Availability math
| Availability | Downtime / year | Downtime / month |
|---|---|---|
| 99% | 3.65 days | 7.3 h |
| 99.9% | 8.8 h | 44 min |
| 99.99% | 53 min | 4.4 min |
| 99.999% | 5.3 min | 26 s |
- Components in series multiply: two 99.9% services both required → 99.8%.
- Redundant components in parallel: 1 − (1 − a)² → two 99% boxes ≈ 99.99%.
Every extra synchronous dependency lowers your ceiling. Redundancy raises it.