This is the companion paper of a talk in the Gems of PODS series, that reviews the development, starting at PODS 1988, of a family of Datalog-like languages with procedural, forward chaining semantics, providing an alternative to the classical declarative, model-theoretic semantics. These languages also provide a unified formalism that can express important classes of queries including fixpoint, while, and all computable queries. They can also incorporate in a natural fashion updates and

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... rminism. Datalog variants with forward chaining semantics have been adopted in a variety of settings, including active databases, production systems, distributed data exchange, and data-driven reactive systems.