Modelling in the contexts of verification and design/11. Structure and Decomposition

The most relevant approach to modelling is decomposition. A prerequesite for decomposition is that we have identified the structure of the system, i.e. its components and their interaction.

However, we argue that there is no unique ``structure of the system. There are many different structures, and each structure depends on the view we take on a system. The plumber has a different view on a system than the software engineer or the power engineer. What views can be taken is for the greatest part education, and to some extent also creativity.

By taking a view on the system we impose structure on the system. Subsequently, we can decompose according to the stucture. There are views that allow for more suitable, or compact, or intuitive decompositions.

Taking a certain view on a system abstracts at the same time from all other views. When we draw a electric circuit, we abstract from the colour of the resistors.

When we model a system we need different views. Some causalities and restrictions can be identified within one view and decomposition, sometimes we need different views. An example for the first is a recipe-based decomposition. There we extract the information what are the basic (processing-)steps to take, what is their order and what timing requirements are there. An example for the latter is a recipe-based decomposition in combination with an instrumental decomposition: when we see that two basic (processing-)steps need the same instruments, we can derive a mutual exclusion requirement for these two basic (processing-)steps.

Below is a list of possible decompositions. As we have not defined the concepts mentioned there, their interpretation remains a bit vague. However, most readers have an own interpretation of these concepts. When elaborating a modelling method we need more precise definitions, for the moment the list is sufficient to illustrate the differences possible. Of course, there are much more views possible. And it also happens that different views are mixed within one view and decomposition.

• functional
• service-based
• process-based
• recipe-based
• workpiece-based
• instrumental