1. ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 6 Slide 1 Requirements Engineering Processes l Processes used to discover, analyze and validate system requirements

2. ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 6 Slide 2 Requirements engineering processes l The processes used for RE vary widely l There are a number of generic activities common to all processes Requirements elicitation Requirements analysis Requirements validation Requirements management

3. ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 6 Slide 3 Elicitation and analysis l Involves technical staff working with customers to find out about: The application domain, The services that the system should provide and The system’s operational constraints l May involve end-users, managers, engineers involved in maintenance, domain experts, trade unions, etc. These are called stakeholders Hospital Example

4. ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 6 Slide 4 Problems of requirements analysis l Stakeholders Don’t know what they really want Express requirements in their own terms Different stakeholders may have conflicting requirements l Organizational and political factors influence requirements l Requirements change during the analysis process. l New stakeholders may emerge

5. ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 6 Slide 5 Viewpoint-oriented elicitation l Stakeholders represent different ways of looking at a problem or problem viewpoints l This multi-perspective analysis is important as there is no single correct way to analyse system requirements

6. ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 6 Slide 6 Banking ATM system (simplified) l Offers services to customers of the bank, owns the system, and other customers l Services include cash withdrawal, message passing (send a message to request a service), ordering a statement and transferring funds

7. ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 6 Slide 7 ATM viewpoints l Bank customers l Representatives of other banks l Hardware and software maintenance engineers l Marketing department l Bank managers and counter staff l Database administrators and security staff l Communications engineers l Personnel department

8. ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 6 Slide 8 Types of viewpoint l Data sources or sinks Viewpoints are responsible for producing or consuming data. l Representation frameworks Viewpoints represent particular types of system model such as real-time systems. l Receivers of services Viewpoints are external to the system and receive services from it.

9. ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 6 Slide 9 Method-based analysis l Widely used approach to requirements analysis. l Depends on the application of a structured method to understand the system l A viewpoint-oriented method (VORD) is used as an example here. l It also illustrates the use of viewpoints

10. ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 6 Slide 10 The VORD method

11. ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 6 Slide 11 VORD process model l Viewpoint identification Discover viewpoints which receive system services and identify the services provided to each viewpoint l Viewpoint structuring Group related viewpoints into a hierarchy. Common services are provided at higher-levels in the hierarchy l Viewpoint documentation Refine the description of the identified viewpoints and services l Viewpoint-system mapping Transform the analysis to an object-oriented design

12. ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 6 Slide 12 VORD standard forms

13. ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 6 Slide 13 Viewpoint identification

14. ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 6 Slide 14 Viewpoint service information

15. ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 6 Slide 15 Viewpoint data/control

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

17. ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 6 Slide 17 Customer/cash withdrawal templates

18. ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 6 Slide 18 Scenarios l Scenarios are descriptions of how a system is used in practice l They are helpful in requirements elicitation as people can relate to these more readily than abstract statement of what they require from a system l Scenarios are particularly useful for adding detail to an outline requirements description

19. ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 6 Slide 19 Scenario descriptions l System state at the beginning of the scenario l Normal flow of events in the scenario l What can go wrong and how this is handled l Other concurrent activities l System state on completion of the scenario

20. ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 6 Slide 20 Event scenarios l Event scenarios may be used to describe how a system responds to the occurrence of some particular event such as ‘start transaction’ l VORD includes a diagrammatic convention for event scenarios. Data provided and delivered Control information Exception processing The next expected event

21. ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 6 Slide 21 Event scenario - start transaction

22. ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 6 Slide 22 Notation for data and control analysis l Ellipses. data provided from or delivered to a viewpoint l Control information enters and leaves at the top of each box l Data leaves from the right of each box l Exceptions are shown at the bottom of each box l Name of next event is in box with thick edges

23. ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 6 Slide 23 Exception description l Most methods do not include facilities for describing exceptions l In this example, exceptions are Timeout. Customer fails to enter a PIN within the allowed time limit Invalid card. The card is not recognised and is returned Stolen card. The card has been registered as stolen and is retained by the machine

24. ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 6 Slide 24 Use cases l Use-cases are a scenario based technique in the UML which identify the actors in an interaction and which describe the interaction itself l A set of use cases should describe all possible interactions with the system l Sequence diagrams may be used to add detail to use-cases by showing the sequence of event processing in the system

25. ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 6 Slide 25 Lending use-case

26. ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 6 Slide 26 Library use-cases

27. ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 6 Slide 27 Catalogue management

28. ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 6 Slide 28 Social and organizational factors l Software systems are used in a social and organizational context. l This can influence or even dominate system requirements l Social and organizational factors are not a single viewpoint but influence all viewpoints l Good analysts must be sensitive to these factors but currently no systematic way to tackle their analysis

29. ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 6 Slide 29 Example l Consider a system which allows senior management to access information without going through middle managers Managerial status. Senior managers may feel that they are too important to use a keyboard. This may limit the type of system interface used Managerial responsibilities. Managers may have no uninterrupted time where they can learn to use the system Organisational resistance. Middle managers who will be made redundant may deliberately provide misleading or incomplete information so that the system will fail

30. ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 6 Slide 30 Requirements validation l Demonstrating that the requirements define the system that the customer really wants l High requirements error costs make validation important l Cost of fixing a requirement error: Phase Cost requirements 2 design 5 coding 15 testing 50 operation 150

31. ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 6 Slide 31 Requirements checking l Validity. Does the system provide the functions which best support the customer’s needs? l Consistency. Are there any requirements conflicts? l Completeness. Are all functions required by the customer included? l Realism. Can the requirements be implemented given available budget and technology l Verifiability. Can the requirements be checked?

32. ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 6 Slide 32 Requirements validation techniques l Requirements reviews Systematic manual analysis of the requirements l Prototyping Using an executable model of the system to check requirements. Covered in Chapter 8 l Test-case generation Developing tests for requirements to check testability l Automated consistency analysis Checking the consistency of a structured requirements description

33. ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 6 Slide 33 Requirements reviews l Regular reviews should be held while the requirements definition is being formulated l Both client and contractor staff should be involved in reviews l Reviews may be formal (with completed documents) or informal. Good communications between developers, customers and users can resolve problems at an early stage

34. ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 6 Slide 34 Review checks l Verifiability. Is the requirement realistically testable? l Comprehensibility. Is the requirement properly understood? l Traceability. Is the origin of the requirement clearly stated? l Adaptability. Can the requirement be changed without a large impact on other requirements?

35. ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 6 Slide 35 Requirements management l Requirements management is the process of managing changing requirements during the requirements engineering process and system development l Requirements are inevitably incomplete and inconsistent New requirements emerge during the process as business needs change and a better understanding of the system is developed Different viewpoints have different requirements and these are often contradictory

36. ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 6 Slide 36 Requirements change l The priority of requirements from different viewpoints changes during the development process l System customers may specify requirements from a business perspective that conflict with end-user requirements l The business and technical environment of the system changes during its development

37. ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 6 Slide 37 Requirements evolution

38. ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 6 Slide 38 Enduring and volatile requirements l Enduring requirements. Stable requirements derived from the core activity of the customer organisation. E.g. a hospital will always have doctors, nurses, etc. May be derived from domain models l Volatile requirements. Requirements which change during development or when the system is in use. In a hospital, requirements derived from health-care policy

39. ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 6 Slide 39 Traceability l Traceability is concerned with the relationships between requirements, their sources and the system design l Source traceability Links from requirements to stakeholders who proposed these requirements l Requirements traceability Links between dependent requirements l Design traceability Links from the requirements to the design

40. ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 6 Slide 40 A traceability matrix

41. ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 6 Slide 41 SE tool support l Requirements storage Requirements should be managed in a secure, managed data store l Change management The process of change management is a workflow process whose stages can be defined and information flow between these stages partially automated l Traceability management Automated retrieval of the links between requirements

42. ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 6 Slide 42 Requirements change management l Should apply to all proposed changes to the requirements l Principal stages Problem analysis. Discuss requirements problem and propose change Change analysis and costing. Assess effects of change on other requirements Change implementation. Modify requirements document and other documents to reflect change