Effective Optimistic Concurrency Control in Multiversion Object Bases

The use of versioned data has proven its value in many areas of Computer Science including concurrency control. In this paper we examine the use of versioned objects in object bases for the purpose of enhancing concurrency. We provide a framework for discussing multi-version objects which includes fundamental definitions, the abstraction of objects as automata and a model of object method executions as transactions. A practical optimistic concurrency control protocol for multiversion objects is then presented within the developed framework. This protocol avoids the high roll back costs associated with optimistic protocols in two ways. First, a less restrictive definition of conflict, compared to other definitions, is used to determine when concurrent executions are invalid. Fewer conflicts means fewer roll backs are necessary. Second, a reconciliation process is described which permits cost-effective recovery from invalid concurrent executions rather than roll back