Most database systems are canonically-online: the state lives on the instances, and you make backups of it, but these are never more canonical than what’s on the local online storage of the cluster (and usually less-so, because it’s offset back in time by at least a few seconds, if not hours.)
When a cluster-node permanent-faults (say, its DC burns down), you lose at least a few seconds of what you — and your customers — thought of as committed data.
In a canonically-at-rest DBMS, the only state that matters is the state in the object store (or other external, highly-replicated durable-storage abstraction.) The reads are an ephemeral caches in front of the canonical at-rest data; and all writes must be pushed down to the at-rest representation before any other nodes in the cluster can see them, and before the write returns as successful to the client.
When a cluster-node permanent-faults (say, its DC burns down), you lose at least a few seconds of what you — and your customers — thought of as committed data.
In a canonically-at-rest DBMS, the only state that matters is the state in the object store (or other external, highly-replicated durable-storage abstraction.) The reads are an ephemeral caches in front of the canonical at-rest data; and all writes must be pushed down to the at-rest representation before any other nodes in the cluster can see them, and before the write returns as successful to the client.