A common way to address scalability requirements of distributed services is to employ server replication and client caching of objects that encapsulate the service state. The performance of such a system could depend very much on the protocol implemented by the system to maintain consistency among object copies. We explore scalable consistency protocols that never require synchronization and communication between all nodes that have copies of related objects. We achieve this by developing a novel approach called local consistency (LC). LC based protocols can provide increased flexibility and efficiency by allowing nodes control over how and when they become aware of updates to cached objects. We develop two protocols for implementing strong consistency using this approach and demonstrate that they scale better than a traditional invalidation based consistency protocol along the system load and geographic distribution dimensions of scale.

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.