Non-Functional Requirements (NFRs)

• Also known as Quality Attributes
• Specify how the system performs its functions. Define how well a system performs its functions rather than what it does
• It's a quality measure on how well the system perform on a particular dimension
• Non-functional requirements DO influence on the architecture
• Quality attributes need to be measurable and testable
• Example of a bad attribute definition: "it must be fast!"
• Quality attributes may contradict one another, we need to make the right tradeoff
• Unrealistic requirements must be called out as soon as possible
• Example: < 100 ms latency between South America and Asia. 100% availability

Examples

• Performance: Response time under 2 seconds for any page load
• Latency: user must be able to check the account balance within 1 s
• Speed: download speeds should be at least 50 Mbit/s
• Efficiency: Optimal use of resources like CPU, memory, and network.
• Reliability: System uptime of 99.9% per month
• Consistency
• Availability: the online store must be available to the users at least 99.9% of the time
• Scalability
• Security: Passwords must be encrypted
• Maintainability: System should allow updates without downtime
• Flexibility
• Deployability: the dev must be able to deploy a new version at least twice a week
• Usability: Interface should be intuitive for first-time users
• Portability: Ability to run the system in different environments or platforms.
• Interoperability: Ability to interact and exchange data with other systems
• Testability
• Observability