Providing undo functionality in groupware has been identified as an important, but difficult problem. Existing solutions show deficiencies like lacking generality, being too complex, being inefficient, or failing to yield acceptable results in common situations. In this paper we describe a new approach which reduces local group undo to a transformation-based method for combining the effects of concurrently issued user commands. Besides transformations we introduce mirror and folding operators in order to build a unique and consistent multidimensional model of the user interaction. In this way the problems of group undo mentioned above can be either overcome or at least reduced.

Concurrency control and group undo are important issues in the design of groupware, especially for interactive group editors. We present an improved version of an existing distributed algorithm for concurrency control that is based on operation transformations. Since the usability of the algorithm relies on its formal correctness, we present a set of necessary and sufficient conditions to be satisfied in order to ensure consistency in a replicated architecture. We identify desirable properties of operation transformations and show how our approach can be employed to implement group undo. The approach has been applied to build a prototypical group editor for text; some experiences gained are presented. Keywords Concurrency Control, Group Editors, Group Undo, Groupware, Interaction Model, Operation Transformation. INTRODUCTION Groupware systems, i.e., multi-user applications for cooperative work [2, 5], present remarkable challenges for application designers. Concurrency control and ...