This research address the problem of providing efficient transactional access to distributed databases. A new mode of locks, termed ordered-shared mode, is proposed that permits data sharing in a constrained manner. A family of locking-based concurrency control protocols is developed by using three modes of locks: shared, ordered-shared, and non-shared. The strictest protocol in this family is the two phase locking protocol for databases, while the most permissive recognizes all conflict- preserving serializable histories. This is the first locking- based concurrency protocol that recognizes the entire class of conflict-preserving serializable histories. Theoretical and empirical studies of these protocols will be conducted during this project. The theoretical comparison is based on the analysis of sets of histories produced by the protocols. The empirical analysis involves building a performance model and a database system test-bed. Distributed database concurrency control protocols based on ordered-shared locks will be developed and implemented on a distributed database test-bed. Finally, the new mode of locks will be used to develop protocols for database system recovery, nested transactions, and abstract data types.
