Software Requirements

1
Software Requirements

• July 9, 2008

2
Objectives

• To introduce the concepts of user and system
  requirements
• To describe functional and non-functional
  requirements
• To explain how software requirements may be
  organised in a requirements document

3
Requirements engineering

• The process of establishing the services that the
  customer requires from a system and the
  constraints under which it operates and is
  developed.
• The requirements themselves are the descriptions
  of the system services and constraints that are
  generated during the requirements engineering
  process.

4
What is a requirement?

• It may range from a high-level abstract statement
  of a service or of a system constraint to a
  detailed mathematical functional specification.
• This is inevitable as requirements may serve a
  dual function
• May be the basis for a bid for a contract -
  therefore must be open to interpretation
• May be the basis for the contract itself -
  therefore must be defined in detail
• Both these statements may be called requirements.

5
Types of requirement

• User requirements
• Statements in natural language plus diagrams of
  the services the system provides and its
  operational constraints. Written for customers.
• System requirements
• A structured document setting out detailed
  descriptions of the systems functions, services
  and operational constraints. Defines what should
  be implemented so may be part of a contract
  between client and contractor.

6
Requirements classification

• Requirements may be functional or non-functional
  (extrafunctional /quality requirement)
• - Functional requirements describe system
• services or functions
• - Non-functional requirements is a constraint
  on
• the system or on the development process

7
Requirements classification

• Stability
• - Stable requirements - not changed during
  development
• -Volatile requirements. Requirements which
  change during development or when the system is
  in use.

8
Requirements classification

• Prioritizing requirements
• - Requirements that absolutely must be met
  (must, shall)
• - Requirements that are highly desirable
  (should)
• - Requirements that are possible but could be
  eliminated (could)

9
Wicked problems

• Most large software systems address wicked
  problems
• Customers do not know exactly what they want
• Developers used to forget the requirements
• Complex problems can never be fully understood
  before the
• system development
• Therefore, requirements are normally both
  incomplete and inconsistent

10
Reasons for inconsistency

• Different users have different requirements and
  priorities. There is a constantly shifting
  compromise in the requirements.
• System end-users and organizations who pay for
  the system have different requirements
• To clarify requirements?

11
Why are requirements important?

• Inconsistent and incomplete requirements are the
  most frequent causes of the system problems

12
Requirement documents

• Requirements definition
• - A statement in natural language plus
  diagrams of the services the system provides and
  its operational constraints. Written for
  customers
• Requirements specification
• - A structured document setting out detailed
  descriptions of the system services. Written as a
  contract between client and contractor

13
Requirement documents

• Software specification
• - A detailed software description which can
  serve as a basis for a design or implementation.
  Written for developers