Axiomatizing probabilistic processes: ACP with generative probabilities

Abstract

This paper is concerned with finding complete axiomatizations of probabilistic processes. We examine this problem within the context of the process algebra ACP and obtain as our end-result the axiom system _prACP ^−-_I , a probabilistic version of ACP which can be used to reason algebraically about the reliability and performance of concurrent systems. Our goal was to introduce probability into ACP in as simple a fashion as possible. Optimally, ACP should be the homomorphic image of the probabilistic version in which the probabilities are forgotten.

We begin by weakening slightly ACP to obtain the axiom system ACP ⁻_I The main difference between ACP and ACP ⁻_I is that the axiom x+δ=x, which does not yield a plausible interpretation in the generative model of probabilistic computation, is rejected in ACP ⁻_I . We argue that this does not affect the usefulness of ACP ⁻_I in practice, and show how ACP can be reconstructed from ACP ⁻_I with a minimal amount of technical machinery.

_prACP ⁻_I is obtained from ACP ⁻_I through the introduction of probabilistic alternative and parallel composition operators, and a process graph model for _prACP ⁻_I based on probabilistic bisimulation is developed. We show that _prACP ⁻_I is a sound and complete axiomatization of probabilistic bisimulation for finite processes, and that _prACP ⁻_I can be homomorphically embedded in ACP ⁻_I as desired.

Our results for ACP ⁻_I and _prACP ⁻_I are presented in a modular fashion by first considering several subsets of the signatures. We conclude with a discussion about the suitability of an internal probabilistic choice operator in the context of _prACP ⁻_I .