基于对象的Any Undo策略

Undo是基于对象的协同图形编辑系统的重要功能,可用于误操作的恢复。Undo的实现极具挑战性,因为分布协同环境下操作具有并发性,Undo的实现与一致性维护密切相关。提出的基于对象的Any Undo策略从操作语意的角度探讨了Undo操作的一致性维护,并给出了具体的实现机制。给出的支持机制都得到了实例证明,其可行性也在原型系统CoDraftPaint中得到初步验证。     

Practical access control management for distributed collaborative editors

The project summarized in this article aims at developing techniques to support access control in Real-Time Distributed Collaborative Editors (RCE). The ever-increasing role of RCE in academic, industry and society comforts the expansion of data sharing and raises growing concerns about controlling access to this data. Indeed, RCE allow for a human–computer–human interaction in a decentralized fashion. Thus, access control for RCE requires a careful design since they need dynamic access changes and low latency access to shared document while maintaining its consistency.In this article, we propose a flexible access control model where the shared document and its authorization policy are replicated in the local memory of each user. To deal with latency and dynamic access changes, we use an optimistic access control technique in such a way that enforcement of authorizations is retroactive. Our model is generic enough to be deployed on the top of the most of existing logging-based collaborative systems. Indeed, it does not entail overheads and it does not affect the convergence of the shared document. We show that naive coordination between updates of both copies can create security holes on the shared document, by permitting illegal modifications or rejecting legal modifications and present our solutions to avoid these problems. Finally, we present a prototype for managing authorizations in collaborative editing work in a decentralized fashion. Thus our model may be deployed easily on mobile devices over P2P networks.     

一种新的多版本增创算法

现有的对象复制技术是实时协同图形编辑系统中符合一致性模型的并发控制策略,但难以解决非几何属性以及三维绘图系统中的并发冲突情况.介绍了一个全新的一致性维护模型——基于版本复制的多版本技术,当并发操作发生冲突时,利用简化规则从目标版本派生出叶子物理版本,分别应用冲突操作到不同的叶子版本,有效地解决了非调和冲突操作的意愿保证.通过研究多版本技术的相关支持算法,在算法MOVIC的基础上,从各个冲突操作的最大共同相容操作着手,提出了基于相容冲突组的快速多版本增创算法FMVIC(Fast Multiple Versions Incremental Creation),减少了相容操作的比较次数,缩小了去冗余操作的范围.     

协同三维几何建模动态访问技术研究*

为解决协同设计过程中几何模型的动态访问问题,提出一种基于Web的协同设计环境下三维几何建模动态访问的分布式体系结构、异类CAD系统之间的动态数据交换组织模式。同时,提出一种面向对象的基于优先权限的乐观并发控制协议,保证模型动态访问在各站点操作的执行结果之间的一致性。在这些技术的共同作用下实现了协同三维几何建模动态访问技术,并通过具体实例验证了该技术的可行性。     

基于四叉树索引的区域版本实时协同绘图系统

随着传统GIS向网络GIS的发展,地图的更新通常需要网络中多个站点相互协作完成,传统的并发控制策略不能很好满足协同GIS中多用户协作感知的要求。为此提出了基于四叉树索引的区域版本并发控制机制,并给出了基于四叉树索引的区域版本实时协同绘图系统的框架和相关问题的解决方案。     

实时协同工作系统中操作转换算法综述

首先分析了实时协同工作系统中的事件关系,讨论了一致性模型及其收敛问题,然后以典型实时协同工作系统中操作转换算法的研究进展为线索,将算法中最核心的调度函数剥离出来,进行了详细的分析 .对意图维护问题进行了解析,将其归结为在立即模式下对并发问题的处理,得出了意图维护问题在算法层面的准则 .最后对基本转换函数和典型转换算法进行了归纳和总结,并对进一步研究方向进行了探讨 .     

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。     

基于色彩地址空间转换与多版本策略的复杂位图协同模型与一致性维护算法

调色编辑在现有位图协同模型下存在数据发送量大、编辑灵活性差、操作重叠易冲突、生成版本多、站点存储压力大等问题,针对这些问题提出了一种面向复杂位图的调色协同编辑模型。协同操作仅发送坐标与色彩变化量的协同操作,具有数据量小、多维编辑粒度、位置可重叠等特性;增加了时空关系的四维位图协同架构,只保存已编辑的像素点数据可减少数据存储量;采用空间转换的副本状态控制算法,通过回溯重构历史版本,可以确保协同操作总是在正确的副本状态上执行;根据当前状态与策略配置,动态生成分支版本的算法,可以解决此前模型分支版本过多的问题。最后,基于CCI（causality-preservation,convergence,intention-preservation）模型进行正确性推论,开发协同编辑器原型Co-Pixel,随机生成的操作集设计实验,对照Adobe Lightroom 模拟实际编辑行为。结果显示提出的模型可有效减少操作产生冲突,降低一致性维护过程中的算法开销,灵活调整用户执行意图维护工作的结果版本。     

A string-wise CRDT algorithm for smart and large-scale collaborative editing systems

With the development of big data and cloud computing, real-time collaborative editing systems have to face new challenges. How to support string-wise operations for smart and large-scale collaborations is one of the key issues in next generation of collaborative editing systems, which is both the core topic of collaborative computing area and the fundamental research of many collaborative systems in science and engineering. However, string-wise operations have troubled the existing collaborative editing algorithms, including Operational Transformation (OT) and Commutative Replicated Data Type (CRDT), for many years. This paper proposes a novel and efficient CRDT algorithm that integrates string-wise operations for smart and massive-scale collaborations. Firstly, the proposed algorithm ensures the convergence and maintains operation intentions of collaborative users under an integrated string-wise framework. Secondly, formal proofs are provided to prove both the correctness of the proposed algorithm and the intentions preserving of string-wise operations. Thirdly, the time complexity of the proposed algorithm has been analyzed in theory to be lower than that of the state of the art OT algorithm and CRDT algorithm. Fourthly, experiment evaluations show that the proposed algorithm outperforms the state of the art OT algorithm and CRDT algorithm.     

Design issues and model for a distributed multi-user editor

The collaborative editing of documents is a very common task nowadays. Writing groups are often distributed over many locations because of the globalization of organizations and the increasing interdisciplinarity of tasks. Since many writers already use computers for their jobs, providing computer support for the collaborative writing process has been identified as an important goal. Numerous tools for computer supported collaborative writing have already emerged but in most cases have not come into widespread usage. In this article the requirements of users for a collaborative editor are analyzed. Providing as much flexibility as possible to the users is identified as a basic need. According to the requirements summary a model for a group editing environment is presented. The model covers cooperative work in local and wide area networks using synchronous and asynchronous cooperation. Finally, an application of the model is presented in the form of the multi-user editing environmentIris.     

考虑工程图形协同应用特点的并发控制研究

并发控制是CSCW系统的关键技术，没有统一标准，通常要根据协同应用的特点来构造。文中以CAD系统内部数据库拓扑结构的一致性作为显示一致性的基础，适时放宽整体一致性模型中的事件定义条件和一致性判定条件，提出一种改进的整体一致性模型。按照该模型的事件关系定义，通过图形对象全局ID标识的一致性维护和添加操作的相关性分析，提出一种添加操作优先的并发控制策略。基于上述模型和策略，研究了一种偏复制式混合体系结构下的悲观和有限乐观的并发控制方法，以及相应的并发控制协议实施技术。工程实践证明，这些并发控制模型、策略、方法和技术充分考虑了工程图形数据结构及其协同应用特点，从而有效地支持设计过程中的人人交互。     

一种维护实时数据库一致性的新方法

本文研究了实时数据库中的一致性要求及现有并发控制技术的缺陷 ,进而提出一种对由现有调度技术生成的事务调度队列进行调整以保证实时数据库外部一致性要求 ,从而获得更佳性能的方法     

Trust-based access control for collaborative systems

In distributed, heterogeneous and network-connected collaborative environments where resources are provided to diverse unknown users for their applications, it is necessary to define access control for resources. Access control for such systems is defined as the ability to authorise or repudiate access to resources by a particular user. Traditional access control solutions are inherently inadequate for collaborative systems because they are effective only in situations where the system knows in advance which users are going to access the resources and what are their access rights so that they can be predefined by the developers or security administrators, but in collaborative systems the number of users as well as their usage on resources is not static. Targeting collaborative systems, a fine grained, flexible, persistent trust-based model for protecting the access and usage of digital resources is defined in this paper using radial basis function neural network (RBFNN). RBFNN classifies the users requesting the resources as trustworthy and non-trustworthy based on their attributes. RBFNN is used for classification because of its ability to generalise well for even unseen data and non-iterative method employed in its training. A proof of concept implementation backed by extensive set of tests on the real data collected for one such collaborative systems, i.e. Enabling Grids for E-Science grid demonstrated that the design is sound for collaborative systems where access of resources are provided to large and unknown users with their variant set of requirements.     

Consistency Control for Synchronous and Asynchronous Collaboration Based on Shared Objects and Activities

We describe a new collaborative technology that bridges the gap between ad hoc collaboration in email and more formal collaboration in structured shared workspaces. Our approach is based on the notion of object-centric sharing, where users collaborate in a lightweight manner but aggregate and organize different types of shared artifacts into semi-structured activities with dynamic membership, hierarchical object relationships, as well as real-time and asynchronous collaboration. We present a working prototype that implements object-centric sharing on the basis of a replicated peer-to-peer architecture. In order to keep replicated data consistent in such a dynamic environment with blended synchronous and asynchronous collaboration, we designed appropriate consistency control algorithms, which we describe in detail. The performance of our approach is demonstrated by means of simulation results.     

基于对象复制的协同绘图并发控制算法

全复制模式下的基于对象的实时协同图形编辑系统支持不同地域的设计人员通过网络同步高效地浏览、操作和修改共享文档和文档中的各种对象，包括图形、图象和文本等，其中操作意愿保证及其并发控制成为复杂而又关键的问题之一，其研究具有挑战性．文中讨论了基于图形对象的并发操作的冲突，提出直接依赖和对象状态概念确定操作的候选对象和相容集合，并改进了对象复制算法．基于此，给出了一个有效的并发控制算法。     

A partially distributed deadlock detection algorithm

In this paper, a partially distributed deadlock detection algorithm [PDDDA] with multiple outstanding requests is presented for use in distributed database systems. This algorithm allows a process to request many resources simultaneously and uses a central controller for detecting multisite deadlocks. The detection of local deadlocks and the maintenance of local deadlock information are performed at each of the local sites. This partially distributed algorithm alleviates the problem of congestion at the central controller in a centralized algorithm and needs fewer messages and smaller storage space than a fully decentralized algorithm. A set of criteria for comparing deadlock detection algorithms are also given and then used to compare PDDDA with a fully decentralized algorithm proposed by Isloor and Marsland.Research reported herein was supported by US Army CECOM, Ft. Monmouth, New Jersey, under Contract No. DAAB07-83-K-K542. The views, opinions, and/or findings contained in this paper are those of the authors and should not be construed as an official Deportment of the Army position, policy or decision.     

协同建模系统中的一种对象引用正确性保证方法

乐观并发控制允许操作并发执行,由此将产生对象引用发生时刻与对象引用使用时刻的几何模型的不一致,结果将导致命名机制的失效而不能保证对象引用的正确性.将引用对象分为可替代与不可替代2种类型,对于可替代对象引用,通过构建对象引用发生时刻的临时几何模型保证该类对象引用的正确性;对于不可替代对象引用,通过恢复对象引用发生时刻的几何模型,然后完成引用该类对象的操作,最后Redo模型恢复过程中被Undo的并发操作来保证该类对象引用的正确性.在原型系统中对提出的方法进行了验证.     

A highly secured and streamlined cloud collaborative editing scheme along with an efficient user revocation in cloud computing

These days with the expanded fame of cloud computing, the interest for cloud-based collaborative editing service is rising. The encryption method is utilized to ensure and secure the data, during the collaborative editing process. In the encryption process, the cloud requires more time to work the collaborative editing. This paper proposes an efficient scheme for reducing the encryption burden over the cooperative users, as the possibilities of cooperative users read and write data by means of any gadget. In the proposed scheme, the encrypted file sent by the data owner is split into smaller segments and stored in the cloud by the cloud service provider (CSP) along with specific tags. Once the cooperative user receives and decrypts the file from the CSP, it modifies and encrypts only the modified segment and resends to the CSP. The CSP after verifying the signature replace the original file segment in the cloud with the modified segment based on the tag information. The scheme that is put forward is performed based on the modified ciphertext-policy hierarchical attribute–based encryption, and the security process is done based on the attribute-based signature schemes. This work employs a proficient attribute updating method to accomplish the dynamic change of users' attributes, consisting granting new attributes, revoking previous attributes, and regranting formerly revoked attributes. A writer's attributes and keys have been revoked, and the stale information cannot be written.     

一种优化组合相似度的协同过滤推荐算法

为了进一步提高相似度计算的准确性,提出了一种优化组合相似度的协同过滤推荐算法。首先,建立用户-项目评分时间矩阵,根据用户对共同评分项目的评分时间先后顺序,计算用户之间的影响力;其次,根据用户对共同评分项目的评分差异,计算评分差异的加权信息熵;最后,将时序行为影响力融入到基于加权信息熵的相似度中,其中融合参数α由随机粒子群优化算法选择。通过与其他相似度计算方法比较,该算法降低了标准平均绝对误差和流行度,在一定程度上降低了数据稀疏性的影响,能更准确地计算相似度,从而提高了推荐质量。     

基于改进蚁群协同算法的枢纽机场场面滑行道优化调度模型

滑行道连接停机位和跑道,是机场场面调度的重要关键环节。基于飞机滑行时的冲突约束和跑道资源的动态分配,采用改进蚁群协同算法与滑动窗口控制相结合的方法,对滑行道进行优化调度。在保证滑行道零冲突、兼顾单个航班滑行时间的前提下,缩小机场进出港航班总滑行时间。对国内某枢纽机场的滑行道调度仿真实验表明,所提出的方法和模型具有明显的优势,可为枢纽机场的场面滑行调度提供决策支持。