了基于数据时态特性的实时事务并发控制算法.该算法根据数据截止期及事务的执行时间估算,改进了事务的验证规则,对事务的提交顺序进行调整,提高了系统的实时性能.理论分析与实验结果表明:该算法降低了事务重启个数及超截止期百分率,性能要优于已有的实时并发控制算法.     

实时数据库中具有时态限制的事务调度算法

在实时数据库中，事务对时态数据对象的访问在很多领域的应用日益广泛．目前实时数据库中的事务调度算法大多仅考虑数据的逻辑一致性，而忽略了满足时态的一致性．本文提出了临时数据截止期的概念，以定理的形式证明了低成本的预测算法的可行性，尽可能早地终止或推迟无法满足时态一致性要求的事务的执行，节省了计算资源以供其它事务执行．在此基础上提出了新的实时数据库中具有时态限制的事务调度算法TDDBPA(Temporary Data Deadline—Based Predictive Algorithm)．通过与目前所知的有效算法比较，本算法在性能指标上(事务失败率、浪费的CPU时间)明显优于这些算法．     

基于挖掘具有时态限制数据语义的实时事务调度算法

在实时数据库中,事务对时态数据对象的访问在很多领域的应用日益广泛。目前实时数据库中的事务调度算法大多仅考虑数据的逻辑一致性,而忽略了满足时态的一致性。论文提出了新的实时数据库中具有时态限制的事务调度算法PSBA(PredictionandSimilarity-BasedAlgorithm)。首先提出了临时数据截止期的概念,以定理的形式证明了低成本地预测算法的可行性,尽可能早地终止或推迟无法满足时态一致性要求的事务的执行,节省了计算资源以供其它事务执行。继而通过挖掘数据语义,利用数据相似性,结合预测算法,进一步提高了调度性能。与目前所知的有效算法比较,该算法在性能指标上(事务失败率、浪费的CPU时间)明显优于这些算法。     

Triggered Updates for Temporal Consistency in Real-Time Databases

A real-time database systemhas temporal consistency constraints in addition to timing constraints.The timing constraints require a transaction to be completedby a specified deadline, and the temporal consistency constraintsrequire that temporal data read by a transaction be up-to-date.If a transaction reads out-of-date data, it will become temporallyinconsistent. A real-time database system consists of differenttypes of temporal data objects, including derived objects. Thevalue of a derived object is computed from a set of other objects,known as the read-set of the derived object. The derived objectmay not always reflect the current state of its read-set; a derivedobject can become out-of-date even if its read-set is up-to-date.Any subsequent transaction reading the derived object will thenbecome temporally inconsistent. In this case, in order to readup-to-date objects, a transaction will have to wait until someother transaction updates the out-of-date object. However, indoing so, the waiting transaction may miss its deadline, particularlyif the update is not periodic but instead arrives randomly. Wepropose to update the outdated objects so that not only is thetemporal consistency improved, but also the number of misseddeadlines does not increase significantly, and as a result thereis an overall improvement in the performance of the system. Wepropose, implement and study a novel approach, to be known astriggered updates, to improve temporal consistency in firm real-timedatabase systems when updates are not periodic. We identify propertiesof triggered updates and explain how they work by giving bothan intuitive and a probabilistic analysis. We present strategiesfor generating triggered updates, discuss their suitability invarious contexts and perform a detailed simulation study to evaluatetheir performance. Results show that it is possible to improvetemporal consistency without degrading the timeliness of real-time database systems to a great deal.     

Wireless real-time on-demand data broadcast scheduling with dual deadlines

In this paper, issues involved in the design of real-time on-demand broadcast system which maintains data temporal constraints are discussed. We propose a new online scheduling algorithm, called RDDS that incorporates access frequency, data size, request-deadline and data-deadline of pending requests for real-time on-demand broadcast system with dual deadlines. Furthermore, the concepts of deferrable requests and non-deferrable requests are introduced, cases of non-deferrable requests are analyzed, and Non-deferrable Request First policy is proposed and integrated into RDDS to form another new algorithm, called RDDS-W. We have performed a series of simulation experiments to evaluate the performance of our algorithms as compared with other previously proposed methods. The experimental results show that our algorithms can substantially outperform other algorithms under a wide range of scenarios, especially when combining with Non-deferrable Request First policy, which improves the performance significantly.     

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.     

Priority Scheduling of Transactions in Distributed Real-Time Databases

One of the most important issues in the design of distributed real-time database system (DRTDBS) is transaction scheduling which consists of two parts: priority scheduling and real-time concurrency control. In the past studies, mostly, these issues are studied separately although they have a very close interaction with each other. In this paper, we propose new priority assignment policies for DRTDBS and study their impact on two typical real-time concurrency control protocols (RT-CCPs), High Priority Two Phase Locking (HP-2PL) and Optimistic Concurrency Control with Broadcast Commit (OCC-BC). Our performance results show that many factors, such as data conflict resolution, degree of data contention and transaction restarts, that are unique to database systems, have significant impact on the performance of the policies which in turn affect the performance of the real-time concurrency control protocols. OCC-BC is more affected by the priority assignment policies than HP-2PL owing to the late detection of conflict. In the design of priority assignment policies, we have found that neither the purely deadline driven policies nor data contention driven policies are suitable for DRTDBS. Our proposed policy, the Mixed Method (MM), which considers both transaction timeliness and data contention, outperforms other policies over a wide range of system parameter settings.     

一种实时数据库事务处理策略的应用

通过对实时事务特点的分析和对实时数据库中事务处理策略的研究,提出了一种把优先级分配策略与定时调度策略相结合的实时事务调度策略,并融合快照机制与优先级分配策略建立了一种新的基于乐观法的并发控制策略。将这种策略应用于一个实时系统的实时数据库,系统的性能和稳定性明显提高。实验结果表明所给出的新策略及方法的正确性和有效性。     

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     

Chronological scheduling of transactions with temporal dependencies

Database applications often impose temporal dependencies between transactions that must be satisfied to preserve data consistency. The extant correctness criteria used to schedule the execution of concurrent transactions are either time independent or use strict, difficult to satisfy real-time constraints. On one end of the spectrum, serializability completely ignores time. On the other end, deadline scheduling approaches consider the outcome of each transaction execution correct only if the transaction meets its real-time deadline. In this article, we explore new correctness criteria and scheduling methods that capture temporal transaction dependencies and belong to, the broad area between these two extreme approaches. We introduce the concepts ofsuccession dependency andchronological dependency and define correctness criteria under which temporal dependencies between transactions are preserved even if the dependent transactions execute concurrently. We also propose achronological scheduler that can guarantee that transaction executions satisfy their chronological constraints. The advantages of chronological scheduling over traditional scheduling methods, as well as the main issues in the implementation and performance of the proposed scheduler, are discussed.     

Queue Waiting Time Aware Dynamic Workflow Scheduling in Multicluster Environments

Workflows are prevailing in scientific computation. Multicluster environments emerge and provide more resources, benefiting workflows but also challenging the traditional workflow scheduling heuristics. In a multicluster environment, each cluster has its own independent workload management system. Jobs are queued up before getting executed, they experience different resource availability and wait time if dispatched to different clusters. However, existing scheduling heuristics neither consider the queue wait time nor balance the performance gain with data movement cost. The proposed algorithm leverages the advancement of queue wait time prediction techniques and empirically studies if the tunability of resource requirements helps scheduling. The extensive experiment with both real workload traces and test bench shows that the queue wait time aware algorithm improves workflow performance by 3 to 10 times in terms of average makespan with relatively very low cost of data movement.     

Effective timestamping in databases

Many existing database applications place various timestamps on their data, rendering temporal values such as dates and times prevalent in database tables. During the past two decades, several dozen temporal data models have appeared, all with timestamps being integral components. The models have used timestamps for encoding two specific temporal aspects of database facts, namely transaction time, when the facts are current in the database, and valid time, when the facts are true in the modeled reality. However, with few exceptions, the assignment of timestamp values has been considered only in the context of individual modification statements. This paper takes the next logical step: It considers the use of timestamping for capturing transaction and valid time in the context of transactions. The paper initially identifies and analyzes several problems with straightforward timestamping, then proceeds to propose a variety of techniques aimed at solving these problems. Timestamping the results of a transaction with the commit time of the transaction is a promising approach. The paper studies how this timestamping may be done using a spectrum of techniques. While many database facts are valid until now, the current time, this value is absent from the existing temporal types. Techniques that address this problem using different substitute values are presented. Using a stratum architecture, the performance of the different proposed techniques are studied. Although querying and modifying time-varying data is accompanied by a number of subtle problems, we present a comprehensive approach that provides application programmers with simple, consistent, and efficient support for modifying bitemporal databases in the context of user transactions. Received: March 11, 1998 / Accepted July 27, 1999     

实时数据库事务的调度与并发控制

实时数据库的核心问题是事务的处理既要确保数据的一致性,又要保证事务的正确性,而它们都与定时相关联。本文对混合实时事务的真实时和非实时部分加以区别定义,建立了混合实时数据库模型,并分析了混合实时事务的调度与并发控制,可以更好地满足实时性时间约束需求,从而提高系统效率。     

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.     

Speculative locking protocols to improve performance for distributed database systems

We have proposed speculative locking (SL) protocols to improve the performance of distributed database systems (DDBSs) by trading extra processing resources. In SL, a transaction releases the lock on the data object whenever it produces corresponding after-image during its execution. By accessing both before and after-images, the waiting transaction carries out speculative executions and retains one execution based on the termination (commit or abort) mode of the preceding transactions. By carrying out multiple executions for a transaction, SL increases parallelism without violating serializability criteria. Under the naive version of SL, the number of speculative executions of the transaction explodes with data contention. By exploiting the fact that a submitted transaction is more likely to commit than abort, we propose the SL variants that process transactions efficiently by significantly reducing the number of speculative executions. The simulation results indicate that even with manageable extra resources, these variants significantly improve the performance over two-phase locking in the DDBS environments where transactions spend longer time for processing and transaction-aborts occur frequently.     

A performance comparison of locking methods with limited wait depth

A number of recent studies have proposed lock conflict resolution methods to improve the performance of standard locking, i.e., strict two-phase locking with the general waiting method. This paper is primarily concerned with the performance of wait depth limited methods with respect to each other and some other methods. The methods considered include the general waiting, wound-wait, and no-waiting methods, symmetric and asymmetric versions of cautious waiting and running priority methods, the wait depth limited (WDL) method, and a modified version of it. In spite of the availability of analytic solutions for most of wait depth limited methods, for reasons given in-the paper, the performance comparison is based on simulation results. The contributions of this study are as follows: 1) modeling assumptions, i.e., a careful definition of transaction restart options; 2) new results concerning the relative performance of wait depth limited methods, which show that a) the running priority method outperforms cautious waiting and may even outperform the WDL method in a system with limited hardware resource, b) WDL outperforms other methods in high lock contention, high capacity systems, and c) modified WDL has a performance comparable to WDL, but incurs less overhead in selecting the abort victim; and 3) contrary to common belief, Tay's Effective Database Size Paradigm for dealing with shared and exclusive locks and/or skewed database accesses in standard locking is applicable to some wait depth limited methods and provides acceptably accurate approximations in others-as long as locking modes for restarted transactions are not resampled     

小卫星地面自动测试系统中实时数据库事务优先级分配算法

小卫星自动化测试是提高测试效率、缩短研制周期、降低研制成本和保证可靠性的重要手段. 针对测试数据量大、类型复杂和实时性要求高等特点, 设计了小卫星自动测试系统实时数据库方案. 建立测试数据相关的实时数据库事务调度模型, 并在该模型基础上设计了异常数据优先的事务优先级分配算法(Abnormal datafirst, ADF). 最后应用数学仿真验证了ADF分配算法的执行效率、正确性和有效性. 本文建立的事务调度模型和优先级分配算法兼顾测试数据自身特点和实时性要求, 尤其注重异常数据的处理, 从而提高了测试的全面性和准确性.     

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。     

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.