Distributed cooperation with action systems

Abstract

Action systems provide a method to program distributed systems that emphasizes the overall behavior of the system. System behavior is described in terms of the possible interactions (actions) that the processes can engage in, rather than in terms of the sequential code that the processes execute. The actions provide a symmetric communication mechanism that permits an arbitrary number of processes to be synchronized by a common handshake. This is a generalization of the usual approach, employed in languages like CSP and Ada, in which communication is asymmetric and restricted to involve only two processes. Two different execution models are given for action systems: a sequential one and a concurrent one. The sequential model is easier to use for reasoning, and is essentially equivalent to the guarded iteration statement by Dijkstra. It is well suited for reasoning about system properties in temporal logic, but requires a stronger fairness notion than it is reasonable to assume a distributed implementation will support. The concurrent execution model reflects the true concurrency that is present in a distributed execution, and corresponds to the way in which the system is actually implemented. An efficient distributed implementation of action systems on a local area network is described. The fairness assumptions of the concurrent model can be guaranteed in this implementation. The relationship between the two execution models is studied in detail in the paper. For systems that will be called fairly serializable, the two models are shown to be equivalent. Proof methods are given for verifying this property of action systems. It is shown that for fairly serializable systems, properties that hold for any concurrent execution of the system can be established by temporal proofs that are conducted entirely within the simpler sequential execution model.