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A CA system is simply one which is not available at all during a network partition, since it is partition-intolerant.

This lack of availability is different from the availability in the A of CAP, since that availability holds only so long as the network is not partitioned (by definition in a CA system).

Such a system might not be considered a distributed system at all (although it may still be distributing load), since a partition-intolerant system is effectively one system as far as the CAP theorem is concerned.

So it's essentially a special case of the CAP theorem, but it is still useful to describe it as CA.

No, it's exactly the same. Availability is a guarantee that all requests are eventually responded to within some time bound, whatever that is. During the partition, availability is violated.

Therefore it's not a CA system, but a C system.


Are you sure? Availability in the CAP theorem is a state, as are (P)artition and (C)onsistency. Your system can't be simultaneously consistent and available in the presence of a network partition. The A in CAP doesn't mean always available. It just means the system can, at best, be any two of the three at a time.


No, it does mean always available - honestly :)

If there is some time period during which requests are not responded to within a time bound, the system is not available then, and further is not a 'highly' or 100% available system. That is what the CAP theorem is talking about.

Consistency, similarly, is not a state but a property that holds across all responses. Either you return a consistent response to all your requests, or you don't. In the context of CAP, there is no middle ground.


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