# System - Availability

Availability is the (elimination|absence) of downtime.

Availability is when a system still work after a node (computer) failure.

Availability means that you can always read and write to the system.

Availability is a SLI resources metrics that shows the percentage of time that the resource responded to requests.

In the real world, there is no such thing as 100% availability. Highly available systems are defined in terms of 9s (99.9%, 99.99%, …). The more 9s, the better.

No system can guarantee availability.

## 3 - High

A high availability requirement implies that the system must replicate data.

## 4 - Function

Availability is a combination of:

• how often things are unavailable - MTBF (mean time between failures)
• and how long they remain that way - MTTR (mean time to recovery).

$$\frac{MTBF}{MTBF+MTTR}$$

Visualisation. See desmos calculator

$$f\left(x,y\right)=\frac{y}{y+x}\left\{x\ge0\right\}\left\{y\ge0\right\}$$