this paper, we propose a general framework for implementing undo in collaborative systems. The framework allows users to individually reverse their own changes, taking into account the possibility of conflicts between different users' operations that may prevent an undo. The proposed framework has been incorporated into DistEdit, a toolkit for building group text-editors. Based on our experience with DistEdit's undo facilities, we discuss several issues that need to be taken into account in using the framework, in order to ensure that a reasonable undo behavior is provided to users. We show that the framework is also applicable to single-user systems, since the operations to undo can be selected not just on the basis of who performed them, but by any appropriate criterion, such as the document region in which the operations occurred or the time interval in which the operations were carried out. Categories and Subject Descriptors: D.2.2 [Software Engineering]: Tools and Techniques -- User Interfaces; H.1.2 [Models and Principles]: User/Machine Systems -- Human Factors; H.2.2 [Database

The ability to undo operations is a standard feature in most single-user interactive applications. However, most current collaborative applications that allow several users to work simultaneously on a shared document lack undo capabilities; those which provide undo generally provide only a global undo, in which the last change made by anyone to a document is undone, rather than allowing users to individually reverse their own changes. In this paper, we propose a general framework for undoing actions in collaborative systems. The framework takes into account the possibility of conflicts between different users' actions that may prevent a normal undo. The framework also allows selection of actions to undo based on who performed them, where they occurred, or any other appropriate criterion. KEYWORDS Undo, collaboration, groupware, conflict analysis. 1 INTRODUCTION The ability to undo operations is a standard and useful feature in most interactive single-user applications. For instance...

In this paper, we investigate the problems associated with the provision of an undo support facility in the context of a synchronous shared or group editor. Previous work on the development of formal models of undo has been restricted to single user systems and has focused on the functionality of undo, as opposed to discussing the support that users require from any error recovery facility. Motivated by new issues that arise in the context of computer supported cooperative work, we aim to integrate formal modelling of undo with an analysis of how users understand undo facilities. Together, these combined perspectives of the system and user lead to concrete design advice for implementing an undo facility. The special issues that arise in the context of shared undo also shed light on the emphasis that should be placed on even single user undo. In particular, we come to regard undo not as a system command to be implemented, but as a user intention to be supported by the system. Keywords:...

Computer simulation is a valuable tool for the design and analysis of complex systems. However, the simulation of large complex systems such as, telecommunication, traffic, manufacturing, combat, training, ecological, engineering, and computer systems, can require massive resources in terms of processing times and memory. The parallel execution of such discrete event simulations offers the potential of substantially reducing this processing time. Optimistic synchronization has been proposed to exploit the inherent parallelism within systems and has been shown to be capable of impressive speedup through parallel execution. Two serious problems with optimistic methods have yet to be adequately resolved. The first is the saving, or logging, of model state information during forward execution, so that rollback to a previous state can be accomplished. The second problem is to solve the first in a way which is transparent to the programmer, i.e. that doesn't substantially complicate model d...

The ability to undo operations is a standard feature in most single-user interactive applications. However, most current collaborative applications that allow several users to work simultaneously on a shared document lack undo capabilities; those which provide undo generally provide only a global undo, in which the last change made by anyone to a document is undone, rather than allowing users to individually reverse their own changes. In this paper, we propose a general framework for undoing actions in collaborative systems. The framework takes into account the possibility of conflicts between different users' actions that may prevent a normal undo. The framework also allows selection of actions to undo based on who performed them, where they occurred, or any other appropriate criterion. The proposed framework have been incorporated in DistEdit, a toolkit for building text group editors. Based on our experience, we discuss methods for maintaining the undo information in a groupware env...