Approaches to concurrency control in distributed data base systems

Whenever multiple users or programs access a data base concurrently, the problem of concurrency control arises. The problem is to synchronize concurrent interactions so that each reads consistent data from the data base, writes consistent data, and is ultimately processed to completion. In a distributed data base this problem is exacerbated because a concurrency control mechanism at one site cannot instantaneously know about interactions at other sites. No fewer than 30 papers on this topic have appeared to date. Our purpose is to survey this literature, concentrating on three approaches—locking, majority consensus, and SDD-1 protocols—which together subsume the bulk of the literature. **