System Design · Junior · Question 41 of 75
What does "99.9%" (three nines) availability mean?
- ✓Correct answer: Allowed downtime is about 8.7 hours per year
- 2The service answers 99.9% of requests in under 100 ms
- 3The service runs at 99.9% of server capacity
Explanation
99.9% ≈ 43 minutes of downtime per month, 99.99% about 4 minutes. Each extra "nine" gets dramatically more expensive. SLOs (internal targets) and SLAs (commitments to customers) are stated in these terms, and an "error budget" helps balance release speed and reliability.