一种动态调整串行化顺序的实时并发控制协议

实时并发控制协议用来控制并发事务间的相互作用,已满足实时数据库的一致性.本文提出一种四阶段实时并发控制协议HTRCC-DASO,协议在读或等待阶段基于事务优先级动态调整串行化顺序,通过阻塞或及早重启低优先级事务,使高优先级事务率先提交并降低事务重启开销.在验证阶段检查事务读写集合的外部一致性和相互一致性并调整活动事务的时间戳间隔,确保满足事务时态一致性和调度冲突可串行化.通过引入相似性概念,提出了HTRCC-DASO的改进版本:SHTRCC-DASO,性能测试结果显示,SHTRCC-DASO协议能保证良好的实时性能.     

数据广播环境下移动实时事务的有效并发控制

在无线数据广播环境下,从移动客户机到数据库服务器的上行带宽非常有限.传统的并发控制协议不适合这种不对称通讯环境.本文结合乐观并发控制协议和时标排序协议提出一种并发控制协议.只读事务满足弱一致性,包含所有更新事务和一个移动只读事务的可串行化图是非循环的.使用本文提出的并发控制协议,移动只读事务能够自主执行,移动更新事务能够较早地检测到数据冲突.模拟试验显示,我们提出的协议相对其它已经存在的并发控制协议来说,能够更好地满足事务截止期.     

移动广播环境中的可串行化并发控制协议

研究了非对称通信数据广播环境中的一致性问题．PVTO协议中数据冲突的检测过于严格，存在很多不必要的事务夭折，限制了其并发度；O—PreH协议中数据冲突的检测过于宽松，必要的事务重启要等待服务器校验，浪费了大量昂贵的上行通信带宽．提出了一种新的可串行化并发控制协议BCC—SR．BCC—SR协议将时标段排序和乐观方法相结合，使用了恰当的校验条件，既避免了不必要的事务夭折，又可尽早处理必要的事务重启，节省了系统资源，提高了并发度．实验结果表明，BCC—SR协议能有效改进移动事务的平均响应时间，更好地满足数据广播环境中高性能事务处理的要求．     

并行数据广播中的亚可串行化并发控制

考虑到移动计算环境的诸多限制以及移动数据库应用的特征,传统冲突可串行化在不对称通信的数据广播环境中显得过于严格而不必要.为此,文中提出一种并行亚可串行化并发控制协议.首先,形式化定义亚可串行化正确性准则,证明亚可串行化虽然比冲突可串行化宽松但能始终保证一致性,给出保证亚可串行化的充分必要条件.进而,提出相应的并行亚可串行化并发控制协议BCC-HSR.实验结果表明,BCC-HSR协议能有效改进移动事务的平均响应时间,更好地满足并行数据广播环境中高性能事务处理的要求.     

基于冲突率预测的自适应并发控制算法

并发控制算法能够保证数据库事务集并发执行的正确性和一致性。为了提高并发事务的执行效率,提出了一种基于冲突率预测的自适应并发控制算法(ACC-PRC)。该算法将并发控制过程分为信息收集和策略选择两个阶段。信息收集阶段利用先验事务队列保证事务执行的可串行化,并且利用循环冲突队列收集系统的事务执行状态。策略选择阶段在循环冲突队列上运用改进的加权移动平均法预测下一阶段冲突率,并根据双向阈值决策下一阶段的并发策略。所提算法在事务到达率较高时能保持良好的事务执行效率,同时能够准确及时地感知冲突率的变化。对比实验表明ACC-PRC算法的综合性能优于HCC算法和ADCC算法。     

基于耦合模式的主动实时并发控制算法

主动实时数据库中触发事务与被触发事务在执行上具有多种耦合模式，传统的并发控制无法对具有复杂耦合模式的事务进行有效调度。通过对不同耦合模式实时要求及事务间冲突关系的分析，提出了新的主动实时并发控制算法(ARTCC-CM)，采用时戳区间策略，在验证阶段检测冲突事务触发度及执行时间，动态调整串行化顺序。理论分析与实验证明，在保证可串行性同时减少了不必要的事务重启，提高了系统性能。     

实时数据库的准一致性可串行化并发控制

可串行化的并发控制对传统应用是合适的,它旨在确保数据的“绝对正确”性,然而在实时数据库中,事务的定时限制往往比数据的“绝对正确”性更重要。为此,本文提出了新颖的准一致性可串行化的并发控制策略,文中着重讨论了准一致性可串行化的有关概念与理论,调度的可准一致性可串行化的判别方法,并发控制协议以及在事务的准一致性可串行化并发执行中不一致性传播放大的控制策略。     

面向用户的移动实时事务并发控制策略

-可串行化、 可串行化、准一致性可串行化等现有面向事务的放松可串行化正确性准则虽能帮助更多无线通信事务满足其截止期要求，但会导致数据的受限不一致。该文针对移动实时环境和应用，形式化定义并分析了 可串行化和 可串行化2种新的面向用户的并发控制策略，它们都比冲突可串行化宽松，但都能始终保证移动用户读取数据的一致性。     

基于级联深度的主动实时乐观并发控制协议

主动实时数据库因结合了时间限制与主动机制而使系统事务的并发控制变得更为复杂。主动规则的引入使事务触发新的事务且在执行上具有多种耦合方式,传统的实时并发控制策略无法对具有复杂执行模式的事务进行有效调度,而基于主动数据库的并发控制机制也没有考虑事务的实时性问题。通过对事务不同耦合方式的实时要求及事务间冲突关系进行分析,提出了新的主动实时数据库乐观并发控制方法,对不同事务级联深度进行评估,结合事务执行的时间信息对冲突事务进行动态调整串行化顺序。理论分析与实验证明,能在保证事务可串行性的同时降低了不必要事务重启个数,更好地满足系统的实时性。     

广播环境中的Q可串行化实时并发控制

研究了实时广播中的数据一致性问题。传统冲突可串行化并发控制协议对非对称通信的广播环境来说过于严格且不必要。为此，提出使用Q可串行化：形式化定义了Q可串行化，证明了它虽比传统冲突可串行化宽松但能始终保证数据一致性，基于可动态调整串行化顺序的时标段排序方法给出了相应的并发控制协议。实验结果表明：新协议可显著降低平均响应时间，使更多移动实时事务满足截止期要求。     

移动数据库事务处理模型研究

移动数据库系统必须提供移动事务处理能力，以满足移动应用的需求。但是，移动计算环境的特点以及一些新型移动应用的出现，使得传统的ACID特性已不足以描述移动事务。本文在分析移动事务与传统数据库中事务的不同之处的基础上，重点讨论移动数据库事务处理模型，包括移动事务处理模型基本要素、特性、分类以及未来发展方向。     

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.     

Efficient validation of mobile transactions in wireless environments

In broadcast environments, the limited bandwidth of the upstream communication channel from the mobile clients to the server bars the application of conventional concurrency control protocols. In this paper, we propose a new variant of the optimistic concurrency control (OCC) protocol that is suitable for broadcast environments. At the server, forward validation of a transaction is done against currently running transactions, including mobile transactions and server transactions. At the mobile clients, partial backward validation of a transaction is done against committed transactions at the beginning of every broadcast cycle. Upon completion of execution, read-only mobile transactions can be validated and committed locally and update mobile transactions are sent to the server for final validation. These update transactions have a better chance of commitment because they have gone through the partial backward validation. In addition to the nice properties of conventional OCC protocols, this protocol provides autonomy between the mobile clients and the server with minimum upstream communication, which is a desirable feature to the scalability of applications running in broadcast environments. This protocol is able to process both update transactions and read-only transactions at the mobile clients at low space and processing overheads.     

Concurrency control in mobile distributed real-time database systems

This paper presents DMVOCC-2PLV (Distributed Multiversion Optimistic Concurrency Control with Two-Phase Local Validation) protocol for processing mobile distributed real-time transactions in mobile broadcast environments. In DMVOCC-2PLV protocol, transaction validation is performed at two levels: local validation and global validation. Local validation of transactions is performed in two phases: local pre-validation and local final validation. At the MHs (Mobile Hosts), all mobile transactions perform local pre-validation of transactions by using a backward validation mechanism. The local pre-validation process is carried out against committed transactions at the server in the last broadcast cycle. Such an early data conflict detection feature can save processing and communication resources. Transactions that survive in local pre-validation must be submitted to the server for local final validation. In global validation distributed update transactions have to check to ensure distributed serializability in all participants. The protocol can eliminate conflicts between mobile read-only and mobile update transactions, and resolve data conflicts flexibly using multiversion dynamic adjustment of serialization order to avoid unnecessary restarts of transactions. Mobile read-only transactions can be committed with no-blocking, and respond time of mobile read-only transactions is greatly reduced. The tolerance of mobile transactions of disconnections from the broadcast channel is increased. The results of simulation experiment show that the new protocol proposed offers better performance in terms of miss rate, restart rate, commit rate and throughput.     

基于移动数据库的事务处理模型的研究

对移动数据库的关键技术之一事务处理进行了分析与探讨。在分析现有移动事务处理模型的基础上，结合研究开发的嵌入式移动数据库系统SwiftDB，提出增加接入代理层的事务处理系统结构，并根据移动计算环境的特点和具体应用需求，分析了移动节点上事务状态，改进两级复制模型的移动事务解决方案。     

移动分布式实时嵌套事务提交

在移动分布式计算环境中,事务移动性和无线网络固有的缺陷使得传统的分布式实时事务管理机制不足以支持移动分布式实时事务的执行,故有必要为移动实时事务研究新的事务处理机制,以提高其成功率.着重研究移动实时事务的提交机制.首先,通过分析移动分布环境中实时事务的特点给出了一个基于功能替代的移动实时嵌套事务模型.然后,提出了一个基于此模型的三层提交结构以及能够保证移动实时事务原子性和结构正确性的三阶段实时提交协议3PRTC(three-phase real-time commit).性能测试表明,所提出的事务模型及其提交机制能够提高实时事务的成功率.     

A concurrency control scheme for mobile transactions in broadcast disk environments

Broadcast disk technique has been often used to disseminate frequently requested data efficiently to a large volume of mobile clients over wireless channels. In broadcast disk environments, a server often broadcasts different data items with differing frequencies to reflect the skewed data access patterns of mobile clients. Previously proposed concurrency control methods for mobile transactions in wireless broadcast environments are focused on the mobile transactions with uniform data access patterns. These protocols perform poorly in broadcast disk environments where the data access patterns of mobile transactions are skewed. In broadcast disk environments, the time length of a broadcast cycle usually becomes large to reflect the skewed data access patterns. This will often cause read-only transactions to access old data items rather than the latest data items. Furthermore, updating mobile transactions will be frequently aborted and restarted in the final validation stage due to the update conflict of the same data items with high access frequencies. This problem will increase the average response time of the update mobile transactions and waste the uplink communication bandwidth. In this paper, we extend the existing FBOCC concurrency control method to efficiently handle mobile transactions with skewed data access patterns in broadcast disk environments. Our method allows read-only transactions to access the more updated data, and reduces the average response time of updating transactions through early aborts and restarts. Our method also reduces the amount of uplink communication bandwidth for the final validation of the update transactions. We present an in-depth experimental analysis of our method by comparing with existing concurrency control protocols. Our performance analysis shows that it significantly decreases the average response time and the amount of uplink bandwidths over existing methods.     

移动广播环境中有效处理实时只读事务

提出了移动广播环境中有效处理实时只读事务的方法。给出了多种多版本广播磁盘组织。采用多版本机制，实现移动只读事务无阻塞提交。通过乐观方法，消除移动只读事务和移动更新事务的冲突。使用多版本动态调整串行次序技术，避免了不必要的事务重启动。在移动主机上如果移动只读事务通过向后有效性确认，则可提交，不需要提交到服务器处理，降低移动只读事务的响应时间。通过模拟仿真对提出的方法进行了性能测试,实验结果表明新方法要优于其他协议。     

移动实时嵌套事务的并发控制

在移动计算环境中，事务移动性和无线网络固有的缺陷使得传统分布式实时事务管理机制不足以支持移动实时事务的执行，故有必要为移动实时事务研究新的事务处理机制以提高其成功率．该文着重研究移动实时事务的并发控制机制．首先，该文给出了一个考虑事务定时限制以及移动性的嵌套事务模型．然后，为减少移动分布式环境中解决数据冲突的开销，该文研究了一种结合优先级夭折和优先级继承的基于封锁的并发控制协议PAI-2PL．当高优先级事务被低优先级事务阻塞时，对于相同家族事务，采用优先级继承方法解决冲突；而对于不同家族事务，则夭折重启低优先级事务．另外，为减少由于断接所引起的无效阻塞，PAI-2PL允许低优先级事务夭折处于断接状态的高优先级事务．通过性能测试，表明所提出的事务模型及并发控制机制能提高实时事务的成功率．     

A Survey of Mobile Transactions

Transaction support is crucial in mobile data management. Specific characteristics of mobile environments (e.g. variable bandwidth, disconnections, limited resources on mobile hosts) make traditional transaction management techniques no longer appropriate. Several models for mobile transactions have been proposed but it is difficult to have an overview of all of them. This paper analyzes and compares several contributions to mobile transactions. The analysis distinguishes two groups of models. The first group includes proposals where transactions are completely or partially executed on mobile hosts. In this group we focus on ACID properties support. The second group considers transactions requested by mobile hosts and executed on the wired network. In this case, ACID properties are not compromised and focus is on supporting mobile host movements during transaction execution. Discussions pointing out limitations, interesting solutions and research perspectives complete this paper.