In real-time collaborative systems, replicated objects, shared by users, are subject to concurrency constraints. In order to satisfy these, various algorithms, qualified as op?timistic, [3, 5, 13, 17, 14, 15, 18], have been proposed that exploit the semantic properties of operations to serialize concurrent operations and achieve copy con?vergence of replicated objects. Their drawback is that they either re?quire a condition on user's operations which is hard to verify when possible to ensure, or they need undo?ing then redoing operations in some situations. The main purpose of this paper is to present two new algorithms that over?come these drawbacks. They are based upon the imple?mentation of a continuous global order which enables that condition to be released, and simplifies the operation inte?gration process. In the second algorithm, thanks to de?ferred broadcast of operations to other sites, this process becomes even more simplified.

In a distributed groupware system, objects shared by users are subject to concurrency and real time constraints. In order to satisfy these, various concurrency control algorithms have been proposed that exploit the semantic properties of operations (C.A. Ellis and S.J. Gibbs, 1989; A. Karsenty and M. Beaudouin-Lafon, 1993; C. Sun et al., 1996). By ordering concurrent operations, they generally guarantee consistency of the different copies of each object. However, in some situations they can result in inconsistent copies, a non respect of user's intentions, and in the need to undo and redo some operations. The principle objective of the paper is to present an algorithm that overcomes these drawbacks in distributed and mobile collaborative environments. The algorithm is based on the notion of user intention, and also on the construction of equivalent histories by exploiting and combining some general semantic properties such as forward/backward transposition.

In a distributed groupware system, objects shared by users are subject to concurrency and real-time constraints.In order to satisfy these, various concurrency control algorithms 141 [ 1 l] have been proposed that exploit the semantic properties of operations.By ordering concurrent operations, they guarantee consistency of the different copies of each object.The drawback of these algorithms is that in some situations they can result in inconsistent copies, a non-respect of user's intentions, and in the need to undo and redo certain operations.The principle objective of the this paper is to present an algorithm that overcomes these problems.The algorithm is based on the notion of user's intention, and also on the construction of equivalent histories by exploiting and combining some general semantic properties such as forward/backward transposition.