Rethinking Consistency Management in Real-time Collaborative Editing Systems

Networked computer systems offer much to support collaborative editing of shared documents among users. Increasing concurrent access to shared documents by allowing multiple users to contribute to and/or track changes to these shared documents is the goal of real-time collaborative editing systems (RTCES); yet concurrent access is either limited in existing systems that employ exclusive locking or concurrency control algorithms such as operational transformation (OT) may be employed to enable concurrent access. Unfortunately, such OT based schemes are costly with respect to communication and computation. Further, existing systems are often specialized in their functionality and require users to adopt new, unfamiliar software to enable collaboration. This research discusses our work in improving consistency management in RTCES. We have developed a set of deadlock-free multi-granular dynamic locking algorithms and data structures that maximize concurrent access to shared documents while minimizing communication cost. These algorithms provide a high level of service for concurrent access to the shared document and integrate merge-based or OT-based consistency maintenance policies locally among a subset of the users within a subsection of the document – thus reducing the communication costs in maintaining consistency. Additionally, we have developed client-server and P2P implementations of our hierarchical document management algorithms. Simulations results indicate that our approach achieves significant communication and computation cost savings. We have also developed a hierarchical reduction algorithm that can minimize the space required of RTCES, and this algorithm may be pipelined through our document tree. Further, we have developed an architecture that allows for a heterogeneous set of client editing software to connect with a heterogeneous set of server document repositories via Web services. This architecture supports our algorithms and does not require client or server technologies to be modified – thus it is able to accommodate existing, favored editing and repository tools. Finally, we have developed a prototype benchmark system of our architecture that is responsive to users’ actions and minimizes communication costs

A component-based collaboration infrastructure

Groupware applications allow geographically distributed users to collaborate on shared tasks. However, it is widely recognized that groupware applications are expensive to build due to coordination services and group dynamics, neither of which is present in single-user applications. Previous collaboration transparency systems reuse existing single-user applications as a whole for collaborative work, often at the price of inflexible coordination. Previous collaboration awareness systems, on the other hand, provide reusable coordination services and multi-user widgets, but often with two weaknesses: (1) the multi-user widgets provided are special-purpose and limited in number, while no guidelines are provided for developing multi-user interface components in general; and (2) they often fail to reach the desired level of flexibility in coordination by tightly binding shared data and coordination services. In this dissertation, we propose a component-based approach to developing group- ware applications that addresses the above two problems. To address the first prob- lem, we propose a shared component model for modeling data and graphic user inter- face(GUI) components of groupware applications. As a result, the myriad of existing single-user components can be re-purposed as shared GUI or data components. An adaptation tool is developed to assist the adaptation process. To address the second problem, we propose a coordination service framework which systematically model the interaction between user, data, and coordination protocols. Due to the clean separation of data and control and the capability to dynamically "glue" them together, the framework provides reusable services such as data distribution, persistence, and adaptable consistency control. The association between data and coordination services can be dynamically changed at runtime. An Evolvable and eXtensible Environment for Collaboration (EXEC) is built to evaluate the proposed approach. In our experiments, we demonstrate two benefits of our approach: (1) a group of common groupware features adapted from existing single- user components are plugged in to extend the functionalities of the environment itself; and (2)coordination services can be dynamically attached to and detached from these shared components at different granules to support evolving collaboration needs

Framework for Real-time collaboration on extensive Data Types using Strong Eventual Consistency

La collaboration en temps réel est un cas spécial de collaboration où les utilisateurs travaillent sur le même élément simultanément et sont au courant des modifications des autres utilisateurs en temps réel. Les données distribuées doivent rester disponibles et consistant tout en étant répartis sur plusieurs systèmes physiques. "Strong Consistency" est une approche qui crée un ordre total des opérations en utilisant des mécanismes tel que le "locking". Cependant, cela introduit un "bottleneck". Ces dix dernières années, les algorithmes de concurrence ont été étudiés dans le but de garder la convergence de tous les replicas sans utiliser de "locking" ni de synchronisation. "Operational Trans- formation" et "Conflict-free Replicated Data Types (CRDT)" sont utilisés dans ce but. Cependant, la complexité de ces stratégies les rend compliquées à intégrer dans des logicielles conséquents, comme les éditeurs de modèles, spécialement pour des data structures complexes comme les graphes. Les implémentations actuelles intègrent seulement des data linéaires tel que le texte. Dans ce mémoire, nous présentons CollabServer, un framework pour construire des environnements de collaboration. Il a une implémentation de CRDTs pour des data structures complexes tel que les graphes et donne la possibilité de construire ses propres data structures.Real-time collaboration is a special case of collaboration where users work on the same artefact simultaneously and are aware of each other’s changes in real-time. Shared data should remain available and consistent while dealing with its physically distributed aspect. Strong Consistency is one approach that enforces a total order of operations using mechanisms, such as locking. This however introduces a bottleneck. In the last decade, algorithms for concurrency control have been studied to keep convergence of all replicas without locking or synchronization. Operational Transformation and Conflict free Replicated Data Types (CRDT) are widely used to achieve this purpose. However, the complexity of these strategies makes it hard to integrate in large software, such as modeling editors, especially for complex data types like graphs. Current implementations only integrate linear data, such as text. In this thesis, we present CollabServer, a framework to build collaborative environments. It features a CRDTs implementation for complex data types such as graphs and gives possibility to build other data structures

A framework for real time collaborative editing in a mobile replicated architecture

Mobile collaborative work is a developing sub-area of Computer Supported Collaborative Work (CSCW). The future of this field will be marked by a significant increase in mobile device usage as a tool for co-workers to cooperate, collaborate and work on a shared workspace in real-time to produce artefacts such as diagrams, text and graphics regardless of their geographical locations. A real-time collaboration editor can utilise a centralised or a replicated architecture. In a centralised architecture, a central server holds the shared document as well as manages the various aspects of the collaboration, such as the document consistency, ordering of updates, resolving conflicts and the session membership. Every user's action needs to be propagated to the central server, and the server will apply it to the document to ensure it results in the intended document state. Alternatively, a decentralised or replicated architecture can be used where there is no central server to store the shared document. Every participating site contains a copy of the shared document (replica) to work on separately. Using this architecture, every user's action needs to be broadcast to all participating sites so each site can update their replicas accordingly. The replicated architecture is attractive for such applications, especially in wireless and ad-hoc networks, since it does not rely on a central server and a user can continue to work on his or her own local document replica even during disconnection period. However, in the absence of a dedicated server, the collaboration is managed by individual devices. This presents challenges to implement collaborative editors in a replicated architecture, especially in a mobile network which is characterised by limited resource reliability and availability. This thesis addresses challenges and requirements to implement group editors in wireless ad-hoc network environments where resources are scarce and the network is significantly less stable and less robust than wired fixed networks. The major contribution of this thesis is a proposed framework that comprises the proposed algorithms and techniques to allow each device to manage the important aspects of collaboration such as document consistency, conflict handling and resolution, session membership and document partitioning. Firstly, the proposed document consistency algorithm ensures the document replicas held by each device are kept consistent despite the concurrent updates by the collaboration participants while taking into account the limited resource of mobile devices and mobile networks. Secondly, the proposed conflict management technique provides users with conflict status and information so that users can handle and resolve conflicts appropriately. Thirdly, the proposed membership management algorithm ensures all participants receive all necessary updates and allows users to join a currently active collaboration session. Fourthly, the proposed document partitioning algorithm provides flexibility for users to work on selected parts of the document and reduces the resource consumption. Finally, a basic implementation of the framework is presented to show how it can support a real time collaboration scenario