A process algebra of communicating shared resources with dense time and priorities

Patrice Marcel Andre Bremond-Gregoire, University of Pennsylvania


The correctness of real-time distributed systems depends not only on the function they compute but also on their timing characteristics. Furthermore, those characteristics are strongly influenced by the delays due to synchronization and resource availability. Process algebras have been used successfully to define and prove correctness of distributed systems. More recently, there has been a lot of activity to extend their application to real-time systems. The problem with most current approaches is that they ignore resource constraints and assume either a total parallelism (unlimited resources) or total interleaving (single resource). Algebra of Communicating Shared Resources (ACSR) is a process algebra designed for the formal specification and manipulation of distributed systems with resource and real-time constraints. A dense time domain provides a more natural way of specifying systems compared to the usual discrete time. Priorities provide a measure of urgency for each action and can be used to ensure that deadlines are met. In ACSR, processes are specified using resource bound, timed actions and instantaneous synchronization events. Processes can be combined using traditional operators such as nondeterministic choice and parallel execution. Specialized operators allow the specification of real-time behavior and constraints. The semantics of ACSR is defined as a labeled transition system. Equivalence between processes is based on the notion of strong bisimulation. A sound and complete set of algebraic laws can be used to transform almost any ACSR process into a normal form. In practice, several specifications may satisfy the same requirements with various degree of desirability. Some may use more resources; some may be faster. In fact, there are many ways to rank processes. We describe a method for defining order relations between execution traces and further expanding the relation to general processes. Monotonicity is an important property of operators as it ensures that ordering is preserved by contexts. We study the conditions that must be satisfied by the trace ordering to ensure monotonicity at the process level, both in the prioritized and unprioritized cases. While most operations are monotonic for a large variety of trace relations, few retain this property in a prioritized setting.

Subject Area

Computer science

Recommended Citation

Bremond-Gregoire, Patrice Marcel Andre, "A process algebra of communicating shared resources with dense time and priorities" (1994). Dissertations available from ProQuest. AAI9427507.