On combining fault tolerance and partial replication with causal consistency

Abstract

The purpose of this paper is to discuss the limitations imposed by introducing fault-tolerance in a partial replication system which aims to provide causal consistency. The general outcome is that, to provide support for indefinite replica-failure, the notion of partial in partial replication becomes not-really-partial-at-all. We prove that to implement causal consistency when indefinite replica-failures are possible, it is impossible to respect the concept of genuine partial replication -- not storing or propagating operations which are on objects a given replica does not replicate locally. In our initial approach to tackle this issue client replicas need only to replicate the operations they depend on which have not yet been marked as durable by the centralised component. We discuss remaining limitations and expected improvements in future work.