2. 2
Introduction
• Requirements form the basis for all
software products
• Requirements engineering is the
process, which enables us to
systematically determine the
requirements for a software product
3. Learning outcome
After this lecture
•You will be able to understand what is requirement
•What is software requirement
•Sources of Requirement
•Levels of requirement
•Importance of software requirement
•Kinds of requirements
•Examples
3
5. 5
Requirement
• Something required, something wanted
or needed
– Webster’s dictionary
• There is a huge difference between
wanted and needed and it should be
kept in mind all the time
6. 6
Software Requirements - 1
• A complete description of what the
software system will do without
describing how it will do it is
represented by the software
requirements
7. 7
Software Requirements - 2
• Software requirements are complete
specification of the desired external
behavior of the software system to be
built
• They also represent External behavior
of the system
9. 9
Software Requirements - 4
• Software requirements may be:
– Part of the bid of contract
– The contract itself
– Part of the technical document, which
describes a product
10. 10
IEEE Definition
• A condition or capability that must be
met or possessed by a system...to
satisfy a contract, standard,
specification, or other formally
imposed document
– IEEE Std 729
11. 11
Sources of Requirements
• Stakeholders
– People affected in some way by the
system
• Documents
• Existing system
• Domain/business area
12. 12
Levels of Software Requirements
• Stakeholders describe requirements at
different levels of detail
– “What versus How”
– “One person’s floor is another person’s
ceiling”
13. 13
What Versus How
User needs
Product space
Actual product’s behavior
Architecture/data flow
Module specifications
Algorithms
Code
What
How
What
How
What
How
What
How
What
How
What
How
14. 14
Importance of Software
Requirements
• The hardest single part of building a
software system is deciding what to
build...No other part of the work so
cripples the resulting system if done
wrong. No other part is difficult to
rectify later
– Fred Brooks
15. 15
Examples of Requirements - 1
• The system shall maintain records of
all payments made to employees on
accounts of salaries, bonuses,
travel/daily allowances, medical
allowances, etc.
16. 16
Examples of Requirements - 2
• The system shall interface with the
central computer to send daily sales
and inventory data from every retail
store
17. 17
Examples of Requirements - 3
• The system shall maintain records of
all library materials including books,
serials, newspapers and magazines,
video and audio tapes, reports,
collections of transparencies, CD-
ROMs, DVDs, etc.
18. 18
Examples of Requirements - 4
• The system shall allow users to search
for an item by title, author, or by
International Standard Book Number
• The system’s user interface shall be
implemented using a web browser
19. 19
Examples of Requirements - 5
• The system shall support at least
twenty transactions per second
• The system facilities which are
available to public users shall be
demonstrable in ten minutes or less
24. 24
Functional Requirements - 2
• Statements of services the system
should provide
– Reaction to particular inputs
– Behavior in particular situations
25. 25
Functional Requirements - 3
• Sequencing and parallelism are also
captured by functional requirements
• Abnormal behavior is also documented
as functional requirements in the form
of exception handling
26. 26
Functional Requirements - 4
• Functional requirements should be
complete and consistent
• Customers and developers usually
focus all their attention on functional
requirements
27. 27
Functional Requirements Example # 1
• The system shall solve a quadratic
equation using the following formula
x = (-b+sqrt(b2
– 4*a*c))/2*a
28. 28
Functional Requirements Example # 2
• The user shall be able to search either
the entire database of patients or select
a subset from it (admitted patients, or
patients with asthma, etc.)
29. 29
Functional Requirements Example # 3
• The system shall provide appropriate
viewers for the user to read documents
in the document store
30. 30
Functional Requirements Example # 4
• Every order shall be allocated a unique
identifier (ORDER_ID) which the user
shall use to access that order
31. 31
Functional Requirements Example # 5
• The system shall allow customers to
return non-perishable items within
fifteen days of the purchase. A
customer must present the original sale
receipt to return an item
32. 32
Comments on Examples
• Notice the level of detail in different
requirements described above. Some
are very detailed compared to others
33. 33
Comments on Examples
• Notice the ambiguity in the
requirement, which uses the term
‘appropriate viewers’
• This requirement does not mention the
formats of documents and types of
viewers, which can be used
34. 34
Comments on Examples
• Notice the ambiguity in the
requirement for solving the quadratic
equation. The requirement does not
speak about the possibility when the
value of ‘a’ is zero
x = (-b+sqrt(b2
– 4*a*c))/2*a
35. 35
Comments on Examples
• Incomplete and ambiguous
requirements are open to multiple
interpretations and assumptions
• This can lead to the development of
poor quality, or faulty, software
products
36. 36
Summary
• Requirements form the basis of all
software engineering projects
• Functional requirements capture the
behavioral aspects/functions of the
proposed automated system
• Functional requirements are the
backbone of all software products
37. 37
References
• ‘Requirements Engineering: Processes and
Techniques’ by G. Kotonya and I.
Sommerville, John Wiley & Sons, 1998
• Software Requirements: Objects, Functions,
and States by A. Davis, PH, 1993
• Software Engineering 6th
Edition, by I.
Sommerville, 2000
• Software Engineering 5th
Edition, by R.
Pressman
