A measure, a target, and a promise
- Uptime: The share of time a service is available, usually shown as a percentage.
- SLO (service level objective): A target, such as 99.9% uptime in a month.
- SLA (service level agreement): A formal customer promise that may offer a bill credit if the agreed level is missed.
What the numbers leave out
In a 30-day month, 99.9% uptime allows about 43 minutes of downtime; 99.99% allows about 4 minutes. A higher target may need spare systems and faster recovery. Uptime alone can miss a broken checkout: a website may load while customers cannot pay.