Asynchronous Observations of Processes

Abstract

Abstract. We study may and must testing-based preorders in an asynchronous setting. In particular, we provide some full abstraction theorems that offer alternative characterizations of these preorders in terms of context closure w.r.t, basic observables and in terms of traces and acceptance sets. These characterizations throw light on the asymmetry between input and output actions in asynchronous interactions and on the difference between synchrony and asynchrony. IntroductionDistributed systems can seldom rely on a global clock, and little assumptions can be made about their relative speed; as a consequence, it is natural to adopt for them an asynchronous communication mechanism. This calls for non-blocking sending primitives that do not oblige producers and consumers to synchronize when exchanging messages, but allow the sender of a message to continue with its task while the message travels to destination. Therefore, for describing distributed systems, a model based on a paradigm that imposes a neat distinction between input and output primitives, in the style of [1] and [17], appears to be a natural choice. In spite of these considerations, the most studied concurrency models in the process algebra community (e.g. [18,3,14,20]) are based on synchronous communications and model process interaction as the execution of simultaneous "complementary actions". Only recently, variants of process algebras based on asynchronous communications have been studied. Two main approaches have been followed to this purpose. They differ in the way (non-blocking) output actions are modelled. These actions are rendered either as state trans]ormers or as processes themselves. The asynchronous variants of ACP [9] and CSP [16] follow the first approach and introduce explicit buffers in correspondence of output channels. This makes outputs non-blocking and immediately executable; their executions make messages available for consumption. The asynchronous variants of ~r-calculus [15,6,12,2] and CCS [21,11,8] follow the second approach and model outputs by creating new concurrent processes. This amounts to modelling an output prefix ~.P as a parallel composition a l PWork partially supported by EEC: HCM project EXPRESS, and by CNR: project "Specifica ad alto livello e verifica formale di sistemi digitali". The third author has been supported by a scholarship from CNR --Comitato Scienza e Tecnologie dell'Informazione.

Modular encoding of synchronous and asynchronous interactions using open Petri nets

“…We adopt the presentation in [22] that allows a non-deterministic choice for input prefixes and silent actions (a feature missing in [6,8]). …”

“…Concretely, as case studies we focus on two paradigmatic process calculi, namely CSP and CCS, and we encode two interaction mechanisms whose connection with nets has received less attention in the literature: the broadcast synchronisation pattern of CSP [1] and the asymmetric message passing of asynchronous CCS [8]. These are instances of the main alternatives concerning the communication pattern, namely, asynchronous message reception vs broadcast synchronisation, respectively.…”

Encoding Asynchronous Interactions Using Open Petri Nets

“…We adopt the presentation in [6] that allows non-deterministic choice for input prefixes (a feature missing in [4,7]). …”

“…Indeed, our initial results are quite encouraging. On the longer run, our hope would then be to lift these equivalences to richer calculi, such as the paradigmatic asynchronous π-calculus [6] and to different behavioural equivalences, including, e.g., failure and testing equivalences [7].…”

“…Specifically, we encode an asynchronous variant of CCS [7] into open nets, in such a way that each process reduction corresponds to a transition firing in the net, and vice versa. The key idea is to exploit openness of places in order to account for name restriction.…”