Chapter 7 Requirements Engineering. Software Engineering: A Practitioner s Approach, 6th edition by Roger S. Pressman

Transcription

1 Chapter 7 Requirements Engineering Software Engineering: A Practitioner s Approach, 6th edition by Roger S. Pressman 1

2 What is requirement engineering? Requirement engineering helps software engineers to better understand the problem they will work to solve. It encompasses the set of tasks that lead to an understanding of what the business impact of the software will be, what the customer wants, and how end users will interact with the software. 2

3 Why is it important? Designing and building an elegant computer program that solves the wrong problem serves no one s needs. That s why it is important to understand what the customer wants before you begin to design and build a computer based system. 3

4 What are the steps? 1 Requirements engineering begins with inception: A task that defines the scope and nature of the problem to be solved. It moves onward to elicitation A task that helps the customer to define what is required, and them elaboration Where basic requirements are refined and modified. As the customer defines the problem, negotiation occurs 4

5 What are the steps? 2 What are the priorities, what is essential, when is it required? Finally, the problem is specified in some manner and then reviewed or validated to ensure that your understanding of the problem and the customers understanding of the problem coincide. 5

6 Requirements Engineering Inception Establish a basic understanding of the problem and the nature of the solution. Elicitation Draw out the requirements from stakeholders. Elaboration Create an analysis model that represents information, functional, and behavioral aspects of the requirements. Negotiation Agree on a deliverable system that is realistic for developers and customers. Specification Describe the requirements formally or informally. Validation Review the requirement specification for errors, ambiguities, omissions, and conflicts. Requirements management Manage changing requirements. 6

7 Inception Ask context free questions Who is behind the request for this work? Who will use the solution (product/system)? What will be the economic benefits? How would you characterize good output from the system? What problems does this solution address? What environment will the product be used in? Are you the right person to answer these questions? Are these question relevant? Can anyone else provide additional information? Should I be asking you anything else? 7

8 Getting Requirements Right The hardest single part of building a software system is deciding what to build. No part of the work so cripples the resulting system if done wrong. No other part is more difficult to rectify later. Fred Brooks The seeds of major software disasters are usually sown within the first three months of commencing the software project. Capers Jones We spend a lot of time the majority of project effort not implementing or testing, but trying to decide what to build. Brian Lawrence 8

9 Eliciting Requirements Why is it so difficult to clearly understand what the customer wants? Scope The boundary of the system is ill defined. Customers/users specify unnecessary technical detail that may confuse rather than clarify objectives. Understanding Customers are not completely sure of what is needed. Customers have a poor understanding of the capabilities and limitations of the computing environment. Customers don t have a full understanding of their problem domain. Customers have trouble communicating needs to the system engineer. Customers omit detail that is believed to be obvious. Customers specify requirements that conflict with other requirements. Customers specify requirements that are ambiguous or untestable. Volatility Requirements change over time. 9

10 Collaborative Requirements Gathering Meetings are attended by all interested stakeholders. Rules established for preparation and participation. Agenda should be formal enough to cover all important points, but informal enough to encourage the free flow of ideas. A facilitator controls the meeting. A definition mechanism (blackboard, flip charts, etc.) is used. During the meeting: The problem is identified. Elements of the solution are proposed. Different approaches are negotiated. A preliminary set of solution requirements are obtained. The atmosphere is collaborative and non threatening. 10

11 Quality Function Deployment Function deployment determines the value (to the customer) of each function required of the system. Information deployment identifies data objects and events. Task deployment examines the behavior of the system. Value analysis determines the priority of requirements. 11

12 Elicitation Work Products Statement of need and feasibility. Statement of scope. List of participants in requirements elicitation. Description of the system s technical environment. List of requirements and associated domain constraints. List of usage scenarios. Any prototypes developed to refine requirements. 12

13 Use Cases A use case scenario is a story about how someone or something external to the software (known as an actor) interacts with the system. Each scenario answers the following questions: Who is the primary actor, the secondary actor(s)? What are the actor s goals? What preconditions should exist before the story begins? What main tasks or functions are performed by the actor? What exceptions might be considered as the story is described? What variations in the actor s interaction are possible? What system information will the actor acquire, produce, or change? Will the actor have to inform the system about changes in the external environment? What information does the actor desire from the system? Does the actor wish to be informed about unexpected changes? 13

14 Elements of the Analysis Model Scenario based elements Use case How external actors interact with the system (use case diagrams; detailed templates) Functional How software functions are processed in the system (flow charts; activity diagrams) Class based elements The various system objects (obtained from scenarios) including their attributes and functions (class diagram) Behavioral elements How the system behaves in response to different events (state diagram) Flow oriented elements How information is transformed as if flows through the system (data flow diagram) 14

15 Use Case Diagram 15

16 Activity Diagram for RE 16

17 Class Diagram 17

18 State Diagram 18

19 Analysis Patterns Pattern name: Captures the essence of the pattern. Intent: What the pattern accomplishes or represents. Motivation: A scenario that illustrates how the pattern solves a problem. Forces and context: External issues (forces) that affect how the pattern is used, and external issues resolved when the pattern is applied. Solution: How the pattern is applied to solve the problem; emphasizes structural and behavioral issues. Consequences: What happens when the pattern is applied; what trade-offs exist during its application. Design: How the pattern can be achieved via known design patterns. Known uses: Examples of uses within actual systems. Related patterns: Patterns related to the named pattern because (1) they are commonly used with the named pattern; (2) they are structurally similar to the named pattern; (3) they are a variation of the named pattern. 19

20 Negotiating Requirements Identify the key stakeholders These are the people who will be involved in the negotiation Determine each of the stakeholders win conditions Win conditions are not always obvious Negotiate Work toward a set of requirements that lead to win win 20

21 Validating Requirements Is each requirement consistent with the objective of the system? Have all requirements been specified at the proper level of abstraction? Is the requirement really necessary? Is each requirement bounded and unambiguous? Does each requirement have attribution? Do any requirements conflict with other requirements? Is each requirement achievable in the system s technical environment? Is each requirement testable, once implemented? Does the model reflect the system s information, function and behavior? Has the model been appropriately partitioned? Have appropriate requirements patterns been used? 21

22 Example CRG Meeting First CRG meeting of the SafeHome project. After Q&A session (inception meeting), stakeholders write a two page product request, which is delivered to those attending the first CRG meeting. Attendees are asked to make a rough list of objects, services, constraints, and performance criteria for the product. At the meeting, a combined list is created in each topic. Subgroups write mini specifications for each list item. Relevant features in mini specifications are added to the list. 22

23 Example CRG Meeting Our research indicates that the market for home management systems is growing at a rate of 40 percent per year. The first SafeHome function we bring to market should be the home security function. Most people are familiar with alarm systems so this would be an easy sell. The home security function would protect against and/or recognize a variety of undesirable situations such as illegal entry, fire, flooding, carbon monoxide levels, and others. It ll use our wireless sensors to detect each situation, can be programmed by the homeowner, and will automatically telephone a monitoring agency when a situation is detected. 23

24 Example CRG Meeting Objects control panel, smoke detectors, window and door sensors, motion detectors, an alarm, an event (sensor has been activated), a display, a PC, telephone numbers, a telephone call, Services configuring the system, setting the alarm, monitoring the sensors, dialing the phone, programming the control panel, reading the display, Constraints System must recognize when sensors are not operating, must be user friendly, must interface directly to a standard phone line, Performance criteria Sensor event should be recognized within one second, an event priority scheme should be implemented, 24

25 Example CRG Meeting Mini specification for Control Panel The Control Panel is a wall mounted unit that is approximately 9 x 5 inches in size. The control panel has wireless connectivity to sensors and a PC. User interaction occurs through a keypad containing 12 keys. A 2 x 2 inch LCD display provides user feedback. Software provides interactive prompts, echo, and similar functions. 25