How to understand the "Availability" of the CAP theorem? As we know, in the CAP theorem, "A" means "Availability". On wikipedia, the explanation of "Availability" is:


  Availability: a guarantee that every request receives a response about
  whether it was successful or failed


However, from the perspective of engineering, there is no ABSOLUTE availability. We can only say that the availability of a system is 5'9'(99.999%), or even 8'9', but we cannot say that the availability of a system is 100%, 100% available system does not exit in reality, even if the system has millions of duplicated nodes, right?

The CAP theorem proves that no system could satisfy the 3 requirements simultaneously. My question is, if a system claims to satisfy both "A" and "P", what is the accurate meaning of this "A"? 6'9' or even higher?
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.