How can a distributed system be consistent and available - CA. Because when partition happens CA is not possible. And if we say partition wont happen then only they are CA, then all systems which are CP or AP will also be CA in case partition wont happen?
3 Answers
Dr. Stonebraker says: The guidance from the CAP theorem is that you must choose either A or C, when a network partition is present. As is obvious in the real world, it is possible to achieve both C and A in this failure mode.
See this for thoughts on why CA can exist:
CA is a specification of the operating range: you specify that the system does not work well under partition or, more precisely, that partitions are outside the operating range of the system.
My background is far from these theoretical considerations and I must say it is highly confusing. I am researching distributed Blockchain systems and I don't see why those "generalized" definitions of C, A, P must always apply. If let's say 5% of nodes fail or are otherwise partitioned, the consensus still functions. If an end user is connected to a partitioned node, the node could let the user know it lost connection. I don't even see how any major Blockchain network is CP without defining conditions such as "if a certain amount of nodes fail or get partitioned, the consensus halts".
It can, but it won't.
The CAP theorem reasons about guarantees when one or more nodes get isolated from the rest of the cluster. In such cases a node has three options which result in the three known CAP trade-offs: i) it keeps responding to any received requests AP; ii) it no longer responds to received requests until it is again able to reach the others CP; iii) it shuts down before receiving any requests to eliminate the partition along with it CA.
In other words you can achieve CA by having your nodes shutting down instead of tolerating the partition but bear in mind that partitions are likely to keep happening hence this will converge to the scenario in which you have a single node in your cluster and I assume this is the opposite of what you want, i.e. having a cluster with multiple nodes is kind of the whole point.
Therefore in practice you end up choosing between CP and CA. See this answer for more illustrative examples.
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