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

Slide 1
Requirements and systems
Software-based
system

User
world
Requirements

An introduction to requirements engineering, 2013

Photo © Liam Quinn

Slide 2
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

Slide 3
Types of requirement
• a user-level facility description,
• a detailed specification of expected
system behaviour,

An introduction to requirements engineering, 2013

Slide 4
• a general system property,
• a specific constraint on the system,

An introduction to requirements engineering, 2013

Slide 5
• information on how to carry out some
computation,
• a constraint on the development of the
system.

An introduction to requirements engineering, 2013

Slide 6
Functional requirements
• Requirements that define a function or
feature of the system
• Something that the system should DO

An introduction to requirements engineering, 2013

Slide 7
• “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

Slide 8
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

Slide 9
• “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
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

Slide 11
• “The system shall implement patient
privacy provisions as set out in the 1998
Data Protection Act”

An introduction to requirements engineering, 2013

Slide 12
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

Slide 13
An introduction to requirements engineering, 2013

Slide 14
• „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

Slide 15
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

Slide 16
“... 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

Slide 17
If the requirements are wrong
• the system may be delivered late
and cost more than originally
expected.

An introduction to requirements engineering, 2013

Slide 18
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

Slide 19
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

Slide 20
• 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
Software-based
system

User
world
Requirements

An introduction to requirements engineering, 2013

Photo © Liam Quinn

Slide 22
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

Slide 23
• Businesses operate in a rapidly
changing environment so their
requirements for system support
are constantly changing.

An introduction to requirements engineering, 2013

Slide 24
• Multiple stakeholders with different
goals and priorities are involved in
the requirements engineering
process.

An introduction to requirements engineering, 2013

Slide 25
• 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

Slide 26
• Requirements are often influenced
by political and organisational
factors that stakeholders will not
admit to publicly.

An introduction to requirements engineering, 2013

Slide 27
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

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Intro to requirements eng.

  • 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 Slide 3
  • 4. Types of requirement • a user-level facility description, • a detailed specification of expected system behaviour, An introduction to requirements engineering, 2013 Slide 4
  • 5. • a general system property, • a specific constraint on the system, An introduction to requirements engineering, 2013 Slide 5
  • 6. • information on how to carry out some computation, • a constraint on the development of the system. An introduction to requirements engineering, 2013 Slide 6
  • 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 Slide 7
  • 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 Slide 8
  • 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 Slide 9
  • 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 Slide 11
  • 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 Slide 13
  • 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 Slide 15
  • 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 Slide 16
  • 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 Slide 17
  • 18. If the requirements are wrong • the system may be delivered late and cost more than originally expected. An introduction to requirements engineering, 2013 Slide 18
  • 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 Slide 19
  • 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 Slide 20
  • 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 Slide 23
  • 24. • Businesses operate in a rapidly changing environment so their requirements for system support are constantly changing. An introduction to requirements engineering, 2013 Slide 24
  • 25. • Multiple stakeholders with different goals and priorities are involved in the requirements engineering process. An introduction to requirements engineering, 2013 Slide 25
  • 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 Slide 26
  • 27. • Requirements are often influenced by political and organisational factors that stakeholders will not admit to publicly. An introduction to requirements engineering, 2013 Slide 27
  • 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