Availability numbers
Play this lessonThis lesson comes with an interactive animation you can break, a live-code challenge and a chapter quiz.High availability is a system's ability to keep running for a long time. We express it as the percentage of time the service is up: 100% would mean it never goes down, and real services land somewhere between 99% and 100%.
A Service Level Agreement (SLA) is a formal promise from a provider to its customers about uptime. Amazon, Google and Microsoft set theirs at 99.9% or higher. Uptime is counted in "nines": the more nines, the better.
| Availability | Downtime per day | Downtime per year |
|---|---|---|
| 99% | 14.40 minutes | 3.65 days |
| 99.9% | 1.44 minutes | 8.77 hours |
| 99.99% | 8.64 seconds | 52.60 minutes |
| 99.999% | 864 milliseconds | 5.26 minutes |
| 99.9999% | 86.4 milliseconds | 31.56 seconds |
Key takeaways
- Each extra nine cuts downtime by 10×.
- SLAs are promises about uptime; cloud providers offer 99.9%+.