Replicated Causal Objects Michel Raynal

High performance distributed computing environments must not only support eecient execution of applications but they must also support communication and cooperation among human users that cooperatively work on complex problems. To support eecient access to objects that support cooperation between users, the objects need to be replicated at nodes where they are frequently accessed. Causal consistency is a consistency criterion particularly attractive for objects shared in cooperative applications. It is based on a causality relation on read and write operations issued by concurrent processes. It only requires that writes that depend on one another be perceived in their dependence order by all processes; so, two independent writes can be perceived in a distinct order by two diierent processes. In message-passing systems causal consistency is usually implemented by a protocol that ensures causal delivery of all messages. In this paper we develop a new protocol for causally consistent shared objects that exploits the write semantics that can be associated with update messages. This protocol allows early delivery of update messages and can discard messages carrying obsolete values. By not requiring that all update messages be delivered in causal order, this protocol improves eeciency, reduces buuering requirements and compares very favorably with protocols that implement causal objects based on causal delivery of all update messages. Associer un type de contr^ ole aux messages pour mettre en oeuvre des objets causaux R esum e : Cet article d eveloppe un nouveau protocole pour impl ementer des objets causalement coh erents dans un syst eme r eparti. Ce protocole exploite la s emantique d' ecriture qui peut ^ etre associ ee aux messages transportant les mises a jour des objets. Par rapport aux protocoles existants, le protocole pr esent e permet de d elivrer au plus t^ ot les les messages de mises a jour et de rejeter les messages qui, lorsqu'ils arrivent, transportent des valeurs obsol etes. En n'exigeant pas la livraison causale de tous les messages ce protocole se r ev ele particuli erement performant et meilleur que les protocoles classiques fond es sur l'ordre causal.