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CS3773 Software Engineering

CS3773 Software Engineering. Lecture 02 Requirements Engineering. Requirements Engineering. Requirements engineering is usually the first stage of software life cycle Requirements engineering is the process of understanding and defining functionalities and constraints of proposed systems

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CS3773 Software Engineering

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  1. CS3773Software Engineering Lecture 02 Requirements Engineering

  2. Requirements Engineering • Requirements engineering is usually the first stage of software life cycle • Requirements engineering is the process of understanding and defining functionalities and constraints of proposed systems • Requirements engineering process produces a document, software requirements specification (SRS) • Customers need a high level specification • Software designers and developers need a more detailed specification UTSA CS3773

  3. Software Requirements • Requirements are desired behaviors • Customers “know” what the system shall do • Software engineers “know” what to built • “Requirements are means of communication with customer and many other stakeholders” -- by Helene Wong, PhD thesis, 1994 • Requirements deal with • Objects • States • Functions UTSA CS3773

  4. Software Requirements Stakeholders • Requirements analysts or system analysts determine requirements • Stakeholders contribute to requirements of systems • Clients • Customers • End-users • Software engineers • Domain experts • Lawyers or auditors • Market researchers UTSA CS3773

  5. Types of Requirements • Functional • What is the system supposed to do • Mapping from input to output • Non-functional (quality) • Usability • Performance • Security • Reliability • Maintainability • Portability UTSA CS3773

  6. Types of Requirements • Process constraints • Resources • Documentation • Standards • Design constraints • Physical environment • Interface • Users UTSA CS3773

  7. Requirements Are Important The hardest single part of building a software system is deciding precisely what to build. No other part of the conceptual work is as difficult as establishing the detailed technical requirements, including all interfaces to people, to machines, and to other software systems. No other part of the work so cripples the resulting system if done wrong. No other part is more difficult to rectify later. -- by Frederick Brooks, “No silver bullet: essence and accidents of software engineering”, 1986. UTSA CS3773

  8. Requirements Are Important • 80% of all software errors are requirements errors • These are software errors detected after unit testing – i.e., in integration testing, in system testing, and after the software is released • Most errors can be traced to unknown, wrong, or misunderstood requirements UTSA CS3773

  9. Requirements Are Important • Requirements usually affect large portions of the implementation; they are rarely encapsulated into modules • Requirements errors may be fundamental assumptions built into the design or code • Expensive requirements errors are often not fixed; they become “features” UTSA CS3773

  10. Requirements Are Important • Requirements errors are expensive to fix UTSA CS3773

  11. Requirements Problems • Over-specification • Under-specification (unintended) • Contradictory requirements • Ambiguous requirements • Unknown requirements • Bad assumptions about environment • Changing requirements UTSA CS3773

  12. Characteristics of Requirements • Correct • Consistent • Complete • Concise • Traceable • Unambiguous • Understandable • Verifiable UTSA CS3773

  13. Requirements Engineering Process • Determine the requirements of a system, and specify what behavior is realized • Work with customers to elicit the requirements • Analyze and model the requirement • Document the requirements in a software requirements specification • Validate the requirements specification UTSA CS3773

  14. Requirements Tasks • Understand problem from each stakeholder's point of view • Extract the essence of the stakeholders' requirements • Negotiate a consistent set of requirements with agreement from all stakeholders; set relative priorities • Record results in an SRS UTSA CS3773

  15. Requirements Elicitation • Elicitation is to gather • Functions that the system should perform • Non-functional requirements that the system should exhibit • Elicitation is critical but difficult • Customers are not good at describing what they want • Software engineers are not good at understanding what customers want • Customers and software engineers speak different languages UTSA CS3773

  16. Requirements Elicitation • Requirements analysts have to understand the system from each stakeholder's point of view • Stakeholders have different views of the system • Requirements analysts resolve conflicting views • Requirements analysts prioritize requirements • Essential requirements • Desirable requirements • Optional requirements UTSA CS3773

  17. Elicitation Techniques Understand problems • For existing system • Review documentation • Observe current system • Questionnaires and Interviews • Apprenticeship • For new systems - brainstorming UTSA CS3773

  18. Analyze Existing System • What is used, what isn't, what's missing • What works well, what doesn't • How the system is used, how it was intended to be used, what new ways we want it to be used • Risks • Users might not be happy with too much change from the old system • Might miss real usage patterns • Might miss obvious possible improvements UTSA CS3773

  19. Analyze Existing System - Review • Review all available documentation • For an automated system, review its requirements specifications and user manuals, as well as development documentation, internal memos, change histories, etc. • For a manual system, review any documented procedures that the workers must follow • Gain knowledge of the system before imposing upon other people's time, before bothering the stakeholders UTSA CS3773

  20. Analyze Existing System - Observation • Identify what aspects to keep and to understand the system you are about to change • System contains a lot of useful functionality that should be included in any future system • Documentation rarely describes a system completely and not up to date and • Current operation of the system may differ significantly from what is described UTSA CS3773

  21. Analyze Existing System - Interview • Questionnaires are useful when information has to be gathered from a large number of people • The answers to questions need to be compared or corroborated. • Ask problem-oriented questions during interview • Interview groups of people together to get synergy UTSA CS3773

  22. Analyze Existing System - Apprentice • The requirements analyst is the apprentice and the user is the master craftsman. • The user can • Describe the task precisely • Explain why the task is done this way • List the exceptions that can occur UTSA CS3773

  23. Brainstorm • Brainstorm is used to gather ideas from every stakeholder and prune ideas • When you have no idea, or too many ideas, sit down and thrash it out, but with some ground rules • Most useful early on, when terrain is uncertain, or when you have little experience, or when novelty is important UTSA CS3773

  24. Brainstorm • Keep the tone informal and non-judgmental • Encourage creativity • Keep the number of participants “reasonable”, if too many, consider a “playoff”-type filtering • Invite back most creative to multiple sessions UTSA CS3773

  25. Brainstorm - the Storm • Generate as many ideas as possible • Quantity, not quality, is goal at this stage • No criticism or debate is permitted • Write down all ideas where all can see • Participants should NOT self-censor or spend too much time wondering if an idea is practical • Original list does not get circulated outside of the meeting UTSA CS3773

  26. Brainstorm – the Calm • Go over the list and explain ideas more carefully • Categorize into “maybe” and “no” by pre-agreed consensus method • Be careful about time Meetings tend to lose focus after 90 to 120 minutes • Review, consolidate, combine, clarify, and expand • Rank the list by priority somehow; choose a winner UTSA CS3773

  27. Brainstorm – Pruning • Vote with threshold • Each person votes up to n times • Keep those ideas with more than m votes • Have multiple rounds thereof with smaller n and m • Vote with campaign speeches • Each person votes up to j < n times • Keep those ideas with at least one vote • Have multiple rounds thereof with smaller j UTSA CS3773

  28. Requirements Analysis • Understand the desired behavior • Interpret the stakeholders' descriptions of requirements • Resolve ambiguities, contradictions, loose ends, etc. • Build models • Use standard notations • Help us to understand the requirements UTSA CS3773

  29. Requirements: What vs. How • Requirements describe purpose and scope of the system • What behavior the customer wants • Not how the behavior is realized • Requirements focus on customer and problems • Understand the customer’s needs • Describe the background and overview of the problem • Requirements represent objects, states, and functions • Requirements include assumptions of the environment UTSA CS3773

  30. Requirements Specification • Specify requirements • Document what is required of the system to be developed • State the requirements from the perspective of the developers • May be a formal document (IEEE-SRS) • Requirements document and specification document are different • Requirements document is a contract • Specification is a detailed guideline for developers UTSA CS3773

  31. Requirements vs. Specification • Requirements document is • A complete list on what customers want • In terms of environment without reference to system • A contract between clients and developers • Specification represents • System’s behavior in terms of the input and output of a system • Which requirements shall be realized by the system • How environment entities are controlled by the system UTSA CS3773

  32. Requirements vs. Specification Specification Data structures and algorithms Requirements Interface Environment System UTSA CS3773

  33. Requirements vs. Specification • Requirements are a collection of statements about phenomena in the environment that we want the system to help make true • A specification is a collection of statements that describe a system’s external behavior as observable through the Interface • A specification refers only to shared phenomena in the interface and what the system shall do • A specification can constrain only shared phenomena that the system itself can control UTSA CS3773

  34. Requirements vs. Specification • Example: a turnstile to the park • Requirements • No one should enter the park without paying an entrance fee • For every entrance fee paid, the system should not prevent a corresponding entry • Specification When a visitor applies a certain amount of force on an unlocked turnstile, the turnstile will rotate till a locked position UTSA CS3773

  35. Requirements Validation • Validate the requirements against stakeholders • Reflect accurately customer’s need • Also create system-level test plans • Validation can be done with techniques • Walkthrough • Review • Prototype • Formal inspection UTSA CS3773

  36. Specification Verification • Verify the specification against requirements • Conforms to the requirement definition • Build the system right • Verification can be done with techniques • Simulation • Consistency checking • Completeness checking • Formal verification: model checking or mathematical reasoning UTSA CS3773

  37. Software Requirements Specifications • Introduction • Overall description • Specific requirements • Requirements table UTSA CS3773

  38. Software Requirements Specification Section 0 • Table of Contents Essential for tracing through use cases, classes, state diagrams • Table of Figures Essential for finding each diagram • List of Tables Essential for finding each table UTSA CS3773

  39. Software Requirements Specification Section 1 Introduction 1.1 Purpose of the SRS e.g., the intended audience 1.2 Scope 1.3 Acronyms, abbreviations, notational conventions 1.4 Overview e.g., the structure of the rest of the SRS document 1.5 References Can be put at the end of the document UTSA CS3773

  40. Software Requirements Specification Section 2 General description 2.1 Product perspective – the environment Any hardware and software components that interact with the system Overview of the interfaces to other component A block diagram would be nice UTSA CS3773

  41. Software Requirements Specification Section 2 General description 2.2 Product functions Overview of the system’s main functions No detail description At the level of use case names 2.3 User characteristics Assumptions about the user UTSA CS3773

  42. Software Requirements Specification Section 2 General description 2.4 General constraints e.g., laws, hardware limitations Any sources of constraints on requirements or design 2.5 Assumptions and Dependencies Assumptions about the environment Any environmental conditions that could cause the system to fail UTSA CS3773

  43. Software Requirements Specification Section 3 Specific requirements 3.1 Functional requirements 3.1.1 Use case diagrams and detail description in tabular format Number each use case for future reference. 3.1.2 Class diagrams 3.1.3 State diagrams 3.1.4 Sequence diagrams In each above section 3.1.x, give English introduction to each diagram to help the reader understand each diagram. UTSA CS3773

  44. Software Requirements Specification Section 3 Specific requirements (continued) 3.1 Functional requirements (continued) 3.1.5 Data dictionary in tabular format • Classes: purpose • Attributes: purpose, range of values • Operations: purpose, parameters, pre/post conditions • Events: purpose, source, destination, parameters UTSA CS3773

  45. Software Requirements Specification Section 3 Specific requirements 3.2 User interface requirements • Screen shots • Purpose of each button, menu options, etc. • List of input/output events • How to navigate among windows UTSA CS3773

  46. Software Requirements Specification Section 3 Specific requirements 3.3 Non-functional requirements Reliability Portability Security … UTSA CS3773

  47. Software Requirements Specification Section 4 Requirements table • Requirement number • Name • Description • Related requirements’ numbers and source • Related use cases’ numbers UTSA CS3773

  48. Reading Assignments • Sommerville’s Book, 8th edition • Chapter 7, “Requirements Engineering Process” • Sommerville’s Book, 9th edition • Chapter 4, “Requirements Engineering” • IEEE Std 830-1998, “IEEE Recommended Practice for Software Requirements Specification” UTSA CS3773

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