Aims

In this chapter, we aim to provide an awareness of the issues involved in constructing and analysing large-scale models. We will introduce modular structuring facilities in VDM-SL as an illustration of the features required in a structuring mechanism. On completion of this chapter, the reader should be able to exploit modular structuring and the potential for re-use in large models.

Introduction

In any course on modelling, one is naturally limited in the size of model which can be developed and presented. However, in any realistic application of modelling technology, questions of scale must be addressed. How can one manage the complexity of developing and analysing a model which contains many related parts?

Before answering this question, it is worth remembering that models should be kept as simple as possible while still capturing the aspects of the system which are felt to be relevant to the analysis. Careful use of abstraction means that many systems can be usefully modelled without encountering problems of scale. However, for some applications, particularly where the product is safety-related, a formally defined language such as VDM-SL must be applied to the production of a substantial model, and so the management of the model's size and complexity becomes a significant issue.

In programming languages, the management of complexity has led to the adoption of modular structuring mechanisms for programs, and this approach has also been applied to VDM-SL. All the models presented so far have been flat in the sense that they have consisted of a series of definitions in a single document.