As suggested in [1], IMO, it is a reasonably good idea for 'synchronized_standby_slots' to DEFAULT to the value of 'synchronous_standby_names'. That way, even if the user missed to configure 'synchronized_standby_slots' explicitly, we would still have reasonable protection in place.
Hmm.
synchronous_standby_names contains application_names, while synchronized_standby_slots contains names of physical replication slots.
These are two different things, and in fact sync replication doesn't even require to use replication slots.
What is worse, even when all standbys use physical replication slots there is no guarantee that values in synchronous_standby_names will match physical slot names.
