Reconciling divergent copies is a problem encountered in distributed systems, groupware, version control systems and personal work involving several mobile computing devices. Published reconciliation methods, whether synchronous or asynchronous, require some ordering facility provided either by a central component (master copy, sequencer) or by a fully distributed mechanism (timestamps, state vectors,...). Nevertheless, scalability is limited. This paper presents an asynchronous algorithm based on Operational Transformations which provides the means to reconcile any number of copies, without this limitation. Copies can be modified (concurrently or not) and then reconciled pair-wise, at any time, regardless of the pair, while their convergence is achieved. Its main advantage is thus to enable free propagation of update operations while ensuring they will be ordered in the same global order.

Wikis, a popular tool for sharing knowledge, are basically collaborative editing systems. However, existing Wiki systems offer limited support for co-operative authoring, and they do not scale well, because they are based on a centralised architecture. This paper compares the well-known centralised MediaWiki system with several peer-to-peer approaches to editing of wiki pages: an operational transformation approach (MOT2), a commutativity-oriented approach (WOOTO) and a conflict resolution approach (ACF). We evaluate and compare them, according to a number of qualitative and quantitative metrics.

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.

The emergence of new criteria specifically adapted to multidatabase systems, in response to constraints imposed by global serializability, leads to restrictive hypotheses in order to ensure correctness of executions. This is the case with the two level serializability presented in [6], that ensures strongly correct executions if transaction programs are Local Database Preserving (LDP). The main drawback of the LDP hypothesis is that it relies on rigorous programming. The principal objective of this paper has been to suppress this drawback while conserving the strong correctness of 2LSR executions We propose defining precisely the notion of value dependencies, and managing them so as not to impose the LDP property.

This paper introduces, as an optimistic concurrency control method, a new certification method by means of intervals of timestamps, usable in a distributed database system. The main advantage of this method is that it allows a chronological commit order which differs from the serialization one (thus avoiding rejections or delays of transactions which occur in usual certification methods or in classical locking or timestamping ones). The use of the dependency graph permits both classifying this method among existing ones and proving it. The certification protocol is first presented under the hypothesis that transactions' certifications are processed in the same order on all the concerned sites; it is then extended to allow concurrent certifications of transactions.