Requirement analysis and specification, software engineering

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

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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