A high concurrency XPath-based locking protocol for XML databases

Providing efficient access to XML documents becomes crucial in XML database systems. More and more concurrency control protocols for XML database systems were proposed in the past few years. Being an important language for addressing data in XML documents, XPath expressions are the basis of several query languages, such as XQurey and XSLT. In this paper, we propose a lock-based concurrency control protocol, called XLP, for transactions accessing XML data by the XPath model. XLP is based on the XPath model and has the features of rich lock modes, low lock conflict and lock conversion. XLP is also proved to ensure conflict serializability. In sum, there are three major contributions in this paper. The proposed XLP supports most XPath axes, rather than simple path expressions only. Conflict conditions and rules in the XPath model are analyzed and derived. Moreover, a lightweighted lock mode, P-lock, is invented and integrated into XLP for better concurrency.     

A Transaction Model for XML Databases

The hierarchical and semistructured nature of XML data may cause complicated update behavior. Updates should not be limited to entire document trees, but should ideally involve subtrees and even individual elements. Providing a suitable scheduling algorithm for semistructured data can significantly improve collaboration systems that store their data—e.g., word processing documents or vector graphics—as XML documents. In this paper we show that concurrency control mechanisms in CVS, relational, and object-oriented database systems are inadequate for collaborative systems based on semistructured data. We therefore propose two new locking schemes based on path locks which are tightly coupled to the document instance. We also introduce two scheduling algorithms that can both be used with any of the two proposed path lock schemes. We prove that both schedulers guarantee serializability, and show that the conflict rules are necessary.     

虚拟样机协同设计并发控制策略研究

针对虚拟样机协同设计中数据量大、事务长、多层嵌套的问题 ,提出了一种基于事务语义的并发控制策略,阐述了并发控制机制中事务结构、事务提交、锁机制和冲突协调等关键性问题。根据数据要求生成复制事务,事务发生改变后 ,其他站点能够实时显示图形、实时读取数据。实例证明 ,基于事务语义的并发控制策略保证协同用户自由地操作共享资源 ,提高了并发控制的可靠性和数据的一致性。     

JPernLite: extensible transaction services for the WWW

Concurrency control is one of the key problems in design and implementation of collaborative systems such as hypertext/hypermedia systems, CAD/CAM systems, and software development environments. Most existing systems store data in specialized databases with built-in concurrency control policies, usually implemented via locking. It is desirable to construct such collaborative systems on top of the World Wide Web, but most Web servers do not support even conventional transactions, let alone distributed (multi-Website) transactions or flexible concurrency control mechanisms oriented toward team work-such as event notification, shared locks, and fine granularity locks. We present a transaction server that operates independently of Web servers or the collaborative systems, to fill the concurrency control gap. By default, the transaction server enforces the conventional atomic transaction model, where sets of operations are performed in an all-or-nothing fashion and isolated from concurrent users. The server can be tailored dynamically to apply more sophisticated concurrency control policies appropriate for collaboration. The transaction server also supports applications employing information resources other than Web servers, such as legacy databases, CORBA objects, and other hypermedia systems. Our implementation permits a wide range of system architecture styles     

同步协同设计中并发控制的研究

为解决协同设计过程中存在的数据一致性不高、数据传输量大、实时性差等问题,通过分析分布式CSCW环境中并发控制的各种方法,并根据同步协同设计特点和用户操作意愿,设计了并发控制网络模型.在该模型中,计算数据传输的最小延时确定主控站点,提出了基于事务优先权的混合并发控制策略,以保证协同用户在操作过程中的并发率.实例证明,以上措施的实现保证用户自由共享资源,提高了并发控制的实时性和数据的一致性.     

载货车辆称重管理系统中并发控制机制的研究和实现

载货车辆称重管理系统在大型厂矿企业中占据重要的地位。在该系统中,通常一些用户拥有相同的使用权限,如果这些用户不加以合理控制会破坏数据库一致性,因此怎样避免数据共享中的冲突通常是在系统开发过程中需要解决的关键问题。本文针对称重管理中多用户共享访问实时数据库数据时出现的问题,探讨了并发控制机制和解决并发控制的方法。     

一种基于乐观方法的安全实时并发控制协议

实时数据库通常应用在一些安全关键类应用中,如电子商务、股票交易、军事指挥系统等。在这样一些应用中,实时数据库系统需同时满足两方面的需求：确保数据安全和尽可能减低实时事务错过截止期的比率。然而,通常这两方面需求是相互冲突的,满足一方面是以牺牲另一方面为代价。本文提出了一种基于乐观方法的安全实时并发控制协议,该协议将安全约束整合到实时乐观并发控制协议中,并能根据应用的需求在安全性和实时性方面进行了适当的折中。性能测试结果显示,该协议在确保数据安全的同时并未明显地降低实时性能。     

基于局部路径加锁的XML并发控制协议

传统XML数据库并发控制协议大多采用基于XML文档树的路径加锁方式,并发度不高。针对该问题,提出一种基于局部路径加锁模型的XML数据库并发控制协议。该协议基于XML的节点编码,利用查询函数对目标节点的祖先后裔加锁情况进行快速判断,并根据操作类型对目标节点或者其父节点进行加锁,从而减少对锁的频繁请求,提高事务并发度以及实现大规模XML数据的有效管理。     

Floor control for multimedia conferencing and collaboration

Floor control allows users of networked multimedia applications to utilize and share resources such as remote devices, distributed data sets, telepointers, or continuous media such as video and audio without access conflicts. Floors are temporary permissions granted dynamically to collaborating users in order to mitigate race conditions and guarantee mutually exclusive resource usage. A general framework for floor control is presented. Collaborative environments are characterized and the requirements for realization of floor control will be identified. The differences to session control, as well as concurrency control and access control are elicited. Based upon a brief taxonomy of collaboration-relevant parameters, system design issues for floor control are discussed. Floor control mechanisms are discerned from service policies and principal architectures of collaborative systems are compared. The structure of control packets and an application programmer's interface are proposed and further implementation aspects are elaborated. User-related aspects such as floor presentation, assignment, and the timely stages of floor-controlled interaction in relation to user-interface design are also presented.     

基于Ajax4jsf的XML设计器

研究了一种支持网络化协同设计、满足不懂XML编程用户设计需求的XML设计器.重点考虑其实时、异步的信息交互能力,基于Ajax4jsf框架设计Web接口,为XML文档批量设计提供技术支持.由Ajax引擎在客户端处理用户请求,减少了多用户并发访问情况下的服务器载荷,通过异步模式获取数据,提升了用户的体验效果.基于模型驱动实现数据在数据库和XML文档间的双向映射,通过解析器DOM实现XML文档与其他应用程序的数据转换.开发了XML设计器的原型系统,并测试其信息交互性能.实验表明,所开发的XML设计器的实时异步信息交互能力有了一定的提高.     

Communication support for distributed collaborative applications

The development of distributed, multimedia, collaborative applications requires the resolution of communication issues such as concurrency control and temporal and causal synchronization of traffic over related data streams. Existing transport and/or session-layer protocols do not include the desired support for multistream, multipoint communication. In this paper, we propose new communication abstractions and mechanisms that facilitate the implementation of the necessary coordination and concurrency control semantics in a collaborative application. We propose a protocol suite called themultiflow conversation protocol (MCP) for the realization of these abstractions and describe its prototype implementation in an internetwork of workstations. The paper also describes our experience with the prototype and results of a performance evaluation.     

An Analysis of Merge Conflicts and Resolutions in Git-Based Open Source Projects

Version control systems such as Git support parallel collaborative work and became very widespread in the open-source community. Whilst Git offers some very interesting features, resolving conflicts that arise during synchronisation of parallel changes is a time-consuming task. In this paper we present an analysis of concurrency and conflicts in official Git repository of four projects: Rails, IkiWiki, Samba and Linux Kernel. We analyse the collaboration process of these projects at specific periods revealing how change integration and conflict rates vary during project development life-cycle. We also analyse how often users decide to rollback to previous document version when the integration process generates conflicts. Finally, we discuss the mechanism adopted by Git to consider changes made on two continuous lines as conflicting.     

协同CAD设计中基于设计历史的冲突检测算法

多位设计者并发地对特征进行操作,具有同步性、分布性和无约束性的特点。并发控制的基本目标就是通过对各用户的操作进行限制,规范化设计时序,保持设计工作的顺利进行。与文字、图像等数据的并发控制相比,CAD特征之间各种复杂的关系（如特征的空间关系、依赖关系等）,使得基于特征的并发控制十分困难。对CAD特征之间各种复杂关系对协同并发操作的影响进行详细分析,并给出复制式协同环境下并发冲突的检测方法和解决方案。     

A distributed concurrency control mechanism for XML data

XML has become a standard for data exchange in many fields of application. Thus, a huge amount of data in this format is spread around Web and is stored in different ways. In order to manage the access of this data, concurrency control techniques have been adopted. Nevertheless, most of these techniques are developed on centralized environments and, approaches for distributed environments do not take into account the specificity of XML data. This paper presents DTX, a mechanism for distributed concurrency control of XML data, based on specific techniques for this kind of data. Aiming to evaluate DTX, experiments were conducted in order to measure its performance.     

一种集成组播代理和操作转换的并发控制方法

摘　要：现有的分布式实时协作系统多采用操作转换的方法来提供并发控制服务,但是在系统数据量很大时,系统性能不高.为解决这一问题,提出了一种新的并发控制方法madOPT:该方法利用对象所定义的操作的语义进行冲突解析,集数据对象的动态载入、数据传输和操作转换功能为一体.madOPT改进了dOPT(distributed operation transformation)算法在操作转换过程中对操作日志的遍历以提高并发控制效率,并将对象属性作为并发控制的粒度,使之支持对图形、图像对象的并发操作;同时结合组播代理,提高数据传输率,使系统整体性能得到改善.     

基于多版本的安全实时并发控制协议

安全实时数据库必须同时满足安全性和实时性,然而这两者有时可能相互冲突.为了解决这种冲突,提出了基于多版本的安全实时并发控制协议.协议为每个数据对象提供两个版本:工作版本和只读版本,根据事务和数据对象的安全级为事务提供不同的数据版本,有效地解决了实时性与安全性之间的冲突,最后给出了协议的正确性证明.     

基于相对位置的分布式实时协同编辑乐观锁并发控制算法

实时分布式协同编辑系统是计算机支持的协同工作的典型应用系统,不少学者对其进行了大量的研究,其难点是协作编辑的一致性、实时性和无约束性。因此,并发控制始终是它的研究热点,目前已提出的协同编辑的并发控制算法有:传统的加锁法、tickle锁、floor控制、可逆执行(Undo/Redo)和操作转换等。传统的加锁法最突出的优点是设计和实现较为简单,能保证具体上下文的语义完整性,但是在分布式协同编辑中无法保证加锁位置的一致性。tickle锁方法适用于分节加锁的实时分布式协同编辑的并发控制,然而无法提供多个编辑者对节内的同时编辑。JCE中的协同编辑采用了floor控制方法,每个协作编者只有获得floor才能进行     

J2EE平台的扩展事务模型支持框架

随着Internet和分布计算的发展，应用趋向于分布、松耦合和复杂化，平面事务模型已经越来越不适应于这些新型的应用，因此出现了多种扩展事务模型．但目前的研究多关注于特定应用环境的模型的提出和分析，同时由于受数据源接口的限制，难以实现事务模型的一些扩展原语，所以真正实现扩展事务模型的系统很少．提出了一种J2EE平台下的扩展事务模型支持框架，通过增强事务管理器的数据并发控制，实现了对常见扩展事务模型的支持．     

Leveraging Java applets: toward collaboration transparency in Java

Widespread use of the Internet is giving rise to the need for collaborative applications that link users at remote sites. Many toolkits support the development of collaboration-aware applications, those developed specifically for cooperative work by multiple users. Another approach is collaboration transparency, the collaborative use of applications originally developed for a single user. When the runtime environment supports collaboration transparency, an application programmer need not write new code to make an application collaborative. Thus, collaboration transparency leverages the existing base of single user applications by extending them to collaborative use. We review options for providing collaboration transparency. We also discuss how collaboration transparency can be incorporated into Sun Microsystems' Java, the most widely used vehicle for developing interactive World Wide Web applications