1. ©Ian Sommerville 2004Software Engineering, 7th edition. Chapter 8 Slide 1 System modeling 1

2. ©Ian Sommerville 2004Software Engineering, 7th edition. Chapter 8 Slide 2 Objectives l To explain why the context of a system should be modelled as part of the RE process l To describe behavioural modelling, data modelling and object modelling l To introduce some of the notations used in the Unified Modeling Language (UML) l To show how CASE workbenches support system modelling

3. ©Ian Sommerville 2004Software Engineering, 7th edition. Chapter 8 Slide 3 System modelling l System modelling helps the analyst to understand the functionality of the system and models are used to communicate with customers. l Different models present the system from different perspectives External perspective showing the system’s context or environment; Behavioural perspective showing the behaviour of the system; Structural perspective showing the system or data architecture.

4. ©Ian Sommerville 2004Software Engineering, 7th edition. Chapter 8 Slide 4 Model types l Data processing model showing how the data is processed at different stages. l Composition model showing how entities are composed of other entities. l Architectural model showing principal sub-systems. l Classification model showing how entities have common characteristics. l Stimulus/response model showing the system’s reaction to events.

5. ©Ian Sommerville 2004Software Engineering, 7th edition. Chapter 8 Slide 5 The Unified Modeling Language (UML) l The UML has become a standard diagrammatic notation for developing models of object-oriented systems. l The UML is a unification of a range of different approaches to OO modelling that were developed in the 1990s. l Its latest version (UML 2) also includes more general modelling constructs. l UML 1 was the most widely used when this chapter was written. I have therefore used this notation here. Consequently, some diagrams such as data-flow diagrams are not expressed in the UML.

6. ©Ian Sommerville 2004Software Engineering, 7th edition. Chapter 8 Slide 6 UML 2 model types l UML 2 has 13 different types of diagram that are used to model different aspects of a system Behavioural diagrams are used to model behavioural features of a system. For example, use-case diagrams, activity diagrams, state machine diagrams Interaction diagrams are used to model interactions between entities in the system. For example, sequence diagrams and communication diagrams. Structure diagrams are used to model the organization of the system. For example, class diagrams, package diagrams, and deployment diagrams.

7. ©Ian Sommerville 2004Software Engineering, 7th edition. Chapter 8 Slide 7 Context models l Context models are used to illustrate the operational context of a system - they show what lies outside the system boundaries. l Social and organisational concerns may affect the decision on where to position system boundaries. l Architectural models show the system and its relationship with other systems.

8. ©Ian Sommerville 2004Software Engineering, 7th edition. Chapter 8 Slide 8 The context of an ATM system

9. ©Ian Sommerville 2004Software Engineering, 7th edition. Chapter 8 Slide 9 Process models l Process models show the overall process and the processes that are supported by the system. l Data flow models may be used to show the processes and the flow of information from one process to another. l These types of diagram are sometimes known as workflow diagrams.

10. ©Ian Sommerville 2004Software Engineering, 7th edition. Chapter 8 Slide 10 Equipment procurement process

11. ©Ian Sommerville 2004Software Engineering, 7th edition. Chapter 8 Slide 11 Behavioural models l Behavioural models are used to describe the overall behaviour of a system. l Two types of behavioural model are: Data processing models that show how data is processed as it moves through the system; State machine models that show the systems response to events. l These models show different perspectives so both of them are required to describe the system’s behaviour.

12. ©Ian Sommerville 2004Software Engineering, 7th edition. Chapter 8 Slide 12 Data-processing models l Data flow diagrams (DFDs) may be used to model the system’s data processing. l These show the processing steps as data flows through a system. l DFDs are an intrinsic part of many analysis methods. l Simple and intuitive notation that customers can understand. l Show end-to-end processing of data.

13. ©Ian Sommerville 2004Software Engineering, 7th edition. Chapter 8 Slide 13 Order processing DFD

14. ©Ian Sommerville 2004Software Engineering, 7th edition. Chapter 8 Slide 14 Data flow diagrams l DFDs model the system from a functional perspective. l Tracking and documenting how the data associated with a process is helpful to develop an overall understanding of the system. l Data flow diagrams may also be used in showing the data exchange between a system and other systems in its environment.

15. ©Ian Sommerville 2004Software Engineering, 7th edition. Chapter 8 Slide 15 Insulin pump DFD

16. ©Ian Sommerville 2004Software Engineering, 7th edition. Chapter 8 Slide 16 UML diagrams l To model workflow or data-flow, you can use UML 2 activity diagrams. l These provide a richer notation that that used here so translation is simple l UML state machine diagrams may be used to model how events are processed by a system

17. ©Ian Sommerville 2004Software Engineering, 7th edition. Chapter 8 Slide 17 State machine models l These model the behaviour of the system in response to external and internal events. l They show the system’s responses to stimuli so are often used for modelling real-time systems. l State machine models show system states as nodes and events as arcs between these nodes. When an event occurs, the system moves from one state to another. l Statecharts are an integral part of the UML and are used to represent state machine models.

18. ©Ian Sommerville 2004Software Engineering, 7th edition. Chapter 8 Slide 18 Statecharts l Allow the decomposition of a model into sub- models (see following slide). l A brief description of the actions is included following the ‘do’ in each state. l Can be complemented by tables describing the states and the stimuli.

19. ©Ian Sommerville 2004Software Engineering, 7th edition. Chapter 8 Slide 19 Microwave oven model

20. ©Ian Sommerville 2004Software Engineering, 7th edition. Chapter 8 Slide 20 Microwave oven state description

21. ©Ian Sommerville 2004Software Engineering, 7th edition. Chapter 8 Slide 21 Microwave oven stimuli

22. ©Ian Sommerville 2004Software Engineering, 7th edition. Chapter 8 Slide 22 Microwave oven operation

23. ©Ian Sommerville 2004Software Engineering, 7th edition. Chapter 8 Slide 23 Key points l A model is an abstract system view. Complementary types of model provide different system information. l Context models show the position of a system in its environment with other systems and processes. l Data flow models may be used to model the data processing in a system. l State machine models model the system’s behaviour in response to internal or external events