Functional vs non-functional requirements

What you take away

• Functional requirements say what the system does; non-functional requirements say how well, and they are what choose the architecture.
• Nine NFRs cover almost everything: scale, latency, availability, consistency, durability, retention, cost, security and geography.
• An SLI is what you measure, an SLO is the target you hold yourself to, an SLA is the contract with consequences.
• Every extra nine of availability multiplies cost and removes design options, so it has to be justified, not assumed.

How the class is laid out

1. The problem
2. The simple version
3. The nine that matter
4. Check your understanding