Distributed Database Concurrency Controls Using Before-Values

Richard Edwin Stearns, Daniel J. Rosenkrantz

Abstract: Associated with the write of a database entity is both the "before" or old value, and the "after" or new value. Concurrency can be increased by allowing other transactions to read the before values of a given transaction. The ramifications of allowing this, particularly on a distributed system in which limited communications is desirable, are investigated. A careful distinction is made between design decisions concerning communications and design decisions concerning the responses to read/write requests. Two schemes for producing such controls are given, one scheme for systems where processes are committed on termination, and the other for systems where committment is made later.