Semantical Considerations on Workflows: An Algebra for Intertask Dependencies

Workflows are composite multitransaction activities occurring in heterogeneous environments. They relax the semantic properties of traditional transactions to accommodate the demands of such environments. It is important that workflows be specified declaratively, reasoned about formally, and scheduled automatically. Declarative approaches based on intertask dependencies are prominent in the literature. However, extant approaches often lack a formal semantics, or fail to meet other important criteria. Also, they do not carefully distinguish event types from instances, a distinction that is crucial when the constraint that tasks are loop-free is relaxed. We propose an approach that gives a rigorous formal semantics for dependencies and meets the above conditions. Our approach uses algebraic expressions to represent dependencies and uses symbolic reasoning to take scheduling decisions. It can form the basis of a programming language for workflows.