In this paper, we introduce the concept of dynamically changing between centralized, replicated, and hybrid collaboration architectures. It is implemented by providing users a function that dynamically changes the mapping between user-interface and program components. We decompose the function into more primitive commands that are executed autonomously by individual users. These commands require a mechanism to dynamically replicate user-interface and program components on a user's site. We present a logging approach for implementing the mechanism that records input (output) messages sent to one incarnation of a program (user-interface) component, and replays the recorded messages to a different incarnation of the component. Preliminary experiments with an implementation of the mechanism show that response and completion times can improve by dynamically changing the architecture to adapt to changes to the set of users in a collaboration session involving a mix of mobile and stationary devices.

Current application-sharing systems support a single architecture for all collaborations, though different systems support different architectures. We have developed a system that supports a wide range of architectural mappings, which include, to the best of our knowledge, all of the existing architectures defined so far including the centralized, replicated, and hybrid architectures. Instead of being bound to a specific I/O protocol such as the X protocol, it is based on a simple abstract I/O protocol to which specific I/O protocols must be mapped by client-supplied code. We have used the system to perform experiments that compare the performance of the centralized and replicated architectures. Our experiments show that the choice of the architecture depends on the computers used by the collaborators, the speed of the connections between the computers, and the cost of the operations performed by the shared application. Under some conditions the centralized architecture gives better performance, while unde r others the replicated architecture gives better performance. Our results contradict the popular belief that the replicated architecture always gives better performance, and show the need for supporting both architectures in a system.

Migrating collaborative applications to or near the workstations of active users can offer better performance in many scenarios. We have developed a client migration mechanism for centralized shared window systems that does not require changes to existing application and system software. It is based on logging input at the old site and replaying it at the new site. This approach raises several difficult questions: How should the log size be kept low? How should response time be kept low while migration is in progress? How should applications that depend on the rate at which input is received be accommodated? How should the transition from the replay phase to the play phase be detected at the new site? How should the software at the old and new sites be synchronized? We have developed a series of alternative approaches for answering these questions and implemented them in the XTV [1] shared window system. In this paper, we motivate, describe, illustrate and evaluate these approaches, an...