Comparison
Eventual consistencyvsStrong consistency
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 →Strong consistency
every read is guaranteed to see the most recent committed write, so you never have to explain why the value went backwards.
A model where reads reflect all prior writes, typically by routing reads to the leader or requiring a quorum. It removes an entire class of "it works on my replica" bugs and costs latency, availability during failover, and cross-region round trips. Most systems need it for a small subset of their data and treat the rest as eventually consistent.
Full entry →