A new distributed optimistic concurrency control method and a comparison of its performance with two-phase locking

Abstract

A distributed optimistic concurrency control (OCC) method followed by locking, such that locking is an integral part of distributed validation and two-phase commit, is presented. This OCC method ensures that a transaction failing its validation will not be reexecuted more than once, in general. Furthermore, deadlocks, which are difficult to handle in a distributed environment, are avoided by serializing lock requests. Implementation details are outlined, and the performance of the schemes is compared with distributed two-phase locking (2PL) through a detailed simulation, which incorporates queuing effects at the devices of the computer systems, buffer management, concurrency control, and commit processing. It is shown that in the case of higher data contention levels, the hybrid OCC method allows a much higher maximum transaction throughput than distributed 2PL. The performance of the method with respect to variable-size transactions is reported. It is shown that by restricting the number of restarts to one, the performance achieved for variable-size transactions is comparable to fixed-size transactions with the same mean size.