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Open Access Publications from the University of California

A model of time dependent behavior in concurrent software systems


A great difficulty in building distributed systems lies in being able to predict what the systems behavior will be. A distributed or communicating system is defined here to be one in in which the hardware consists of a set of processors each with their own memory, connected by some communication medium (there is no shared memory), and the software is assumed to be of the CSP (Hoare's Communicating Sequential Processes) type.

In the past few years some theories have been proposed to model features of communicating systems. Milner's Calculus of communicating Systems (CCS), Winskel's Synchronization Trees (ST), Hennessy's Acceptance Trees (AT), and Hoare and Brookes's theory of communicating processes are examples of formal models of such systems. All of these models concentrate on modelling observable properties of a system.

Event Dependency Trees (EDT) is a new representation of communicating systems that models the time dependent nature of such systems. None of the representations mentioned above explicitly represent time but time is precisely the factor that introduces so much variability and complexity into such software and systems. EDT provides a representation based on trees and a set of operations over the EDT trees that can be used to produce deadlock-free software. The model supplies potentially important information for the design and construction of distributed, parallel software systems.

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