Chapter Summary

Chapter 14:

Beyond Functionality: Software Quality Attributes

© Karl E. Wiegers

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Quality Distinguishes Products

 Functional requirements (features) may only satisfy a customer

 Quality attributes make customers “enjoy” your product (an old-fashioned view of quality)

 Quality attributes are often critical to product usefulness (the architecture-centric view)

 Quality attributes are harder to elicit from customers—they don’t usually think to volunteer them

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Software Quality Attributes

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1. Quality Attributes For Users

 Availability ◦ Uptime / (Uptime + Downtime) ◦ Downtime can be scheduled or unscheduled ◦ Driven by business and safety needs ◦ Could be time-dependent

 Efficiency ◦ Minimal consumption of resources ◦ Needs to consider spikes, growth

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 Flexibility ◦ Ease of adding new capabilities ◦ Essential for incremental, iterative development

 Integrity ◦ Blocking unwanted access ◦ Enforcing privacy ◦ Preventing data loss

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 Interoperability ◦ Exchange of data with other systems ◦ File and message formats

 Reliability ◦ Operation without failure ◦ Related to availability ◦ Trade off impact severity against development

cost.

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 Robustness ◦ Continued operation when faced with invalid

inputs, unexpected conditions

 Usability ◦ Can be very subjective ◦ Effort to provide inputs, use outputs ◦ Often specified in terms of task times

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2. Quality Attributes For Developers

 Maintainability ◦ Defect correction ◦ Feature enhancement ◦ Related to flexibility ◦ Understandable, simple design

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 Portability ◦ Preparing for variations in operating environment ◦ Hardware platform, operating system, database, GUI.

 Reusability ◦ Using components in other applications ◦ Modular, independent, well-documented, generic

software

 Testability ◦ Ease of looking for, finding defects ◦ Affects design guidelines for complexity

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3. Performance Requirements

 Speed of functions ◦ Response time ◦ Throughput ◦ Capacity ◦ Timing deadlines

 Can, however, affect system safety

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Summary of Quality Attributes

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4. Documenting Quality Attributes (quantifiable and verifiable)

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5. Quality Attribute Relationships (Trade-offs)

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6. Implementing Nonfunctional Requirements

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Integrity, interoperability, robustness, usability, safety

Functional requirements

Availability, efficiency, flexibility, performance, reliability

System architecture

Interoperability, usability Design constraint

Flexibility, maintainability, portability, reliability, reusability, testability, usability

Design guideline

Portability Implementation constraint

END

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