Comparison
CAP theoremvsEventual consistency
CAP theorem
the network between your two datacentres broke and you had to decide whether to refuse writes or accept divergence.
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.
Full entry →Eventual consistency
you write to one node, read from another a moment later, get the old value, and it is correct a second after that.
A guarantee that replicas converge if writes stop, with no bound stated on how long that takes. It is what asynchronous replication gives you, and it is fine for most read paths. It is not fine anywhere a user reads back their own write, which is why read-your-writes is usually bolted on top rather than assumed.
Full entry →