Consistency models

A consistency model is the contract a storage system gives you about what reads can return when many clients read and write the same data at once, often through different replicas. The strongest common one, linearizability, makes the system behave as if there were a single copy and each operation took effect instantly at some moment between its start and finish, so once a write completes, everyone sees it. Weaker models relax this: sequential consistency keeps one agreed order but may ignore real time, causal consistency only promises that causes are seen before their effects, and eventual consistency only promises that replicas converge once writes stop. Weaker models are faster and stay available more often, but push more surprises onto the application, like a reply showing up before the post it answers.