Hierarchical Coloured Petri Nets

This chapter shows how a CPN model can be organised as a set of modules, in a way similar to that in which programs are organised into modules. There are several reasons why modules are needed. Firstly, it is impractical to draw a CPN model of a large system as a single net, since it would become very large and inconvenient. Although the net can be printed on a set of separate sheets and glued together, it would be difficult to get an overview and it would be time-consuming to produce a nice layout. Secondly, the human modeller needs abstractions that make it possible to concentrate on only a few details at a time. CPN modules can be seen as black boxes, where modellers, when they desire, can forget about the details within modules. This makes it possible to work at different abstraction levels, and hence we shall also refer to CPN models with modules as hierarchical CPN models. Thirdly, there are often system components that are used repeatedly. It would be inefficient to model these components several times. Instead, a module can be defined once and used repeatedly. In this way there is only one description to read, and one description to modify when changes are necessary.