How to understand the "Availability" of the CAP theorem?
Your question is mixing two somewhat orthogonal interpretations of the term &quot;Availability&quot;.

The &quot;reliability theory&quot;/&quot;engineering&quot; interpretation, where availability is basically
the ratio of time the system is not failed.

The &quot;distributed computing&quot; interpretation, which is centered around the idea that

&quot;every request received by a non-failing node in the system must result in a response&quot;



So, let's consider a system of two nodes, that crash at midnight, and are resume operation at noon, every day.
So in the first interpretation we have 50% availability, that is, no nines at all. However, the system can still fulfill the second interpretation.