Backend & systems·Consistency and replication
the network between your two datacentres broke and you had to decide whether to refuse writes or accept divergence.
CAP theorem
The result that a distributed store cannot be simultaneously consistent and available while the network is partitioned. In practice it is a statement about what you do during a partition, not a menu of three options you pick two from: partitions happen, so you are choosing C or A. The everyday version of the question is whether your failover refuses writes or accepts them on both sides.