Lazy replication: exploiting the semantics of distributed services

Abstract

Distributed services such as mail or bulletin boards must be highly available. High availability requires that the data managed by the service be replicated. A major challenge in managing replicated data is ensuring consistency among the copies of the data. One way to guarantee consistency is to force operations to take effect in the same order at all sites. This approach, however, is often expensive. We have designed a new method for constructing logically centralized, highly available services to be used in a distributed environment. Our method is intended for services that appear to clients to be logically centralized: in spite of the service's distributed implementation, it has the same observable behavior as a single copy. We take the semantics of the application implemented by the service into account in order to weaken implementation constraints and thus improve response time and increase availability; constraints can be relaxed as long as clients cannot observe the difference.