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Chapter 7Requirements Engineering

Chapter 7Requirements Engineering

Software Engineering: A Practitioner’s Approach, 6th editionby Roger S. Pressman

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Requirements EngineeringRequirements 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.

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.

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InceptionInception 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?

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?

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Getting Requirements RightGetting 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

“ 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

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Eliciting RequirementsEliciting 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.

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.

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Collaborative Requirements GatheringCollaborative 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.

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.

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Quality Function DeploymentQuality 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.

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.

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Elicitation Work ProductsElicitation 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.

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.

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Use-CasesUse-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?

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?

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Elements of the Analysis ModelElements 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)

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)

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Use-Case DiagramUse-Case Diagram

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Activity Diagram for REActivity Diagram for RE

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Class DiagramClass Diagram

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State DiagramState Diagram

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Analysis PatternsAnalysis PatternsPattern name:Pattern name: Captures the essence of the pattern. Captures the essence of the pattern.

Intent:Intent: What the pattern accomplishes or represents. What the pattern accomplishes or represents.

Motivation:Motivation: A scenario that illustrates how the pattern solves a problem. A scenario that illustrates how the pattern solves a problem.

Forces and context:Forces and context: External issues (forces) that affect how the pattern is External issues (forces) that affect how the pattern is used, and external issues resolved when the pattern is applied. used, and external issues resolved when the pattern is applied.

Solution:Solution: How the pattern is applied to solve the problem; emphasizes How the pattern is applied to solve the problem; emphasizes structural and behavioral issues.structural and behavioral issues.

ConsequencesConsequences: What happens when the pattern is applied; what trade-: What happens when the pattern is applied; what trade-offs exist during its application.offs exist during its application.

DesignDesign: How the pattern can be achieved via known design patterns.: How the pattern can be achieved via known design patterns.

Known usesKnown uses: Examples of uses within actual systems.: Examples of uses within actual systems.

Related patternsRelated patterns: Patterns related to the named pattern because: Patterns related to the named pattern because

(1)(1) they are commonly used with the named pattern;they are commonly used with the named pattern;

(2)(2) they are structurally similar to the named pattern;they are structurally similar to the named pattern;

(3)(3) they are a variation of the named pattern.they are a variation of the named pattern.

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Negotiating RequirementsNegotiating 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”

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”

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Validating RequirementsValidating 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?

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?

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Example CRG MeetingExample 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.

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.

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Example CRG MeetingExample 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.

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.

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Example CRG MeetingExample 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, …

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, …

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Example CRG MeetingExample 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.

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.