基于Agent的移动事务模型及实现

连接断开和数据一致性问题是移动数据库系统研究中的关键难点之一,目前研究的乐观复制方法,要么会带来不可接受的回滚事务数量,要么对数据的安排和操作加以严格的限制,很难实际应用.文中针对移动环境连接不稳定和资源有限的特点,提出了一个基于Agent的移动事务模型AMTM.阐述了移动Agent技术用于移动环境下事务处理的优点,设计了一个基于Agent的三层移动事务处理的原型系统,并简要说明了该系统在实现上采取的关键技术.通过实验分析比较了基于Agent方法与乐观复制方法各自的优缺点和适用范围.     

乐观并发协议在移动计算环境中的应用

在移动计算环境中,基本乐观并发协议的事务处理存在并发度低和重启率高等问题。为此,提出一种适用于移动计算环境的乐观事务并发协议――基于数据读写时间戳的乐观并发控制协议。仿真实验表明,该协议可以提高事务处理的并发度,降低事务重启概率,与基本乐观事务并发控制协议和基于时间戳的乐观事务并发控制协议相比,该协议更适用于移动计算环境。     

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。     

一种基于移动环境的混合乐观实时事务处理策略

随着移动计算技术的快速发展，移动环境下涉及到实时事务处理的应用需求正逐渐增长．由于移动环境下固有特性：高的网络延迟、频繁的断接性和移动性等，采用传统的事务处理技术，在移动环境下很难满足事务的截止期要求．提出了一种基于高优先级两段锁的混合乐观实时事务并发控制协议(HORTCC—SHP2PL)．该协议在移动实时事务处理中将乐观并发控制和高优先级两段锁结舍起来，利用两阶段提交协议实现移动实时事务的全局提交．为了进一步减少移动实时事务重启的数目．在并发控制协议中引入了相似性的概念．仿真实验显示．与分布式高优先级两段锁(HP2PL)比较，HORTCC—SHP2PL明显地减少了实时事务错过截止期的比率，提高了事务的并发度，能更好的满足移动实时事务截止期的要求．     

移动数据库系统乐观事务处理策略

在移动数据库系统中，计算平台的移动性，频繁的断接性以及长事务等特性使得传统的事务处理模式不再适用，为了解决移动数据中的事处处理问题，提出了一种新的移动事务处理模型－乐观两阶段提交移动事务模型（O2PC－MT），该模型采用乐观并发控制与两阶段提交协议相结合的方法，对移动事务的长事务特性提供了灵活与有效的支持，此外，该模型允许移动计算机分多发送中务操作，且在事务执行过程中可以任意移动，从而提供了对交互式事务及随意移动性的支持，实验结果表明，与基于两段锁协议及其变形的其它移动事务处理模型相比，O2PC－MT提高了系统的事务吞吐率并改善了系统的总体性能。     

更新分发环境下移动事务处理的节能策略设计

能量的损耗和事务的一致性是更新分发环境下移动应用所需要考虑的两个主要问题,但传统的节能技术并不支持对移动客户端移动事务的并发控制,而现有的基于数据广播的移动事务处理方法主要针对保持数据一致性而设计,并未考虑事务处理过程中的节能问题.将分布式索引技术应用于移动事务处理,提出一种在更新分发环境下同时实现移动事务并发控制和节能处理的有效方法.实验结果表明使用该方法比未应用索引技术前节能84.5%.     

移动计算中基于权值的自适应事务处理模型

在综合分析现有事务模型的基础上，提出了一种根据事务提交所处的网络环境和事务是否访问热点数据，自动选择子事务处理模型的基于权值的自适应移动事务处理模型——AMTMBW。实验证明，新的事务处理模型在移动事务的撤销率，移动事务的执行时间等方面与其他模型相比，其性能都有较大的提高。     

移动数据库的事务处理关键技术研究与分析

移动事务的移动性、频繁断接性等特点使传统的事务处理技术已不能适应移动环境,因此设计一个适应移动数据库环境的事务模型是移动事务处理关键.本文在分析移动事务的特点和移动事务处理典型结构基础上,主要论述设计移动事务模型必须要解决的移动事务的过区切换、断接处理、同步复制、故障恢复等四个关键技术的研究现状和原理分析.     

广播环境下移动事务处理中的节能问题研究

当前移动计算技术研究中索引技术与移动事务处理方法相对独立,即传统的索引技术并不支持对移动客户端移动事务的并发控制,而现有的基于数据广播的移动事务处理方法主要针对保持数据一致性而设计,并未考虑事务处理过程中的节能问题。将分布式索引技术应用于移动事务处理,提出一种在数据广播环境下同时实现移动事务并发控制和节能处理的有效方法。实验结果表明该方法的平均调谐时间为未应用索引技术前的15.5%。     

移动事务并发控制技术的研究

为有效降低移动计算环境下事务处理的重启动率、提高事务的吞吐量,在研究传统并发控制协议的基础上,提出了一种数据锁乐观并发控制协议DLOCC,DLOCC协议结合了锁并发控制协议与乐观并发控制协议的优点,利用数据项的更新次数和访问概率将服务器上的海量数据进行分类,依据不同的数据类型采用相应的并发控制协议.模拟实验结果表明,DLOCC协议性能优于OCC-TI协议,有效地提高了系统资源利用率.     

Using optimistic atomic broadcast in transaction processing systems

Atomic broadcast primitives are often proposed as a mechanism to allow fault-tolerant cooperation between sites in a distributed system. Unfortunately, the delay incurred before a message can be delivered makes it difficult to implement high performance, scalable applications on top of atomic broadcast primitives. Recently, a new approach has been proposed for atomic broadcast which, based on optimistic assumptions about the communication system, reduces the average delay for message delivery to the application. We develop this idea further and show how applications can take even more advantage of the optimistic assumption by overlapping the coordination phase of the atomic broadcast algorithm with the processing of delivered messages. In particular, we present a replicated database architecture that employs the new atomic broadcast primitive in such a way that communication and transaction processing are fully overlapped, providing high performance without relaxing transaction correctness.     

K锁算法—一种数据库上的可适应并发控制策略

本文提出了一种基于悲观和乐观思想的并发控制算法——K锁算法.K锁算法利用K锁机制,在一定程度上允许冲突执程的并发执行.和乐观算法比较,它减少了在系统的执程冲突概率较大时执程的重执次数,和悲观算法比较,它提高了冲突执程的执行并发度.     

移动分布式数据库中的实时事务处理

提出了移动事务实时提交协议(MTRTC)和多版本乐观并发控制协议(MVOCC)处理移动分布式实时事务。MVOCC有效性检查分为局部有效性检查和全局有效性检查，采用动态调整串行次序，避免了不必要的事务重启动，改善了只读事务的响应时间。MTRTC是一个实时提交协议，减少了通信信息。实验结果表明结合MVOCC和MTRTC事务处理协议优于其它事务处理协议。     

Analysis of concurrency-coherency control protocols for distributedtransaction processing systems with regional locality

A system structure and protocols for improving the performance of a distributed transaction processing system when there is some regional locality of data reference are presented. A distributed computer system is maintained at each region, and a central computer system with a replication of all databases at the distributed sites is introduced. It provides the advantage of distributed systems principally for local transactions, and has the advantage of centralized systems for transactions accessing nonlocal data. Specialized protocols keep the copies at the distributed and centralized systems consistent without incurring the overhead and delay of generalized protocols for fully replicated databases. The advantages achievable through this system structure and the tradeoffs between protocols for concurrency and coherency control of the duplicate copies of the databases are studied. An approximate analytic model is used to estimate the system performance. It is found that the performance is sensitive to the protocol and that substantial performance improvement can be obtained as compared with distributed systems     

多副本分布式事务处理关键技术及典型数据库系统综述

数据复制是分布式数据库提高可用性的重要手段,通过在不同区域放置数据库的部分副本,还可以提高本地读写操作的响应速度,增加副本数量也会提升读负载的线性扩展能力.考虑到这些优良特性,近年来国内外都出现了众多多副本分布式数据库系统,包括Google Spanner、CockroachDB、TiDB、OceanBase等一系列主流的工业界系统,也出现了包括Calvin、Aria、Berkeley Anna等一系列优秀的学术界系统.然而,多副本数据库带来诸多收益的同时,也带来了一致性维护、跨节点事务、事务隔离等一系列挑战.总结分析现有的复制架构、一致性维护策略、跨节点事务并发控制等技术,对比几个代表性多副本数据库系统之间在分布式事务处理方面上的差异与共同点,并在阿里云环境下搭建跨区域的分布式集群环境,对几个代表性系统的分布式事务处理能力进行了实验测试分析.     

Blocking reduction for distributed transaction processing within MANETs

Atomic commit protocols for distributed transactions in mobile ad-hoc networks have to consider message delays and network failures. We consider ad-hoc network scenarios, in which participants hold embedded databases and offer services to other participants. Services that are composed of several other services can access and manipulate data of physically different databases. In such a scenario, distributed transaction processing can be used to guarantee atomicity and serializability throughout all databases. However, with problems like message loss, node failure, and network partitioning, mobile environments make it hard to get estimations on the duration of a simple message exchange. In this article, we focus on the problem of setting up reasonable time-outs when guaranteeing atomicity for transaction processing within mobile ad-hoc networks, and we show the effect of setting up “wrong” time-outs on the transaction throughput and blocking time. Our solution, which does not depend on time-outs, shows a better performance in unreliable networks and remarkably reduces the amount of blocking.     

Distributed optimistic concurrency control methods forhigh-performance transaction processing

There is an ever-increasing demand for more complex transactions and higher throughputs in transaction processing systems leading to higher degrees of transaction concurrency and, hence, higher data contention. The conventional two-phase locking (2PL) Concurrency Control (CC) method may, therefore, restrict system throughput to levels inconsistent with the available processing capacity. This is especially a concern in shared-nothing or data-partitioned systems due to the extra latencies for internode communication and a reliable commit protocol. The optimistic CC (OCC) is a possible solution, but currently proposed methods have the disadvantage of repeated transaction restarts. We present a distributed OCC method followed by locking, such that locking is an integral part of distributed validation and two-phase commit. This method ensures at most one re-execution, if the validation for the optimistic phase fails. Deadlocks, which are possible with 2PL, are prevented by preclaiming locks for the second execution phase. This is done in the same order at all nodes. We outline implementation details and compare the performance of the new OCC method with distributed 2PL through a detailed simulation that incorporates queueing effects at the devices of the computer systems, buffer management, concurrency control, and commit processing. It is shown that for higher data contention levels, the hybrid OCC method allows a much higher maximum transaction throughput than distributed 2PL in systems with high processing capacities. In addition to the comparison of CC methods, the simulation study is used to study the effect of varying the number of computer systems with a fixed total processing capacity and the effect of locality of access in each case. We also describe several interesting variants of the proposed OCC method, including methods for handling access variance, i.e., when rerunning a transaction results in accesses to a different set of objects