Service availability ($A=T_{\mathrm{up}}/T$)

For an observation period of duration $T>0$, let $T_{\mathrm{up}}$ be the total time during which a service meets a specified condition. Its measured service availability is $A=T_{\mathrm{up}}/T$. The condition must say which operation can be performed: accepting a new user and serving an already admitted user can have different dependencies. A mandatory service dependency can reduce service availability even when all data copies remain intact.