The Nature of Synchronisation

In each of the current stochastic process algebras all non-competitive interactions between components or agents are modelled using a single combinator, variously called the parallel, synchronisation or cooperation operator. This paper aims to compare the definitions of this combinator which have been used; in particular, looking at the different ways in which rates are associated with the actions which result from such interactions. The implications of the chosen definitions, from a modelling point of view, will be described. When we consider concrete systems rather than abstract representations many different types of interactions between systems are exhibited. Some of these possible interactions are presented in the latter half of the paper and we analyse the extent to which these can be captured using the combinators available in the SPA languages. To conclude some observations about current modelling practice are made together with suggestions of potential extensions to the set of combinators.