Issues in Integrating Active Rules into Database Systems
Cochrane, Roberta J.
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An essential feature of next-generation database systems is the ability to define and process rules that respond to database events. We address several issues involved in fully integrating such active rules into multi-user database systems. We do this by investigating two very different database rule systems: the Update Dependency Language, which uses a tentative goal-oriented search strategy, and the Starburst Rule System, which uses a forward-chaining irrevocable control strategy.<P>For the Update Dependency Language, we formally define the language and define safety requirements for its conditions and procedures. We analyze the lacking requirements for two different execution strategies: one that uses a depth-first search and one that uses a concurrent search. We show that it is incorrect to release shared locks on failed subpaths before a successful path is found. However, we show that two-phase locking can be relaxed to allow the early release of exclusive locks along failed subpaths.<P>For the Starburst Rule System, we describe the components that handle recovery in the presence of system- generated and user-requested rollbacks. We investigate the problem of maintaining rule priorities in the Starburst Rule System and others, describing the requirements and implementation of a priority system that combines user-defined priorities and system-generated default priorities. To support an environment in which users can modify rules during normal database operations, we define consistency requirements for rule definition operations and present a solution based on hierarchical locking that maintains these consistency requirement in a multi-user environment.