Concepts for optimistic concurrency control in centralized and distributed database systems

Optimistic synchronization methods are considered as a promising alternative to locking algorithms for concurrency control, especially for applications with moderate conflict probability. This paper aims at providing a survey of some existing proposals for optimistic synchronization in centralized and distributed database systems, as well as describing a number of new and improved techniques. After an introduction we review the original BOCCscheme for centralized database systems and describe two heavily improved alternatives (the FOCC and the BOCC + approach) and also a combination with locking. We then show how these basic algorithms can be used for synchronization in distributed database systems. Here the validations can be performed either at a central site or in a distributed way where the latter approach introduces some extra problems to solve. Furthermore, three important optimizations for optimistic synchronization protocols are discussed: placing transactions before the end of the serialization order in case of conflict, use of a multiversion concept and restriction to level-2-consistency. © 1988 De Gruyter Oldenbourg. All rights reserved.