Explains what is meant by requirements and requirements engineering

1. Requirements and Requirements
Engineering
Ian Sommerville
http://www.youtube.com/watch?v=Ec0s0z5uXQ8
An introduction to requirements engineering, 2013
Slide 1

2. Requirements and systems
Software-based
system
User
world
Requirements
An introduction to requirements engineering, 2013
Photo © Liam Quinn
Slide 2

3. What are system requirements?
• Requirements are defined during the
early stages of a system development
as a specification of what should be
implemented or as a constraint of
some kind on the system.
An introduction to requirements engineering, 2013
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4. Types of requirement
• a user-level facility description,
• a detailed specification of expected
system behaviour,
An introduction to requirements engineering, 2013
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5. • a general system property,
• a specific constraint on the system,
An introduction to requirements engineering, 2013
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6. • information on how to carry out some
computation,
• a constraint on the development of the
system.
An introduction to requirements engineering, 2013
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7. Functional requirements
• Requirements that define a function or
feature of the system
• Something that the system should DO
An introduction to requirements engineering, 2013
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8. • “If a patient is known to be allergic to a
particular medication, then prescription
of that medication shall result in a
warning message being issued to to the
prescriber”
An introduction to requirements engineering, 2013
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9. Non-functional requirements
• Requirements that define some
observable characteristics or quality of
the system
• Availability, reliability, safety, security,
resilience, …
An introduction to requirements engineering, 2013
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10. • “The system shall be available to all
clinics during normal working hours
(Mon-Fri, 0830-1730). Downtime during
normal working hours shall not exceed 5
seconds in any one day”
An introduction to requirements engineering, 2013
Slide 10

11. Domain requirements
• Requirements that are derived from
characteristics of the application domain
or rules and regulations that apply in
that domain
An introduction to requirements engineering, 2013
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12. • “The system shall implement patient
privacy provisions as set out in the 1998
Data Protection Act”
An introduction to requirements engineering, 2013
Slide 12

13. What is requirements
engineering?
• Requirements engineering covers
all of the activities involved in
discovering, documenting, and
maintaining a set of requirements for
a computer-based system.
An introduction to requirements engineering, 2013
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14. An introduction to requirements engineering, 2013
Slide 14

15. • „engineering‟ implies that systematic
and repeatable techniques should be
used to ensure that system
requirements are complete, consistent,
relevant, etc.
An introduction to requirements engineering, 2013
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16. Are requirements important?
“The principal problem areas in software
development and production are the
requirements specification and the
management of customer requirements”
An introduction to requirements engineering, 2013
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17. “... difficulties with requirements are the
key root-cause of the safety-related
software errors that have persisted until
integration and system testing”
An introduction to requirements engineering, 2013
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18. If the requirements are wrong
• the system may be delivered late
and cost more than originally
expected.
An introduction to requirements engineering, 2013
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19. If the requirements are wrong
• the customer and end-users may not
satisfied with the system,
• they may not use its facilities or may
even decide to scrap it altogether.
An introduction to requirements engineering, 2013
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20. If the requirements are wrong
• the system may be unreliable in use
with regular system errors and crashes
disrupting normal operation.
An introduction to requirements engineering, 2013
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21. • If the system continues in use, the costs
of maintaining and evolving the system
are very high.
An introduction to requirements engineering, 2013
Slide 21

22. Software-based
system
User
world
Requirements
An introduction to requirements engineering, 2013
Photo © Liam Quinn
Slide 22

23. Difficulties with requirements
• Stakeholders don‟t know what they want
from a new system
• It is very difficult to imagine how future
systems might work
An introduction to requirements engineering, 2013
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24. • Businesses operate in a rapidly
changing environment so their
requirements for system support
are constantly changing.
An introduction to requirements engineering, 2013
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25. • Multiple stakeholders with different
goals and priorities are involved in
the requirements engineering
process.
An introduction to requirements engineering, 2013
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26. • System stakeholders do not have clear
ideas about what they need
• They can only describe their
requirements in a vague and ambiguous
way
An introduction to requirements engineering, 2013
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27. • Requirements are often influenced
by political and organisational
factors that stakeholders will not
admit to publicly.
An introduction to requirements engineering, 2013
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28. Summary
•
Requirements define what a system should do and
the desirable qualities of that system
•
Requirements engineering is a set of systematic
techniques for defining and validating system
requirements
•
Requirements engineering is difficulties and problems
with requirements are the major cause of system
problems and project failures
An introduction to requirements engineering, 2013
Slide 28