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。     

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.     

无线移动环境中的拉式实时并发控制机制

考虑到诸如带宽、移动性、频繁断接等限制,传统可串行化并发控制协议显得过于严格而无法满足移动实时数据库环境的要求。该文首先形式化定义并分析了弱可串行化正确性准则,弱可串行化比传统冲突可串行化宽松且能始终保证数据库的一致性。进而给出了一种适宜于移动实时数据库的弱可串行化并发控制协议WMHP-2PL。最后性能实验证实了新协议的有效性。     

广播环境中的移动实时事务并发控制

研究了实时数据广播中的一致性问题，由于广播环境的通信不对称性，传统的可串行化并发控制协议显得过于严格，不必要，不可行，为此，提出使用伪可串行化并发控制；形式化定义了伪可串行化的概念，证明了它虽经传统可串行宽松但能保证数据一致性，给出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．     

数据广播中т-可串行化移动实时事务并发控制

研究了实时数据广播环境中的一致性维护问题。由于广播环境的非对称通讯特性,传统可串行化并发控制方法过于严格而不必要;现有放松可串行化方法都会导致数据库受限不一致、使各个用户读取受限不一致数据。提出了使用т-可串行化并发控制,首先形式化定义了т-可串行化正确性准则,证明了它虽比传统可串行化宽松但能始终保证数据一致性,使各个用户始终读取一致性数据,进而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。     

数据广播中T-可串行化移动实时事务并发控制

研究了实时数据广播环境中的一致性维护问题。由于广播环境的非对称通讯特性,传统可串行化并发控制方法过于严格而不必要;现有放松可串行化方法都会导致数据库受限不一致、使各个用户读取受限不一致数据。提出了使用?子-可串行化并发控制,首先形式化定义了?子-可串行化正确性准则,证明了它虽比传统可串行化宽松但能始终保证数据一致性,使各个用户始终读取一致性数据,进而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。     

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。     

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.     

移动实时事务预留广播空间的验证协议

在无线数据广播环境中,从移动客户机到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信环境.本文提出一种可变乐观并发控制协议,服务器周期性向移动客户机广播数据对象,同时把一个广播周期分成若干个子周期,在两个连续的子周期间,一个保留空间被预留来存放服务器更新事务在第一个子周期开始后修改的所有数据对象,移动只读事务通过比较服务器更新事务提交的写集合和自己读集合来自主进行一致性验证,如果只读事务不能通过部分验证,它不会被草率地夭折并重新启动,相反,应用一个改进的向前验证策略,帮助移动只读事务争取更多提交机会.最后,本文进行充分的实验对提出的算法性能进行了评价.     

Effects of broadcast errors on concurrency control in wireless broadcasting environments

In wireless broadcasting environments, mobile clients cannot receive data reliably over broadcast channels because a reliable transmission protocol is not applicable to the channels. If such broadcast errors are not properly handled by a concurrency control algorithm, it could lead to a fatal effect especially when clients are permitted to issue update transactions. However, the effects of broadcast errors on concurrency control have been little researched. In this paper, we have proposed a concurrency control algorithm to support update transactions issued by mobile clients and evaluated the performance of the algorithm by focusing the effects of broadcast errors with an analytic model. The analytic results show our algorithm is efficient in resolving the broadcast errors.     

Efficient Dissemination of Transaction-Consistent Data in Broadcast Environments

In this paper, we present a novel protocol for disseminating data in broadcast environments such that view consistency, a useful correctness criterion for broadcast environments, is guaranteed. Our protocol is based on concurrency control information that is constructed by the server and is broadcasted at the beginning of each broadcast cycle. The concurrency control information mainly captures read-from relations among update transactions. A salient feature of the protocol is that the concurrency control information is small in size, but precise enough for reducing unnecessary abortion of mobile transactions. The small-sized concurrency control information implies low communication overhead on broadcasting system. In addition, the computation overheads imposed by the algorithm on the server and the clients are low. We also address the reliability issue of wireless communication and the incorporation of a prefetching mechanism into our protocol. Simulation results demonstrate the superiority of our protocol in comparison with existing methods. Furthermore, we have extended our protocol to deal with local view consistency which requires that all mobile transactions submitted by the same client observe the same serial order of update transactions     

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．     

实时广播环境中的分布式校验并发控制机制

本文设计了一种新的分布式阶段校验的移动实时事务并发控制协议，校验任务由移动客户机与服务器共同分担，允许获得更高的并发度，可更早的检测到数据冲突以减少处理与通讯资源的浪费，性能实验结果表明，新协议有效可行，能帮助更多的实是事故满足截止期要求。     

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.     

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.