Mutually Consistent Recording in Asynchronous Computations

A global record (i.e. a set of local records, one for each process of an asyn-chronous computation) abstracts what is usually called global state, global checkpoint or global snapshot in particular problems. This paper concentrates on consistency of global records. First, a general model of asynchronous computations, including the classical shared memory model and several message passing models, is introduced. Then, under this general model, a necessary and suucient condition, stating whether an arbitrary set of local records can be included in some consistent global record, is proved. Finally, it is shown that, when a simple strategy (derived as a consequence of the previous theorem) is followed by each process of an asynchronous computation, then all local records taken by processes belong to consistent global records. Such a result can be used to design eecient snapshoting or checkpointing algorithms in asynchronous computations. the HCM project \CABERNET" (3702) and the ESPRIT BRA project "BROADCAST" (6360). Accepted as short presentation in 15th ACM Symposium on Principles of Distributed Computing (PODC'96, may 1996, Philadelphia) under the title "About recording of asynchronous computation". Enregistrements mutuellement coh erents dans les calculs asynchrones R esum e : Un enregistrement global (c'est-a-dire un ensemble d'enregistrements locaux, un pour chaque processus d'un calcul asynchrone) recouvre les notions ha-bituelles d' etat global, point de contr^ ole global, ou encore instantan e, rencontr ees dans dii erents probl emes particuliers. On introduit d'abord un mod ele g en eral de calcul asynchrone, englobant les mod eles classiques de m emoire partag ee et de communication par messages. Dans ce cadre g en eral, on etablit ensuite une caract erisation des ensembles d'enregistre-ments locaux pouvant faire partie d' enregistrements globaux coh erents. Ennn, on montre que si tous les processus d'un calcul asynchrone appliquent une certaine strat egie (d eduite simplement de la caract erisation pr ec edente), alors tous les enre-gistrements locaux peuvent appartenir a des enregistrements globaux coh erents. Une telle strat egie peut ^ etre utilis ee pour construire des algorithmes eecaces de calcul d'enregistrements globaux coh erents dans les calculs asynchrones.