The primary replica is constantly monitoring the state of it’s secondaries. With the use of a continuous ping, the primary node always knows if the secondaries are up or down.
It’s when SQL detects that one of it’s synchronous replicas goes offline is when interesting things can happen.
So here’s the discussion that came up, if a synchronous replica goes offline for whatever reason, SQL won’t be able to commit any transactions and that means we can be confident that the secondary is up to date, right?
Read on to learn the answer. Which is “no.” But David explains why, so you should read that instead of just having me say it.