Group communication is a useful abstraction in the development of highly available distributed and communication-oriented applications in wide area networks (WANs). The most important aspects of this abstraction are the dynamic maintenance of group membership and its diverse semantics for interleaving membership change notifications within the flow of regular messages. In this paper we propose a new architecture for a scalable group membership service for wide area environments. Our architecture provides two different service levels and their semantics, each geared to different applications with different needs: The congress membership service which provides simple semantics of membership approximation, and the moshe service, which extends congress, provides full virtual synchrony semantics. The novelty of our design is in its client-server approach, which allows lightweight clients to benefit from advanced membership services. Furthermore, our design supports the coexistence of full...

In this work we present a novel protocol for total ordering of messages in asynchronous distributed environments prone to machine and communication link failures. Using the protocol as a building block, we constructed a Totally Ordered Group Communication (TOGC) system, i.e., a group communication service with a totally ordered multicast primitive. TOGC is a powerful infrastructure for building distributed fault-tolerant applications such as totally ordered broadcast, consistent object replication, distributed shared memory, Computer Supported Cooperative Work (CSCW) applications and distributed monitoring and display applications. An important contribution of the total ordering protocol described in this work is its ability to dynamically adjust the message delivery flow to changes in the transmission rates of the participating processes. The adaptation is accomplished by assigning delivery priorities (weights) to messages according to sender transmission rates. The priorities are det...

The ubiquitous nature of communication in computer networks, firmly manifested in the Internet era, provided a context for the introduction of different collaborative tools widely accepted by the on-line community, such as textual chats, white boards, shared editors, video conference systems, shared spaces for the exchange of multimedia documents or even simple e-mail based collaborative systems.

Maestro is a middleware support tool for distributed multimedia and collaborative computing applications. These applications share a common need for managing multiple subgroups while providing possibly different quality-of-service guarantees for each of these groups. Maestro’s functionality maps well into these requirements, and can significantly shorten the development time of such applications. In this paper, we report on Maestro, and demonstrate its utility in implementing several multimedia and collaborative computing applications. In particular, we provide a detailed description of the implementation of IMUX, a pseudo X-server (proxy) for collaborative computing applications that is based on Maestro. Copyright © 1999 John Wiley & Sons, Ltd. KEY WORDS: distributed multimedia; collaborative computing; middleware; group communication; quality of service

This paper is a review of the field of computer-supported cooperative work (CSCW) with respect to digital libraries. The literature surveyed covers both library & information science and computer science. An overview of the field of CSCW is provided including requirements capture, ethnography, interfaces, toolkits, organisational memory etc. Collaboration is interpreted in a wide sense and systems supporting user-staff (e.g. remote reference) and user-user (e.g. collaborative filtering) interactions are described.

Coordination is the fundamental issue in computer-mediated process-centered teamwork. This paper focuses on how to manage coordination in our Web-based cooperative environment implemented in Java. In particular, attention is drawn to the coordination information storage with using the relational and objected-oriented databases. Comparison of using two types of databases is addressed as the result of experiments which is in favour of the object-oriented database.

The key performance parameter here is the response time observable by the user, rather than the number of operations per second as in noninteractive application systems. n Distributed user community. Cooperating users must be able to work on different machines connected over the Internet with nonnegligible and nondeterministic latency. While fiber-optic communication technologies offer virtually unlimited Internet bandwidth, the communication latency over an intercontinental connection cannot be reduced much below 100 milliseconds due to the speed limit of electronic and light signals. It is therefore communication latency, rather than bandwidth, that challenges the design of Internet-based, response-sensitive systems and calls for latency-tolerant or latencyhiding technical solutions. n Unconstrained operation. Multiple users must be allowed to edit the document freely at any time to facilitate a natural cooperative infor18

The following work focuses on the requirements for a Collaborative Augmented Reality System with the usage of Internet technologies to create new possibilities for collaborative teaching. In most learning environments, learning is imparted through seeing and hearing. In this paper, Collaborative AR Discussion and personal experience are also integrated in order to achieve a quicker transmission of study material and an increase in memory efficiency. We have analyzed the scenarios occur most frequently in service and maintenance and also the literature of Tele-Teaching. The needed requirements were derived and as a result integrated in a test system. This collaborative game simulates a web-based training and teaching environment in Collaborative AR as a new approach for teachers and trainees and describes scenarios and research perspectives for distance education and training at the same location. The game was used as a testbed to clarify the problems associated with AR and Collaborative Learning.

In this paper we consider three types of computer-based systems that are employed individually and in pairs to achieve learning objectives: Computer-Supported Learning Systems, Collaborative Systems, and Immersive Presence Systems. When these systems are integrated together, higher-order learning objectives that have typically required co-located interactions can be supported in a distributed fashion. The objective of this work is to report on educational uses of the intersection of these three different types of systems in the combination space we refer to as Computer Supported Collaborative Learning requiring Immersive Presence (CSCLIP). Educational uses of CSCLIP technology and their features are identified. We then provide a discussion and comparison of the features common to CSCLIP systems. Within this discussion, we develop the criteria necessary for comparison of current CSCLIPs and some key requirements of implementation of future CSCLIPs.