It is, in fact, not the same for every DB. Databases vary significantly in their default and maximum guarantees for successfully acknowledged writes; Mongo chooses fairly weak ones. By contrast, consider systems like VoltDB, which aims for strict serializability by default, or etcd, which offers linearizability/sequential reads by default.
> etcd, which offers linearizability/sequential reads by default
Not quite true - you have to opt-in to quorum reads in etcd. Otherwise it just returns the first read (which used to be the default in k8s and caused a lot of grief).
Ah, yes, sorry! I forgot exactly how that all shook out. I thiiink that prevents dirty reads, and with client monotonicity you can recover sequential consistency? Not sure how many clients actually do that; maybe they just do regular reads at RC...
