7.1 Introduction

This chapter is concerned with modelling requirements. The diagram that is most frequently used for modelling requirements in the Unified Modelling Language (UML) is the use case diagram, although, technically speaking, any diagram may be used. In this section of the book, we will be concentrating on modelling requirements using use case diagrams. See References 1 to 3 for more discussion on use case models.

Use case diagrams, it may be argued, are perhaps one of the simplest of all diagrams, at least on the surface. However, they are perhaps the most misunderstood of all the UML diagrams. This is due in part to their inherent simplicity. Because the diagrams look so simple people often assume that very little effort is involved in generating the diagrams. This could not be further from the truth!

Requirements engineering is the discipline of engineering that is concerned with capturing, analysing and modelling requirements. In this book we are looking purely at modelling requirements with some degree of analysis. How these requirements are arrived at in the first place is entirely up to the engineer. Indeed, many different techniques for requirements capture will be mentioned, but will not be covered in any detail, as this is beyond the scope the book.

Before the UML can be related to requirements, it is important to understand some of the basic concepts behind requirements engineering. These concepts will be introduced at a...