An introduction to requirements engineering, 2013 Slide 1

Requirements and Requirements Engineering

Ian Sommerville

http://www.youtube.com/watch?v=Ec0s0z5uXQ8

An introduction to requirements engineering, 2013 Slide 2

Requirements and systems

User world

Software-based system

Requirements

Photo © Liam Quinn

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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.

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Types of requirement

• a user-level facility description,

• a detailed specification of expected system behaviour,

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• a general system property,

• a specific constraint on the system,

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• information on how to carry out some computation,

• a constraint on the development of the system.

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Functional requirements

• Requirements that define a function or feature of the system

• Something that the system should DO

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• “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”

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Non-functional requirements

• Requirements that define some observable characteristics or quality of the system

• Availability, reliability, safety, security, resilience, …

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• “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”

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Domain requirements

• Requirements that are derived from characteristics of the application domain or rules and regulations that apply in that domain

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

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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.

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• ‘engineering’ implies that systematic and repeatable techniques should be used to ensure that system requirements are complete, consistent, relevant, etc.

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Are requirements important?

“The principal problem areas in software development and production are the requirements specification and the management of customer requirements”

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“... difficulties with requirements are the key root-cause of the safety-related software errors that have persisted until integration and system testing”

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If the requirements are wrong

• the system may be delivered late and cost more than originally expected.

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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.

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If the requirements are wrong

• the system may be unreliable in use with regular system errors and crashes disrupting normal operation.

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• If the system continues in use, the costs of maintaining and evolving the system are very high.

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User world

Software-based system

Requirements

Photo © Liam Quinn

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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

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• Businesses operate in a rapidly

changing environment so their requirements for system support are constantly changing.

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

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• System stakeholders do not have clear ideas about what they need

• They can only describe their requirements in a vague and ambiguous way

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

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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