Sorry I wasn't clear in my original post.

I'm thinking about the application layer. If you have an application that writes data to a table, it's typical to run multiple instances of that application to support scale and reliability requirements.

If I send a write to one instance, how does it communicate and synchronise that write with the other application instances?

I ask because this can be a tricky thing to do, especially when consensus is required, as consensus algorithms such as Raft/Paxos require a number of network roundtrips which will introduce latency, and actually account for much of that latency in the database examples given in some cases.