A scalable technique for implementing multiple consistency levels for distributed objects

Abstract

In large scale distributed systems, caching and replication could greatly speedup access and increase availability. Consistency of replicated state can be guaranteed by forcing operations to occur in the same order at all sites. However some applications can preserve correctness with weaker consistency requirements leading to better performance. We propose an object lifetime based mutual consistency detection mechanism that is used to implement multiple consistency levels. This mechanism provides scalable implementations because caching overheads at client nodes depend only on the accesses done at the node. A contribution of this paper is the separation of the the mutual consistency detection mechanism from the policy that decides the desired consistency guarantees. This allows multiple consistency levels to coexist, thus improving system performance through the use of weaker consistency levels when possible. Besides improving performance, the mechanism allows for a graceful weakening of consistency requirements when stronger requirements cannot be maintained, as in the case of disconnection that can be experienced in mobile environments. The mutual consistency detection mechanism also provides a uniform way of hoarding a mutually consistent set of objects during voluntary disconnection.