1. ©Ian Sommerville 2000Software Engineering, 6/e, Chapter 71 System models l Abstract descriptions of systems whose requirements are being analysed

2. ©Ian Sommerville 2000Software Engineering, 6/e, Chapter 72 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 2000Software Engineering, 6/e, Chapter 73 Topics covered l Context models l Behavioural models l Data models l Object models l CASE workbenches

4. ©Ian Sommerville 2000Software Engineering, 6/e, Chapter 74 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 2000Software Engineering, 6/e, Chapter 75 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 2000Software Engineering, 6/e, Chapter 76 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 2000Software Engineering, 6/e, Chapter 77 Model types l Data processing model showing how the data is processed at different stages l Composition model (a.k.a. entity-relationship-attribute 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 2000Software Engineering, 6/e, Chapter 78 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 2000Software Engineering, 6/e, Chapter 79 The context of an ATM system

10. ©Ian Sommerville 2000Software Engineering, 6/e, Chapter 710 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

11. ©Ian Sommerville 2000Software Engineering, 6/e, Chapter 711 Equipment procurement process

12. ©Ian Sommerville 2000Software Engineering, 6/e, Chapter 712 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 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 Both of these models are required for a description of the system’s behaviour

13. ©Ian Sommerville 2000Software Engineering, 6/e, Chapter 713 Data-processing models l Data flow diagrams (DFDs) 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

14. ©Ian Sommerville 2000Software Engineering, 6/e, Chapter 714 Order processing DFD

15. ©Ian Sommerville 2000Software Engineering, 6/e, Chapter 715 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

16. ©Ian Sommerville 2000Software Engineering, 6/e, Chapter 716 CASE toolset DFD

17. ©Ian Sommerville 2000Software Engineering, 6/e, Chapter 717 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

18. ©Ian Sommerville 2000Software Engineering, 6/e, Chapter 718 Microwave oven model

19. ©Ian Sommerville 2000Software Engineering, 6/e, Chapter 719 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

20. ©Ian Sommerville 2000Software Engineering, 6/e, Chapter 720 Microwave oven state description

21. ©Ian Sommerville 2000Software Engineering, 6/e, Chapter 721 Microwave oven stimuli

22. ©Ian Sommerville 2000Software Engineering, 6/e, Chapter 722 Statecharts: Complexity l Rapid increase of possible states renders charts too complex l For larger systems, should decompose entire system state into hierarchy of “superstates” and detailed, local statecharts

23. ©Ian Sommerville 2000Software Engineering, 6/e, Chapter 723 Microwave oven operation

24. ©Ian Sommerville 2000Software Engineering, 6/e, Chapter 724 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

25. ©Ian Sommerville 2000Software Engineering, 6/e, Chapter 725 Software design semantic model

26. ©Ian Sommerville 2000Software Engineering, 6/e, Chapter 726 Data dictionaries l Data dictionaries are lists of all of the names used in the system models. Descriptions of the entities, relationships and attributes are also included l Advantages Support name management and avoid duplication Store of organisational knowledge linking analysis, design and implementation l Many CASE workbenches support data dictionaries

27. ©Ian Sommerville 2000Software Engineering, 6/e, Chapter 727 Data dictionary entries

28. ©Ian Sommerville 2000Software Engineering, 6/e, Chapter 728 Object models l Object-oriented data 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

29. ©Ian Sommerville 2000Software Engineering, 6/e, Chapter 729 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 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

30. ©Ian Sommerville 2000Software Engineering, 6/e, Chapter 730 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. ©Ian Sommerville 2000Software Engineering, 6/e, Chapter 731 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 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

32. Library class hierarchy

33. User class hierarchy

34. ©Ian Sommerville 2000Software Engineering, 6/e, Chapter 734 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

35. ©Ian Sommerville 2000Software Engineering, 6/e, Chapter 735 Multiple inheritance

36. ©Ian Sommerville 2000Software Engineering, 6/e, Chapter 736 Object aggregation l Aggregation model shows how classes which are collections are composed of other classes l Similar to the part-of relationship in semantic data models

37. ©Ian Sommerville 2000Software Engineering, 6/e, Chapter 737 Object aggregation

38. ©Ian Sommerville 2000Software Engineering, 6/e, Chapter 738 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

39. ©Ian Sommerville 2000Software Engineering, 6/e, Chapter 739 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