Optimistic Replication and Resolution

Abstract

The term "Optimistic Replication" is prevalent in the distributed systems and distributed algorithms literature. The database literature prefers "Lazy Replication." 2 Definition Data replication places physical copies of a shared logical item onto different sites. Optimistic replication (OR) allows a program at some site to read or update the local replica at any time. An update is tentative because it may conflict with a remote update. Such conflicts are resolved after the fact, in the background. Replicas may diverge occasionally but are expected to converge eventually (see entry on Eventual Consistency). OR avoids the need for distributed concurrency control prior to using an item. It allows a site to execute even when remote sites have crashed, when network connectivity is poor or expensive, or while disconnected from the network. Disconnected operation, the capability to compute while disconnected from a data source, e.g., in mobile computing, requires OR. In computer-supported cooperative work, OR enables a user to temporarily insulate himself from other users. 3 Historical background The first historical instance of OR is Johnson's and Thomas's replicated database (1976). 1 Usenet News (1979) was an important and inspirational development. News supports a large-scale ever-growing database of (read-only) items, posted by users all over the world. A Usenet site connects infrequently (e.g., daily) with