Rollback-dependency Trackability: an Optimal Characterization and Its Protocol

Considering a checkpoint and communication pattern, the Rollback Dependency Track-ability (RDT) property stipulates that there is no hidden dependency between local checkpoints. In other words, if there is a dependency between two checkpoints due to a non-causal sequence of messages , then there must exist a causal sequence of messages that \doubles" the non-causal one and that establishes the same dependency. This paper provides a minimal characterization of the RDT property. This characterization deenes the smallest set of non-causal sequences of messages that have to be doubled in order to ensure the RDT property. Then, we consider the family of communication-induced checkpointing protocols that ensure on-they the RDT property. Assuming processes take local checkpoints independently (called basic checkpoints), protocols of this family direct them to take on-they additional local checkpoints (called forced checkpoints) in order that the resulting checkpoint and communication pattern satisses the RDT property. A new protocol belonging to that family is presented and it is shown that this protocol is optimal in terms of the number of forced checkpoints and in terms of the size of data structures it requires. The protocol attains this goal by a subtle tracking of causal dependencies on already taken checkpoints; this tracking is then used to prevent the occurrence of hidden dependencies. Finally a set of non-optimal protocols are derived from the optimal one. These derivations show a tradeoo between the size of the control information required by a protocol and the number of forced checkpoints it takes. It is interesting to note that this set includes many of the communication-induced checkpointing protocols proposed in the literature. La propri et e RDT: une caract erisation et un protocole optimaux R esum e : Dans une ex ecution r epartie avec points de contr^ ole, la propri et e RDT stipule qu'il n'y a pas de d ependance cach ee entre les points de contr^ ole locaux. Autrement dit, toute d ependance entre deux points de contr^ ole locaux due a une s equence de messages non causale doit ^ etre \doubl ee" par une d ependance causale. Dans cet article, on propose une caract erisation minimale de la pro-pri et e RDT. Cette caract erisation d eenit le plus petit sous-ensemble de s equences non-causales dont le doublement implique celui de toutes les s equences non-causales. On s'int eresse a une famille de protocoles qui assurent la propri et e RDT, …