= Service availability
{title2=$A=T_{\mathrm{up}}/T$}
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.
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