Availability is the percentage of the time that a system is up, functional, and reachable. Crashes, network failures, or extremely high load (for example, from a DDoS attack) that prevents the system from responding can all affect its availability.
Usually, it's a good idea to strive for as high a level of availability as possible. You may stumble upon the term counting the nines, as availability is often specified as 99% (two nines), 99.9% (three), and so on. Each additional nine is much harder to obtain, so be careful when making promises. Take a look at the following table to see how much downtime you could afford if you specified it on a monthly basis:
| Downtime/month | Uptime |
|---|---|
| 7 hours 18 minutes | 99% (“two nines”) |
| 43 minutes 48 seconds | 99.9% (“three nines”) |
| 4 minutes 22.8 seconds | 99.99% (“four nines”) |
| 26.28 seconds | 99.999% (“five nines”) |
| 2.628 seconds | 99.9999% (“... |