Algorithm for concurrency control in replicated databases.

A replicated database system is a distributed database system in which some data objects are stored redundantly at multiple sites to improve the reliability of the system. Without proper control mechanisms, the consistency of a replicated database system could be violated. A new scheme to increase the reliability as well as the degree of concurrency is described. It allows transactions to operate on a data object so long as one or more token copies are available. The scheme also exploits the fact that, for recovery reasons, there are two values for one data object. It is proved that transactions executed according to this scheme preserve consistency.