1. ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 7 Slide 1 System models l Abstract descriptions of systems whose requirements are being analysed

2. ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 7 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 2000 Software Engineering, 6th edition. Chapter 7 Slide 3 Topics covered l 7.1 Context models l 7.2 Behavioural models l 7.3 Data models l 7.4 Object models l 7.5 CASE workbenches

4. ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 7 Slide 4 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

5. ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 7 Slide 5 Structured methods l Structured methods incorporate system modelling as an inherent part of the method l Methods define a set of models, a process for deriving these models and rules and guidelines that should apply to the models l CASE tools support system modelling as part of a structured method

6. ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 7 Slide 6 Method weaknesses l They do not model non-functional system requirements l They do not usually include information about whether a method is appropriate for a given problem l The may produce too much documentation l The system models are sometimes too detailed and difficult for users to understand

7. ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 7 Slide 7 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

8. ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 7 Slide 8 Context models l Context models are used to illustrate the boundaries of a system l Social and organisational concerns may affect the decision on where to position system boundaries l Architectural models show the a system and its relationship with other systems

9. ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 7 Slide 9 The context of an ATM system

10. ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 7 Slide 10 Process models l Process models show the overall process and the processes that are supported by the system

11. ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 7 Slide 11 Equipment procurement process

12. ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 7 Slide 12 Process / Behavioural models l Data flow models may be used to show the processes and the flow of information from one process to another

13. ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 7 Slide 13 Behavioural models l Behavioural models are used to describe the overall behaviour of a system l Two types of behavioural model are shown here Data flow models show how data is processed as it moves through the system State machine models that show the systems response to events l Both of these models may be required for a full description of the system’s behaviour

14. ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 7 Slide 14 Data-flow models l Data flow diagrams are used to model the system’s data processing l These show the processing steps as data flows through a system l Intrinsic part of many analysis methods l Simple and intuitive notation that customers can understand l Show end-to-end processing of data

15. ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 7 Slide 15 Order processing DFD

16. ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 7 Slide 16 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

17. ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 7 Slide 17 CASE toolset DFD

18. ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 7 Slide 18 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

19. ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 7 Slide 19 Microwave oven model

20. ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 7 Slide 20 Microwave oven state description

21. ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 7 Slide 21 Microwave oven stimuli

22. ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 7 Slide 22 Microwave oven operation

23. ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 7 Slide 23 Semantic data models l Used to describe the logical structure of data processed by the system l Entity-relation-attribute model sets out the entities in the system, the relationships between these entities and the entity attributes l Widely used in database design. Can readily be implemented using relational databases l No specific notation provided in the UML but objects and associations can be used

24. ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 7 Slide 24 Software design semantic model

25. ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 7 Slide 25 Data dictionaries l Data dictionaries are lists of all of the names used in the system models. Processes, entities, relationships, attributes etc … Descriptions are also included l Advantages Support name management and avoid duplication Store of organisational knowledge linking analysis, design, implementation, and maintenance l Many CASE workbenches support data dictionaries

26. ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 7 Slide 26 Data dictionary entries

27. ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 7 Slide 27 Object models l Object models describe the system in terms of object classes l An object class is an abstraction over a set of objects with common attributes and the services (operations) provided by each object l Various object models may be produced Inheritance models Aggregation models Interaction models

28. ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 7 Slide 28 Object models l Natural ways of reflecting the real-world entities manipulated by the system l More abstract entities are more difficult to model using this approach l Modeling involves identifying object classes, their behavior, and their relationships l Object class identification is recognised as a difficult process requiring a deep understanding of the application domain l Object classes reflecting domain entities are reusable across systems

29. ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 7 Slide 29 The Unified Modeling Language l Devised by the developers of widely used object- oriented analysis and design methods l Has become an effective standard for object-oriented modelling l Notation – for Class Diagrams Object classes are rectangles with the name at the top, attributes in the middle section and operations in the bottom section Relationships between object classes (known as associations) are shown as lines linking objects Inheritance is referred to as generalisation and is shown ‘upwards’ rather than ‘downwards’ in a hierarchy

30. ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 7 Slide 30 Inheritance models l Organise the domain object classes into a hierarchy l Classes at the top of the hierarchy reflect the common features of all classes l Object classes inherit their attributes and services from one or more super-classes. These may then be specialised as necessary l Class hierarchy design is a difficult process if duplication in different branches is to be avoided

31. Library class hierarchy

32. User class hierarchy

33. ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 7 Slide 33 Multiple inheritance l Rather than inheriting the attributes and services from a single parent class, a system which supports multiple inheritance allows object classes to inherit from several super-classes l Can lead to semantic conflicts where attributes/services with the same name in different super-classes have different semantics l Makes class hierarchy reorganisation more complex

34. ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 7 Slide 34 Multiple inheritance

35. ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 7 Slide 35 Object aggregation l Aggregation model shows how classes which are collections are composed of other classes

36. ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 7 Slide 36 Object aggregation

37. ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 7 Slide 37 Object behaviour modelling l A behavioural model shows the interactions between objects to produce some particular system behaviour that is specified as a use-case l Sequence diagrams (or collaboration diagrams) in the UML are used to model interaction between objects

38. ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 7 Slide 38 Issue of electronic items

39. ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 7 Slide 39 CASE workbenches l A coherent set of tools that is designed to support related software process activities such as analysis, design or testing l Analysis and design workbenches support system modelling during both requirements engineering and system design l These workbenches may support a specific design method or may provide support for a creating several different types of system model

40. ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 7 Slide 40 An analysis and design workbench

41. ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 7 Slide 41 Analysis workbench components l Diagram editors l Model analysis and checking tools l Repository and associated query language l Data dictionary l Report definition and generation tools l Forms definition tools l Import/export translators l Code generation tools

42. ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 7 Slide 42 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

43. ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 7 Slide 43 Key points l Semantic data models describe the logical structure of data which is imported to or exported by the systems l Object models describe logical system entities, their classification and aggregation l Object models describe the logical system entities and their classification and aggregation l CASE workbenches support the development of system models