1. Chapter 6+7 in textbook 1 Chapter 4 Software Requirements 1

2. 2 Please join Tawasol Group number : 92705

3. Objectives 3 To understand the concept of software requirements To understand the difference between functional and non functional requirements Understand the importance of getting it right. Understand how requirements are documented (the SRS document) Understand the requirements engineering process Understand how to discover requirements Understand how to validate requirements Understand how to manage requirements 3

4. Overview 4 What are software requirements? Functional requirements Non functional requirements Domain requirements User and system requirements Interface specification Why is it important to get it right? The SRS document The requirements engineering process. Feasibility study Requirements elicitation Requirements specification Requirements validation Requirements management 4

5. What are Software Requirements 5 “The descriptions of the system services and constraints that are generated during the requirements engineering process.” Developed during the first phase in the software development life cycle.

6. The LIBSYS case study 6 A library system that provides a single interface to a number of databases of articles in different libraries. Users can search for, download and print these articles for personal study.

7. Requirements types 7 1. Functional requirements Statements of services the system should provide, how the system should react to particular inputs and how the system should behave in particular situations. 2. Non-functional requirements constraints on the services or functions offered by the system such as timing constraints, constraints on the development process, standards, etc. 3. Domain requirements Requirements that come from the application domain of the system and that reflect characteristics of that domain.

8. Functional Requirements 8 Describe functionality or system services. Depend on the type of software, expected users and the type of system where the software is used. Functional requirements can be User requirements are high-level statements of what the system should System requirements should describe the system services in detail.

9. Examples 9 1. The user shall be able to search either all of the initial set of databases or select a subset from it. 2. The system shall provide appropriate viewers for the user to read documents in the document store. 3. Every order shall be allocated a unique identifier (ORDER_ID) which the user shall be able to copy to the account’s permanent storage area.

10. Problems 10 Problems arise when requirements are not precisely stated (Ambiguous) Ambiguous requirements may be interpreted in different ways by developers and users. Consider the term ‘appropriate viewers’ User intention - special purpose viewer for each different document type; Developer interpretation - Provide a text viewer that shows the contents of the document.

11. Non-Functional Requirements 11 These define system properties, constraints, and process requirements

12. System Properties 12 Reliability, Response time Maintainability Storage requirements.

13. Constraints 13 I/O device capability, Data representations

14. Process Requirements 14 Mandating a particular CASE system, Programming language or Development method.

15. 15 Which do you think is more critical, A functional or non-functional requirement and why? Have a look at the following video

16. Europeana Project 16

17. Non-functional classifications 17 Product requirements Requirements which specify that the delivered product must behave in a particular way e.g. execution speed, reliability, etc. Organisational requirements Requirements which are a consequence of organisational policies and procedures e.g. process standards used, implementation requirements, etc. External requirements Requirements which arise from factors which are external to the system and its development process e.g. interoperability requirements, legislative requirements, etc.

18. Types of Non-functional requirements 18

19. Examples 19 Product requirement The user interface for LIBSYS shall be implemented as simple HTML without frames or Java applets. Organisational requirement The system development process and deliverable documents shall conform to the process and deliverables defined in XYZCo-SP-STAN-95. External requirement The system shall not disclose any personal information about customers apart from their name and reference number to the operators of the system.

20. Verifiable Non-functional requirements 20 Non-functional requirements may be very difficult to state precisely Imprecise requirements may be difficult to verify. Therefore we need a statement using some measure that can be objectively tested.

21. Example 21 The system shall be easy to use Better expressed as: Experienced users shall be able to use all the system functions after a total of two hours training. After this training, the average number of errors made by experienced users shall not exceed two per day.

22. Requirements measures 22

23. Domain Requirements 23 Derived from the application domain and describe system characteristics and features that reflect the domain. Domain requirements may be new functional requirements, constraints on existing requirements or define specific computations.

24. Library system domain requirements There shall be a standard user interface to all databases which shall be based on the Z39.50 standard (library standard - design constraint ). Because of copyright restrictions, some documents must be deleted immediately on arrival. Depending on the user’s requirements, these documents will either be printed locally on the system server for manually forwarding to the user or routed to a network printer.

25. Train protection system The deceleration of the train shall be computed as: D train = D control + D gradient where D gradient is 9.81ms 2 * compensated gradient/alpha and where the values of 9.81ms 2 /alpha are known for different types of train.

26. Class Activity 26 Working with your team, identify the type of requirements listed on the sheet. When you are done discuss your decisions with the rest of the class. Check each requirement, if you got it right give yourself 1 point. Compute your total. The winning teams will get a 0.5 (out of 2 in-class marks)

27. User Requirements 27 Written for customers Statements in natural language Describe the services the system provides and its operational constraints. May include diagrams or tables Should describe functional and non-functional requirements Should be understandable by system users who don’t have detailed technical knowledge. We provide a definition for a user requirement.

28. System Requirements 28 Statements that set out detailed descriptions of the system’s functions, services and operational constraints. Defines what should be implemented so may be part of a contract between client and contractor. Intended to be a basis for designing the system. Can be illustrated using system models We provide a specification for a system requirement.

29. Guidelines for writing requirements 29 Invent a standard format and use it for all requirements. Use language in a consistent way. Use shall for mandatory requirements, should for desirable requirements. Use text highlighting to identify key parts of the requirement. Avoid the use of computer jargon.

30. Definition and Specifications 30

31. Interface Specification 31 Most systems must operate with other systems and the operating interfaces must be specified as part of the requirements.

32. Why is it important to get it right? 32 If you don’t do it right you will build a very elegant software solution that solves the wrong problem.  the result is project failure (wasted time, and money, personnel frustration, and customer dissatisfaction.

33. 33

34. Representing Requirements The SRS document 34 The Software Requirements Specification (SRS) document is the official statement of what is required of the system developers.

35. The SRS document 35 Should include both a definition of user requirements and a specification of the system requirements. It is NOT a design document. As far as possible, it should set WHAT the system should do rather than HOW it should do it.

36. Users of the SRS 36

37. Structure of the SRS 37 Preface Introduction Glossary User requirements definition System architecture (high level) System requirements specification System models System evolution Appendices Index

38. Requirements Engineering Process 38 “The process of establishing the services that the customer requires from a system and the constraints under which it operates and is developed.” The result is a specification :“representing the requirements in a manner that ultimately leads to successful software implementation.

39. Requirements engineering 39 Involves the following processes Feasibility Study  Feasibility Report Requirements elicitation  System Models Requirements Specification  user and system requirements Requirements validation  Requirements document (SRS) + Requirements Management

40. Requirements Engineering Process 40

41. The Feasibility Study 41 A feasibility study decides whether or not the proposed system is worthwhile. A short focused study that checks If the system contributes to organisational objectives; If the system can be engineered using current technology and within budget; If the system can be integrated with other systems that are used.

42. Requirements Elicitation 42 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. May involve end-users, managers, engineers involved in maintenance, domain experts, trade unions, etc. These are called stakeholders. “refer to any person or group who will be affected by the system, directly or in directly.”

43. Requirements discovery 43 Sources of information Documentation System stakeholders Interviews (close, open) Understand and critique real-life examples. (scenarios, prototypes) Observation (ethnography) Specifications of similar systems

44. Stakeholders for LIBSYS 44 Can you identify possible stakeholders for the LIBSYS? Library manager Article providers Students Staff (library, finance, maintenance etc..)

45. 45

46. Analysis : System models 46 Why? The model aids the analyst in understanding the system, thereby makes the requirements analysis easier and more systematic. The model becomes the focal point of review. The model becomes foundation for design. Produced during requirements analysis. More on modeling in chapter 5.

47. Requirements specification 47 It is the final work product produced by the requirements engineer. The SRS document It serves as a foundation for the software design and implementation.

48. Requirements Validation 48 The process of ensuring that the requirements actually define the system that the customer wants. Why is it important? The cost of fixing a requirements problem by making a system change is much greater than repairing design or coding errors.

49. Validation checks 49 Validity checks ◦ Is this what the user wants? ◦ Does the system provide the functions which best support the customer’s needs? Consistency checks ◦ Requirements should not conflict Completeness checks ◦ Requirements should define all functions and constraints ◦ Are all functions required by the customer included? Realism checks ◦ Ensure they could be implemented ◦ Can the requirements be implemented given available budget and technology Verifiability ◦ Requirements should be written so that they are verifiable, you should be able to write tests for each requirement.

50. Validation techniques 50 Requirements reviews A team of reviewers manually check the requirements. Prototyping An executable model of the system is demonstrated to customers. Test-case generation Requirements should be testable. If tests are difficult or impossible to design, this means that the requirements will be difficult to implement. Developing tests before any code is written is used in ----?

51. Requirements Management 51 Why? Requirements for large software systems are always changing. Because the problem can not be fully specified, the requirements are usually incomplete and bound to change. During the software process the stakeholders understanding of the problem is constantly changing After the system is installed, new requirements will emerge. What? It is the process of understanding and controlling changes to system requirements.

52. Requirements Management Planning 52 Requirements identification Each requirement must be uniquely identified A change management process Assess the impact and cost of change Traceability policies Define the relationship between requirements CASE tool support

53. Class Activity 53 Working with your team, list the most important characteristics of good requirements. When you are done discuss your decisions with the rest of the class. Check each characteristic, if you got it right give yourself 1 point. Compute your total. The winning teams will get a 0.5 (out of 2 in-class marks)

54. Good Requirements Explanation Characteristic Everything the software is supposed to do and responses of the software to all classes of input data are specified in the SRS Complete The requirement does not contradict any other requirement Consistent Every requirement in the SRS represents something required in the final system. Correct The requirement is concisely stated without recourse to technical jargon, acronyms (unless defined elsewhere in the Requirements document). Every requirement has one and only one interpretation. Unambiguous There is a cost effective method that can check whether the final software meets the requirement. Verifiable