I am honestly kinda curious how exactly you manage your servers and keep them up to date, I feel like there is a linux patch every week or so, which would usually require a reboot. Do you all deploy live patching, how you become aware of critical stuff in your otherwise busy lives, RSS?
How do you handle the database schema changes during updates? Have two databases and disable replication during the update? How do you sync changes that occur to the backup while/after the main is upgraded?
I think OP is asking about system updates. Zero time application deployment is a different thing.
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From my experience database schema changes require for all connections to drop, but they tend to happen a lot less compared to other updates.
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Not much of an addition, but you’re absolutely right, in most systems that are expected to be highly available, there’s standard maintenance times, an agreement in place, and no critical use of the system is permitted to be scheduled in that regular time period. Any deployments are limited to that window, in case a rollback is necessary, data sync, etc.
All of that is in addition to the type of high availability stuff you’re describing.