Availability
What is availability?
Availability is the percentage of time that some service or infrastructure is accessible to clients and is operated upon under normal conditions. For example, if a service has 100% availability, it means that the said service functions and responds as intended (operates normally) all the time.
可用性（Availability） 指的是某项服务或基础设施在正常条件下可供客户端访问并正常运行的时间比例。例如，如果某项服务的可用性为 100%，这意味着该服务始终按照预期运行并响应请求（即始终正常运行）。
Measuring availability
Mathematically, availability, A, is a ratio. The higher the A value, the better. We can also write this up as a formula:
$$
A (in \ percent) = \frac{ (Total \ Time − Amount \ Of \ Time \ Service \ Was \ Down)}{Total \ Time} * 100
$$
We measure availability as a number of nines. The following table shows how much downtime is permitted when we’re using a given number of nines.
The Nines of Availability
| Availability Percentages versus Service Downtime | |||
|---|---|---|---|
| Availability % | Downtime per Year | Downtime per Month | Downtime per Week |
| 90% (1 nine) | 36.5 days | 72 hours | 16.8 hours |
| 99% (2 nines) | 3.65 days | 7.20 hours | 1.68 hours |
| 99.5% (2 nines) | 1.83 days | 3.60 hours | 50.4 minutes |
| 99.9% (3 nines) | 8.76 hours | 43.8 minutes | 10.1 minutes |
| 99.99% (4 nines) | 52.56 minutes | 4.32 minutes | 1.01 minutes |
| 99.999% (5 nines) | 5.26 minutes | 25.9 seconds | 6.05 seconds |
| 99.9999% (6 nines) | 31.5 seconds | 2.59 seconds | 0.605 seconds |
| 99.99999% (7 nines) | 3.15 seconds | 0.259 seconds | 0.0605 seconds |
Availability and service providers
Each service provider may start measuring availability at different points in time. Some cloud providers start measuring it when they first offer the service, while some measure it for specific clients when they start using the service. Some providers might not reduce their reported availability numbers if their service was not down for all the clients. The planned downtimes are excluded. Downtime due to cyberattacks might not be incorporated into the calculation of availability. Therefore, we should carefully understand how a specific provider calculates their availability numbers.
可用性与服务提供商 不同的服务提供商可能从不同的时间点开始计算可用性。一些云服务提供商会从首次提供该服务时开始计算，而另一些则会从特定客户开始使用该服务时计算。
某些提供商在服务并未对所有客户宕机的情况下，可能不会降低其报告的可用性指标。此外，计划内的停机时间通常不计入宕机时间，而由于网络攻击导致的宕机时间可能也不会纳入可用性计算。
因此，我们需要仔细了解具体服务提供商是如何计算其可用性数值的。