Weakly-Persistent Causal Objects in Dynamic Distributed Systems

Abstract

In the context of clients accessing a read/write shared object, persistency of a written value is a property stating that a value written into the object is always available unless overwritten by a successive write operation. This property can be easily guaranteed in a static distributed system provided that either a subset of processes implementing the object does not crash or processes can crash and then recover being able to retrieve their last state. Unfortunately the enforcing of this property in a potentially large scale and dynamic distributed system (e.g. a P2P system) is far from being trivial when considering the case in which processes implementing the object may fail or leave at any time without notifying any other process (i.e., the last state might not be retrievable). The paper introduces the notion of weak persistency that guarantees persistency of values when a system becomes quiescent (arrivals and departures subside). An implementation of a weakly-persistent object ensuring causal consistency is provided along with its correctness proof. The interest of causal consistency lies in the fact that, contrarily to atomic consistency, it can be maintained even during non-quiescent periods of the distributed system (i.e., when persistency is not guaranteed)