Achieving robustness in distributed database systems

The problem of concurrency control in distributed database systems in which site and communication link failures may occur is considered. The possible range of failures is not restricted; in particular, failures may induce an arbitrary network partitioning. It is desirable to attain a high “level of robustness” in such a system; that is, these failures should have only a small impact on system operation. A level of robustness termed maximal partial operability is identified. Under our models of concurrency control and robustness, this robustness level is the highest level attainable without significantly degrading performance. A basis for the implementation of maximal partial operability is presented. To illustrate its use, it is applied to a distributed locking concurrency control method and to a method that utilizes timestamps. When no failures are present, the robustness modifications for these methods induce no significant additional overhead.