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。     

一种面向混合实时事务调度的并发控制协议

首先给出了一个两层结构的混合实时数据库系统模型,其中支持采用非定期任务调度算法来改进系统的性能．进一步,针对这种模型下混合事务的数据一致性问题,提出了一种新的并发控制协议——MCC-DATI．该协议采用动态优先级驱动的调度算法,通过限制非定期的软实时事务对硬实时事务的阻塞时间,保证硬实时事务的可调度性;同时,采用非定期任务调度算法以及基于时间戳间隔的动态串行化顺序调整机制来减少软实时事务的截止期错失率．仿真实验表明,相对于先前的混合事务的并发控制协议,该协议在不同的系统负载与截止期约束下都能够改进系统的性能。     

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     

A multigranularity locking model for RDF

The advent of Linked Data is spurring the deployment of applications that use the RDF data model at the information tier. In addition to querying RDF data, there is also the requirement for online updates with suitable concurrency control. Client sessions in Web applications are organized as transactions involving requests that read and write shared data. Executing concurrently, these sessions may invalidate each other’s data. This paper presents a locking model, which is a variant of multigranularity locking protocol (MGL), to address isolation between transactions that manipulate RDF data. Four “hierarchically” related granules are defined, as well as new read/write operations and their corresponding lock modes, specifically created for the RDF data model. These new operations allow greater concurrency than the classical read/write operations in relational databases. We assessed the performance of the proposed locking model through model simulation.     

On-line multiversion database concurrency control

This paper presents a new model for studying the concurrency vs. computation time tradeoffs involved in on-line multiversion database concurrency control. The basic problem that is studied in our model is the following: Given:a current database system state which includes information such as which transaction previously read a version from which other transaction; which transaction has written which versions into the database; and the ordering of versions previously written; anda set of read and write requests of requesting transactions. Question: Does there exist a new database system state in which the requesting transactions can be immediately put into execution (their read and write requests satisfied, or in the case of predeclared writeset transactions, write requests are guaranteed to be satisfied) while preserving consistency under a given set of additional constraints? (The amount of concurrency achieved is defined by the set of additional constraints). In this paper we derive “limits” of performance achievable by polynomial time concurrency control algorithms. Each limit is characterized by a minimal set of constraints that allow the on-line scheduling problem to be solved in polynomial time. If any one constraint in that minimal set is omitted, although it could increase the amount of concurrency, it would also have the dramatic negative effect of making the scheduling problem NP-complete; whereas if we do not omit any constraint in the minimal set, then the scheduling problem can be solved in polynomial time. With each of these limits, one can construct an efficient scheduling algorithm that achieves an optimal level of concurrency in polynomial computation time according to the constraints defined in the minimal set.     

Evaluation of concurrency control strategies for mixed soft real-time database systems

Previous research in real-time concurrency control mainly focuses on the schedulability guarantee of hard real-time transactions and the reduction of the miss rate of soft real-time transactions. Although many new database applications have significant response time requirements, not much work has been done in the joint scheduling of traditional non-real-time transactions and soft real-time transactions. In this paper, we study the concurrency control problems in mixed soft real-time database systems, in which both non-real-time and soft real-time transactions exist simultaneously. The objectives are to identify the cost and the performance tradeoff in the design of cost-effective and practical real-time concurrency control protocols, and to evaluate their performance under different real-time and non-real-time supports. In particular, we are interested in studying the impacts of different scheduling approaches for soft real-time transactions on the performance of non-real-time transactions. Instead of proposing yet another completely new real-time concurrency control protocol, our objective is to design an efficient integrated concurrency control method based on existing techniques. We propose several methods to integrate the well-known two phase locking and optimistic concurrency control with the aims to meet the deadline requirements of soft real-time transactions and, at the same time, to minimize the impact on the performance of non-real-time transactions. We have conducted a series of experiments based on a sanitized version of stock trading systems to evaluate the performance of both soft real-time and non-real-time transactions under different real-time supports in the system.     

Real-time index concurrency control

Real time database systems are expected to rely heavily on indexes to speed up data access and thereby help more transactions meet their deadlines. Accordingly, high performance index concurrency control (ICC) protocols are required to prevent contention for the index from becoming a bottleneck. We develop real time variants of a representative set of classical B-tree ICC protocols and, using a detailed simulation model, compare their performance for real time transactions with firm deadlines. We also present and evaluate a real time ICC protocol called GUARD-link that augments the classical B-link protocol with a feedback based admission control mechanism. Both point and range queries, as well as the undos of the index action transactions are included in the study. The performance metrics used in evaluating the ICC protocols are the percentage of transactions that miss their deadlines and the fairness with respect to transaction type and size. Experimental results show that the performance characteristics of the real time version of an ICC protocol could be significantly different from the performance of the same protocol in a conventional (nonreal time) database system. In particular, B-link protocols, which are reputed to provide the best overall performance in conventional database systems, perform poorly under heavy real time loads. The new GUARD-link protocol, however, although based on the B-link approach, delivers the best performance (with respect to all performance metrics) for a variety of real time transaction workloads, by virtue of its admission control mechanism. GUARD-link provides close to ideal fairness in most environments     

面向更新密集型应用的内存数据库高效检查点技术

面向更新密集型应用的内存数据库系统,其检查点技术应符合几个关键的要求,包括检查点操作对正常事务处理的干扰尽可能小、能够处理存取倾斜状况、支持数据库系统的快速恢复、提供恢复过程中的系统可用性等.该文提出一种事务一致的分区检查点技术,采用基于元组的动态多版本并发控制机制,避免了读写事务的加锁冲突,提高系统吞吐能力;检查点操作以只读事务形式实现,存多版本并发控制下,避免检查点操作对正常事务处理的堵塞;由于检查点文件是事务一致的,只需要记录事务的Redo 日志信息,在系统恢复过程中,只需要对日志文件进行一遍扫描处理,加快恢复过程;基于优先级的数据分区装载和恢复,使得恢复过程中新事务的数据存取请求迅速得到满足,保证了恢复过程中的系统可用性.由于采用两级版本管理机制以及动态版本共享技术,多版本管理的空间开销降低到可以接受的水平.实验结果表明,文中提出的检查点技术方案获得比模糊检查点技术高27%的系统吞吐量,同时版本管理的空间开销在可接受的范围之内,满足高性能应用的要求.     

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.     

并行实时数据库系统中的一种自适应并发控制模式

根据事务的实时性及关键性分类，事务分为硬实时、固实时、软实时和非实时事务。传统的并发控制仅考虑其中一到两种事务，且将固／软实时事务统一为软实时事务考虑，带有很大的不精确性，这将降低系统吞吐率。为此，本文提出了一种自适应并发控制模式。该模式采用两级并发控制体系，一级是主并发控制器，探测不同类事务间可能存在
在的类间数据冲突；二级是从并发控制器，探测同类事务间可能存在的类内数据冲突，由四类从并发控制器实现。根据各类事务的不同特征，各控制器采用的策略不同，因而具有极大的自适应性并能最终提高系统的吞吐率，但需要增加额外的硬件开销。     

An energy-efficient mobile transaction processing method using random back-off in wireless broadcast environments

Broadcast is widely accepted as an efficient technique for disseminating data to a large number of mobile clients over a single or multiple channels. Due to the limited uplink bandwidth from mobile clients to server, conventional concurrency control methods cannot be directly applied. There has been many researches on concurrency control methods for wireless broadcast environments. However, they are mostly for read-only transactions or do not consider exploiting cache. They also suffer from the repetitive aborts and restarts of mobile transactions when the access patterns of mobile transactions are skewed. In this paper, we propose a new optimistic concurrency control method suitable for mobile broadcast environments. To prevent the repetitive aborts and restarts of mobile transactions, we propose a random back-off technique. To exploit the cache on mobile clients, our method keeps the read data set of mobile transactions and prefetches those data items when the mobile transactions are restarted. As other existing optimistic concurrency control methods for mobile broadcast environments does, it works for both read-only and update transactions. Read-only transactions are validated and locally committed without using any uplink bandwidth. Update transactions are validated with forward and backward validation, and committed after final validation consuming a small amount of uplink bandwidth. Our performance analysis shows that it significantly decreases uplink and downlink bandwidth usage compared to other existing methods.     

On consistent reading of entire databases

Many applications need to read an entire database in a consistent way. This global-reading of an entire database formulated as a global-read transaction (GRT) is not a trivial issue since it will cause a high degree of interference to other concurrent transactions. Conventional concurrency control protocols are obviously inadequate in handling the long-lived GRT. Previous studies proposed additional tests, namely, the color test and the shade test, to handle conflicts between the GRT and update transactions. However, we discovered that both algorithms can bring about nonserializable schedules of transactions. We propose an enhanced algorithm directly built on the two algorithms to guarantee the serializability of transactions.     

Algorithm to Process Read-only Transactions in Real-time Environments

In this study, we investigate a different approach to maintaining serializability in real-time database systems (RTDBS) such that concurrency among transactions can be increased. The study is motivated by the dominance of read only transactions (ROTs) in many real-time applications. Given the knowledge about the read/write characteristics of transactions, it can be more efficient and effective to process ROTs separately from update transactions (UTs). In particular, we have devised an independent algorithm to process ROTs while a conventional concurrency control protocol such as optimistic concurrency control (OCC) can be employed to process UTs. Using a separate algorithm to process ROTs can reduce the interference between UTs and ROTs. The undesirable overhead caused by transaction restart and blocking due to concurrency control can be alleviated. Consequently, the timeliness of the system can be improved. The performance of using this approach is examined through a series of simulation experiments. The results showed that the performance of ROTs in terms of miss rate and restart rate is improved significantly while the performance of UTs is also improved slightly. As a result, separate processing of ROTs is a viable approach that achieves better performance and resource utilization than using solely the OCC protocol, one of the best performing protocols in the literature of real-time database.     

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。     

一种新的混合实时事务并发控制算法

单一的实时事务并发控制策略因为对事务性能以及事务对数据的访问方式有着特殊限制而无法满足不同类型事务同时并存的混合实时数据库的要求.针对不同类型实时事务特征,提出了一种新的混合实时事务并发控制算法,对不同类型实时事务采用不同并发控制策略,具有极强的针对性和自适应性,算法同时通过分析数据的相关语义,利用数据相似性定义,合理放宽可串行化的正确性标准,在优先考虑硬实时事务的前提下,尽可能增加软实时事务成功提交的比例以提高系统整体性能.仿真实验结果证明MRTT_CC算法性能良好.     

Concurrency control in database systems

Ideas that are used in the design, development, and performance of concurrency control mechanisms have been summarized. The locking, time-stamp, optimistic-based mechanisms are included. The ideas of validation in the optimistic approach are presented in some detail. The degree of concurrency and classes of serializability for various algorithms have been presented. Questions that relate arrival rate of transactions with degree of concurrency and performance have been briefly presented. Finally, several useful ideas for increasing concurrency have been summarized. They include flexible transactions, adaptability, prewrites, multidimensional time stamps, and relaxation of two-phase locking     

A neural-based concurrency control algorithm for database systems

Concurrency control (CC) algorithms guarantee the correctness and consistency criteria for concurrent execution of a set of transactions in a database. A precondition that is seen in many CC algorithms is that the writeset (WS) and readset (RS) of transactions should be known before the transaction execution. However, in real operational environments, we know the WS and RS only for a fraction of transaction set before execution. However, optional knowledge about WS and RS of transactions is one of the advantages of the proposed CC algorithm in this paper. If the WS and RS are known before the transaction execution, the proposed algorithm will use them to improve the concurrency and performance. On the other hand, the concurrency control algorithms often use a specific static or dynamic equation in making decision about granting a lock or detection of the winner transaction. The proposed algorithm in this paper uses an adaptive resonance theory (ART)-based neural network for such a decision making. In this way, a parameter called health factor (HF) is defined for transactions that is used for comparing the transactions and detecting the winner one in accessing the database objects. HF is calculated using ART2 neural network. Experimental results show that the proposed neural-based CC (NCC) algorithm increases the level of concurrency by decreasing the number of aborts. The performance of proposed algorithm is compared with strict two-phase locking (S2PL) algorithm, which has been used in most commercial database systems. Simulation results show that the performance of proposed NCC algorithm, in terms of number of aborts, is better than S2PL algorithm in different transaction rates.     

多级多版本数据库管理系统全局串行化

多级调度应该保证事务历史可串行化,满足多级安全特性,不会引入隐通道,并保证高级别事务不会因为无限等待而"饿死".与其他多级数据管理系统调度机制相比,多级多版本时戳调度机制满足上述要求,但该机制存在两个问题,一是事务可能读旧版本,二是要求调度器是可信进程.提出一种多级多版本全局时戳调度机制(MLS_MVGTO),以及依据事务快照生成其全局时戳的基本步骤.给出了预知只读事务信息时的两种改进方法.MLS_MVGTO机制生成的事务历史可串行化,不引入隐通道等,并且该方法避免引入一个全局可信的调度器,并通过对只读事务的深入分析,允许事务读新版本.     

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．     

Modular synchronization in distributed, multiversion databases:version control and concurrency control

A version control mechanism is proposed that enhances the modularity and extensibility of multiversion concurrency control algorithms. The multiversion algorithms are decoupled into two components: version control and concurrency control. This permits modular development of multiversion protocols and simplifies the task of proving the correctness of these protocols. A set of procedures for version control is described that defines the interface with the version control component. It is shown that the same interface can be used by the database actions of both two-phase locking and time-stamp concurrency control protocols to access multiversion data. An interesting feature of the framework is that the execution of read-only transactions becomes completely independent of the underlying concurrency control implementation. Unlike other multiversion algorithms, read-only transactions in this scheme do not modify any version-related information, and therefore do not interfere with the execution of read-write transactions. The extension of the multiversion algorithms to a distributed environment becomes very simple