A new paradigm for the design of concurrent systems

A concurrent system is usually understood as a collection of processes that interact through some communication mechanisms. The active components of such a system are the processes; their interaction is described in terms of messages and operations on shared memory. The principal process communication mechanism of Ada, the rendezvous, is a high-level mechanism based on synchronizing messages.In this position paper we argue that appropriate design methods and languages for real-time systems should be based on a notion of interaction that is compatible with the corresponding concepts in the implementation language but provides a higher level of abstraction. It is claimed that existing methods do not satisfy this requirement with respect to Ada. A new paradigm is therefore suggested that reverses the traditional setting as follows. The active components of the system are not the processes but the interactions in which they participate. Such interactions, called joint actions, can be executed whenever they are enabled, i.e. when the required processes are free and willing to participate in these actions.This change of viewpoint has an effect on the kind of entities that are described and refined in the design process. In particular, refinement of joint actions changes the granularity of the system. i.e. the atomicity of events being considered. Another novelty introduced by joint actions is the application of the production language paradigm to the design of concurrent systems. As joint actions are a generalization of the rendezvous, Ada is a suitable implementation language for this approach.