Design of Optimistic Methods for Concurrency Control in Database Sharing Systems

Database Sharing refers to a local multiprocessor architecture where all processors share a common database at the disk level. Transaction systems running in such an environment intend to support high transaction rates with short response times as well as high availability and modular growth. In order to achieve these goals, Database Sharing (DB-sharing) requires an efficient syn­ chronization component to control the processors' accesses to the shared database. In this paper, we concentrate ourselves on optimistic methods for concurrency control in DB-sharing environ­ ments that promise less synchronization messages per transaction than locking algorithms. We describe a new distributed protocol called broadcast validation where the validations for a transaction are simultaneously started at multiple processors by a broadcast message. Such a parallel validation scheme is prerequisite for reaching short response times and high transaction rates. The use of timestamps permits simple and fast validations as well as an integrated solution to the so-called buller invalidation problem that is introduced by the DB-sharing architecture. Further improve­ ments of our algorithm are proposed in order to reduce the syn­ chronization overhead and to allow a combination with a distributed locking protocol which is advisable for applications with higher conflict probability. The communication and validation overhead of our algorithms is quantified by simple estimates.