vu-re-lecture-01 software engineering.ppt
- 1. 1 Software Requirements Engineering (CS 708) Dr. Ghulam Ahmad Farrukh
- 2. 2 Introduction • Requirements form the basis for all software products • Requirements engineering is the process, which enables us to systematically determine the requirements for a software product
- 4. 4 Requirement • Something required, something wanted or needed – Webster’s dictionary • There is a huge difference between wanted and needed and it should be kept in mind all the time
- 5. 5 Software Requirements - 1 • A complete description of what the software system will do without describing how it will do it is represented by the software requirements
- 6. 6 Software Requirements - 2 • Software requirements are complete specification of the desired external behavior of the software system to be built • They also represent External behavior of the system
- 7. 7 Software Requirements - 3 • Software requirements may be: – Abstract statements of services and/or constraints – Detailed mathematical functions
- 8. 8 Software Requirements - 4 • Software requirements may be: – Part of the bid of contract – The contract itself – Part of the technical document, which describes a product
- 9. 9 IEEE Definition • A condition or capability that must be met or possessed by a system...to satisfy a contract, standard, specification, or other formally imposed document – IEEE Std 729
- 10. 10 Sources of Requirements • Stakeholders – People affected in some way by the system • Documents • Existing system • Domain/business area
- 11. 11 Levels of Software Requirements • Stakeholders describe requirements at different levels of detail – “What versus How” – “One person’s floor is another person’s ceiling”
- 12. 12 What Versus How User needs Product space Actual product’s behavior Architecture/data flow Module specifications Algorithms Code What How What How What How What How What How What How
- 13. 13 Importance of Software Requirements • The hardest single part of building a software system is deciding what to build...No other part of the work so cripples the resulting system if done wrong. No other part is difficult to rectify later – Fred Brooks
- 14. 14 Examples of Requirements - 1 • The system shall maintain records of all payments made to employees on accounts of salaries, bonuses, travel/daily allowances, medical allowances, etc.
- 15. 15 Examples of Requirements - 2 • The system shall interface with the central computer to send daily sales and inventory data from every retail store
- 16. 16 Examples of Requirements - 3 • The system shall maintain records of all library materials including books, serials, newspapers and magazines, video and audio tapes, reports, collections of transparencies, CD- ROMs, DVDs, etc.
- 17. 17 Examples of Requirements - 4 • The system shall allow users to search for an item by title, author, or by International Standard Book Number • The system’s user interface shall be implemented using a web browser
- 18. 18 Examples of Requirements - 5 • The system shall support at least twenty transactions per second • The system facilities which are available to public users shall be demonstrable in ten minutes or less
- 19. 19 Kinds of Software Requirements
- 20. 20 Kinds of Software Requirements • Functional requirements • Non-functional requirements • Domain requirements • Inverse requirements • Design and implementation constraints
- 22. 22 Functional Requirements - 1 • Statements describing what the system does • Functionality of the system
- 23. 23 Functional Requirements - 2 • Statements of services the system should provide – Reaction to particular inputs – Behavior in particular situations
- 24. 24 Functional Requirements - 3 • Sequencing and parallelism are also captured by functional requirements • Abnormal behavior is also documented as functional requirements in the form of exception handling
- 25. 25 Functional Requirements - 4 • Functional requirements should be complete and consistent • Customers and developers usually focus all their attention on functional requirements
- 26. 26 Functional Requirements Example # 1 • The system shall solve a quadratic equation using the following formula x = (-b+sqrt(b2 – 4*a*c))/2*a
- 27. 27 Functional Requirements Example # 2 • The user shall be able to search either the entire database of patients or select a subset from it (admitted patients, or patients with asthma, etc.)
- 28. 28 Functional Requirements Example # 3 • The system shall provide appropriate viewers for the user to read documents in the document store
- 29. 29 Functional Requirements Example # 4 • Every order shall be allocated a unique identifier (ORDER_ID) which the user shall use to access that order
- 30. 30 Functional Requirements Example # 5 • The system shall allow customers to return non-perishable items within fifteen days of the purchase. A customer must present the original sale receipt to return an item
- 31. 31 Comments on Examples • Notice the level of detail in different requirements described above. Some are very detailed compared to others
- 32. 32 Comments on Examples • Notice the ambiguity in the requirement, which uses the term ‘appropriate viewers’ • This requirement does not mention the formats of documents and types of viewers, which can be used
- 33. 33 Comments on Examples • Notice the ambiguity in the requirement for solving the quadratic equation. The requirement does not speak about the possibility when the value of ‘a’ is zero x = (-b+sqrt(b2 – 4*a*c))/2*a
- 34. 34 Comments on Examples • Incomplete and ambiguous requirements are open to multiple interpretations and assumptions • This can lead to the development of poor quality, or faulty, software products
- 35. 35 Summary • Requirements form the basis of all software engineering projects • Functional requirements capture the behavioral aspects/functions of the proposed automated system • Functional requirements are the backbone of all software products
- 36. 36 References • ‘Requirements Engineering: Processes and Techniques’ by G. Kotonya and I. Sommerville, John Wiley & Sons, 1998 • Software Requirements: Objects, Functions, and States by A. Davis, PH, 1993 • Software Engineering 6th Edition, by I. Sommerville, 2000 • Software Engineering 5th Edition, by R. Pressman