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Unit-4
Requirement analysis
and Specification
Requirements Engineering
• Requirement: A function, constraint or other
property that the system must provide to fill the
needs of the system’s intended user(s)
• Engineering: implies that systematic and repeatable
techniques should be used
• Requirement Engineering means that requirements for a
product are defined, managed and tested systematically
Requirements Engineering Tasks
• Inception —Establish a basic understanding of the problem
and the nature of the solution.
• Elicitation —Draw out the requirements from stakeholders.
• Elaboration (Highly structured)—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.
Requirements Engineering-I
• Inception—ask a set of questions that establish …
– basic understanding of the problem
– the people who want a solution
– the nature of the solution that is desired, and
– the effectiveness of preliminary communication and
collaboration between the customer and the developer
• Elicitation—elicit requirements from all stakeholders
• Elaboration—create an analysis model that identifies
data, function and behavioral requirements
• Negotiation—agree on a deliverable system that is
realistic for developers and customers
Requirements Engineering-II
• Specification—can be any one (or more) of the following:
– A written document
– A set of models
– A formal mathematical
– A collection of user scenarios (use-cases)
– A prototype
• Validation—a review mechanism that looks for
– errors in content or interpretation
– areas where clarification may be required
– missing information
– inconsistencies (a major problem when large products or systems are
engineered)
– conflicting or unrealistic (unachievable) requirements.
• Requirements management
Inception
• Identify stakeholders
– “who else do you think I should talk to?”
• Recognize multiple points of view
• Work toward collaboration
• The first questions
– Who is behind the request for this work?
– Who will use the solution?
– What will be the economic benefit of a successful
solution
– Is there another source for the solution that you need?
Eliciting Requirements
• meetings are conducted and attended by both software engineers
and customers
• rules for preparation and participation are established
• an agenda is suggested
• a "facilitator" (can be a customer, a developer, or an outsider)
controls the meeting
• a "definition mechanism" (can be work sheets, flip charts, or wall
stickers or an electronic bulletin board, chat room or virtual forum)
is used
• the goal is
– to identify the problem
– propose elements of the solution
– negotiate different approaches, and
– specify a preliminary set of solution requirements
Use QFD to
prioritize
requirem ents
inform ally
prioritize
requirem ents
form al prioritization?
Create Use-cases
yes no
Elic it requirem ent s
write scenario
define actors
com plete tem plate
draw use-case
diagram
Conduct FAST
m eetings
Make lists of
functions, classes
Make lists of
constraints, etc.
Quality Function Deployment
• Function deployment determines the “value” (as
perceived by 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 relative priority of
requirements
Elicitation Work Products
• a statement of need and feasibility.
• a bounded statement of scope for the system or product.
• a list of customers, users, and other stakeholders who
participated in requirements elicitation
• a description of the system’s technical environment.
• a list of requirements (preferably organized by function)
and the domain constraints that apply to each.
• a set of usage scenarios that provide insight into the use of
the system or product under different operating
conditions.
• any prototypes developed to better define requirements.
Building the Analysis Model
• Elements of the analysis model
– Scenario-based elements
• Functional—processing narratives for software functions
• Use-case—descriptions of the interaction between an “actor” and
the system
– Class-based elements
• Implied by scenarios
– Behavioral elements
• State diagram
– Flow-oriented elements
• Data flow diagram
Use-Cases
• A collection of user scenarios that describe the thread of usage of a system
• Each scenario is described from the point-of-view of an “actor”—a person or
device that interacts with the software in some way
• 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 extensions 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?
Use-Case Diagram
homeowner
Arms/ disarms
system
Accesses system
via Internet
Reconfigures sensors
and related
system features
Responds to
alarm event
Encounters an
error condition
system
administrator
sensors
Class Diagram
Sensor
name/id
type
location
area
characteristics
identify()
enable()
disable()
reconfigure()
State Diagram
Reading
Comman
ds
System status = “ready”
Display msg = “enter cmd”
Display status = steady
Entry/subsystems ready
Do: poll user input panel
Do: read user input
Do: interpret user input
State name
State variables
State activities
Analysis Patterns
Pattern name: A descriptor that captures the essence of the pattern.
Intent: Describes what the pattern accomplishes or represents
Motivation: A scenario that illustrates how the pattern can be used to address
the problem.
Forces and context: A description of external issues (forces) that can affect
how the pattern is used and also the external issues that will be resolved when
the pattern is applied.
Solution: A description of how the pattern is applied to solve the problem with
an emphasis on structural and behavioral issues.
Consequences: Addresses what happens when the pattern is applied and
what trade-offs exist during its application.
Design: Discusses how the analysis pattern can be achieved through the use
of known design patterns.
Known uses: Examples of uses within actual systems.
Related patterns: On e or more analysis patterns that are related to the
named pattern because (1) it is commonly used with the named pattern; (2) it
is structurally similar to the named pattern; (3) it is a variation of the named
pattern.
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”
Functional and non-functional requirements
• 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.
• Non-functional requirements
– constraints on the services or functions offered by the
system such as timing constraints, constraints on the
development process, standards, etc.
• Domain requirements
– Requirements that come from the application domain
of the system and that reflect characteristics of that
domain
Requirements Engineering Tasks
1. Inception
2. Elicitation
3. Elaboration
4. Negotiation
5. Specification
6. Validation
7. Management
SSystem
RRequirements
SSpecification
Specifying the SpecificationsSpecifying the Specifications
Software Requirements Specification
• It contains a complete information description, a
detailed functional description, a representation of
system behavior, an indication of performance
requirements and design constraints, appropriate
validation criteria, and other information pertinent to
requirements.
Format of SRS:
Introduction
Information
Functional Description
Behavioral
"requirement" ≠ "specification"
• Requirement – understanding
between customer and supplier
• Specification – what the software
must do
• Requirements that are not in the SRS
– Costs
– Delivery dates
– Acceptance procedures
– etc
Uses of the SRS
• Design
• Validation
• Customer Contract – rarely
Role of SRS
1. “The SRS must correctly define all of the
software requirements, but no more.”
2. “The SRS should not describe design,
verification, or project management details,
except for required design constraints.”
1. Unambiguous
2. Complete
3. Verifiable
4. Consistent
5. Modifiable
6. Traceable
7. Usable during the Operation and Maintenance Phase
Characteristics of a Good SRS
Desired SRS CharacteristicsDesired SRS Characteristics
• Complete
• Consistent
• Changeable
• Traceable
SRS Table of Contents
1. Introduction
1. Purpose
2. Scope
3. Definitions
4. References
5. Overview
2. General Description
1. Product Perspective
2. Product Functions
3. User Characteristics
4. General Constraints
5. Assumptions and Dependencies
3. Specific Requirements
3. Specific Requirements
3.1 Functional Requirements
3.1.1 Func Req 1
3.1.1.1 Introduction
3.1.1.2 Inputs
3.1.1.3 Processing
3.1.1.4 Outputs
3.1.2 Func Req 2
…
3.2 External Interface Requirements
3.2.1 User Interface
3.2.2 Hardware Interfaces
3.2.3 Software Interfaces
3.2.4 Communication Interfaces
3.3 Performance Requirements
3.4 Design Constraints
3.4.1 Standards Compliance
3.4.2 Hardware Limitations
3.5 Attributes
3.5.1 Security
3.5.2 Maintainability
3.6 Other Requirements
3.6.1 Database
Problems Without SRS
• Without developing the SRS document, the system would
not be implemented according to customer needs.
• Software developers would not know whether what they
are developing is what exactly is required by the
customer.
• Without SRS document, it will be very difficult for the
maintenance engineers to understand the functionality of
the system.
• It will be very difficult for user document writers to write
the users’ manuals properly without understanding the
SRS document.
Requirement ModelingRequirement Modeling
Tools for modeling requirements
• Use Cases
• State Diagrams
• UI Mockups
• Storyboards
• Prototypes
Data Flow Diagram
Data Flow Diagram
Feasibility Study
• Economic feasibility
– cost/benefit analysis
• Technical feasibility
– hardware/software/people, etc.
• Legal feasibility
• Alternatives
– there is always more than one way to do it
Four Types of feasibility
Technical feasibility
Is the project possible with current
technology?
What technical risk is there?
Availability of the technology:
Is it available locally?
Can it be obtained?
Will it be compatible with other systems?
Economic feasibility
Is the project possible, given resource
constraints?
What are the benefits?
Both tangible and intangible
Quantify them!
What are the development and
operational costs?
Are the benefits worth the costs?
Schedule feasibility
Is it possible to build a solution
in time to be useful?
What are the consequences of delay?
Any constraints on the schedule?
Can these constraints be met?
Operational feasibility
If the system is developed, will
it be used?
Human and social issues…
Potential labour objections?
Manager resistance?
Organizational conflicts and policies?
Social acceptability?
legal aspects and government regulations?

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Requirement analysis and specification, software engineering

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  • 5. Requirements Engineering • Requirement: A function, constraint or other property that the system must provide to fill the needs of the system’s intended user(s) • Engineering: implies that systematic and repeatable techniques should be used • Requirement Engineering means that requirements for a product are defined, managed and tested systematically
  • 6. Requirements Engineering Tasks • Inception —Establish a basic understanding of the problem and the nature of the solution. • Elicitation —Draw out the requirements from stakeholders. • Elaboration (Highly structured)—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.
  • 7. Requirements Engineering-I • Inception—ask a set of questions that establish … – basic understanding of the problem – the people who want a solution – the nature of the solution that is desired, and – the effectiveness of preliminary communication and collaboration between the customer and the developer • Elicitation—elicit requirements from all stakeholders • Elaboration—create an analysis model that identifies data, function and behavioral requirements • Negotiation—agree on a deliverable system that is realistic for developers and customers
  • 8. Requirements Engineering-II • Specification—can be any one (or more) of the following: – A written document – A set of models – A formal mathematical – A collection of user scenarios (use-cases) – A prototype • Validation—a review mechanism that looks for – errors in content or interpretation – areas where clarification may be required – missing information – inconsistencies (a major problem when large products or systems are engineered) – conflicting or unrealistic (unachievable) requirements. • Requirements management
  • 9. Inception • Identify stakeholders – “who else do you think I should talk to?” • Recognize multiple points of view • Work toward collaboration • The first questions – Who is behind the request for this work? – Who will use the solution? – What will be the economic benefit of a successful solution – Is there another source for the solution that you need?
  • 10. Eliciting Requirements • meetings are conducted and attended by both software engineers and customers • rules for preparation and participation are established • an agenda is suggested • a "facilitator" (can be a customer, a developer, or an outsider) controls the meeting • a "definition mechanism" (can be work sheets, flip charts, or wall stickers or an electronic bulletin board, chat room or virtual forum) is used • the goal is – to identify the problem – propose elements of the solution – negotiate different approaches, and – specify a preliminary set of solution requirements
  • 11. Use QFD to prioritize requirem ents inform ally prioritize requirem ents form al prioritization? Create Use-cases yes no Elic it requirem ent s write scenario define actors com plete tem plate draw use-case diagram Conduct FAST m eetings Make lists of functions, classes Make lists of constraints, etc.
  • 12. Quality Function Deployment • Function deployment determines the “value” (as perceived by 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 relative priority of requirements
  • 13. Elicitation Work Products • a statement of need and feasibility. • a bounded statement of scope for the system or product. • a list of customers, users, and other stakeholders who participated in requirements elicitation • a description of the system’s technical environment. • a list of requirements (preferably organized by function) and the domain constraints that apply to each. • a set of usage scenarios that provide insight into the use of the system or product under different operating conditions. • any prototypes developed to better define requirements.
  • 14. Building the Analysis Model • Elements of the analysis model – Scenario-based elements • Functional—processing narratives for software functions • Use-case—descriptions of the interaction between an “actor” and the system – Class-based elements • Implied by scenarios – Behavioral elements • State diagram – Flow-oriented elements • Data flow diagram
  • 15. Use-Cases • A collection of user scenarios that describe the thread of usage of a system • Each scenario is described from the point-of-view of an “actor”—a person or device that interacts with the software in some way • 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 extensions 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?
  • 16. Use-Case Diagram homeowner Arms/ disarms system Accesses system via Internet Reconfigures sensors and related system features Responds to alarm event Encounters an error condition system administrator sensors
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  • 20. State Diagram Reading Comman ds System status = “ready” Display msg = “enter cmd” Display status = steady Entry/subsystems ready Do: poll user input panel Do: read user input Do: interpret user input State name State variables State activities
  • 21. Analysis Patterns Pattern name: A descriptor that captures the essence of the pattern. Intent: Describes what the pattern accomplishes or represents Motivation: A scenario that illustrates how the pattern can be used to address the problem. Forces and context: A description of external issues (forces) that can affect how the pattern is used and also the external issues that will be resolved when the pattern is applied. Solution: A description of how the pattern is applied to solve the problem with an emphasis on structural and behavioral issues. Consequences: Addresses what happens when the pattern is applied and what trade-offs exist during its application. Design: Discusses how the analysis pattern can be achieved through the use of known design patterns. Known uses: Examples of uses within actual systems. Related patterns: On e or more analysis patterns that are related to the named pattern because (1) it is commonly used with the named pattern; (2) it is structurally similar to the named pattern; (3) it is a variation of the named pattern.
  • 22. 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”
  • 23. Functional and non-functional requirements • 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. • Non-functional requirements – constraints on the services or functions offered by the system such as timing constraints, constraints on the development process, standards, etc. • Domain requirements – Requirements that come from the application domain of the system and that reflect characteristics of that domain
  • 24. Requirements Engineering Tasks 1. Inception 2. Elicitation 3. Elaboration 4. Negotiation 5. Specification 6. Validation 7. Management
  • 26. Software Requirements Specification • It contains a complete information description, a detailed functional description, a representation of system behavior, an indication of performance requirements and design constraints, appropriate validation criteria, and other information pertinent to requirements. Format of SRS: Introduction Information Functional Description Behavioral
  • 27. "requirement" ≠ "specification" • Requirement – understanding between customer and supplier • Specification – what the software must do • Requirements that are not in the SRS – Costs – Delivery dates – Acceptance procedures – etc
  • 28. Uses of the SRS • Design • Validation • Customer Contract – rarely
  • 29. Role of SRS 1. “The SRS must correctly define all of the software requirements, but no more.” 2. “The SRS should not describe design, verification, or project management details, except for required design constraints.”
  • 30. 1. Unambiguous 2. Complete 3. Verifiable 4. Consistent 5. Modifiable 6. Traceable 7. Usable during the Operation and Maintenance Phase Characteristics of a Good SRS
  • 31. Desired SRS CharacteristicsDesired SRS Characteristics • Complete • Consistent • Changeable • Traceable
  • 32. SRS Table of Contents 1. Introduction 1. Purpose 2. Scope 3. Definitions 4. References 5. Overview 2. General Description 1. Product Perspective 2. Product Functions 3. User Characteristics 4. General Constraints 5. Assumptions and Dependencies 3. Specific Requirements
  • 33. 3. Specific Requirements 3.1 Functional Requirements 3.1.1 Func Req 1 3.1.1.1 Introduction 3.1.1.2 Inputs 3.1.1.3 Processing 3.1.1.4 Outputs 3.1.2 Func Req 2 … 3.2 External Interface Requirements 3.2.1 User Interface 3.2.2 Hardware Interfaces 3.2.3 Software Interfaces 3.2.4 Communication Interfaces 3.3 Performance Requirements 3.4 Design Constraints 3.4.1 Standards Compliance 3.4.2 Hardware Limitations 3.5 Attributes 3.5.1 Security 3.5.2 Maintainability 3.6 Other Requirements 3.6.1 Database
  • 34. Problems Without SRS • Without developing the SRS document, the system would not be implemented according to customer needs. • Software developers would not know whether what they are developing is what exactly is required by the customer. • Without SRS document, it will be very difficult for the maintenance engineers to understand the functionality of the system. • It will be very difficult for user document writers to write the users’ manuals properly without understanding the SRS document.
  • 36. Tools for modeling requirements • Use Cases • State Diagrams • UI Mockups • Storyboards • Prototypes
  • 39. Feasibility Study • Economic feasibility – cost/benefit analysis • Technical feasibility – hardware/software/people, etc. • Legal feasibility • Alternatives – there is always more than one way to do it
  • 40. Four Types of feasibility Technical feasibility Is the project possible with current technology? What technical risk is there? Availability of the technology: Is it available locally? Can it be obtained? Will it be compatible with other systems? Economic feasibility Is the project possible, given resource constraints? What are the benefits? Both tangible and intangible Quantify them! What are the development and operational costs? Are the benefits worth the costs? Schedule feasibility Is it possible to build a solution in time to be useful? What are the consequences of delay? Any constraints on the schedule? Can these constraints be met? Operational feasibility If the system is developed, will it be used? Human and social issues… Potential labour objections? Manager resistance? Organizational conflicts and policies? Social acceptability? legal aspects and government regulations?