The way I have things setup, the app servers are independent from the database. Meaning an instance that is on the app server 1 may be on the database server 5 and another on the same app server maybe on database server 2. See https://masto.host/infrastructure/
When I run the upgrades, I run them instance by instance but on all app servers simultaneously. v3.5.0 upgrade is very database intensive and can take several minutes on larger instances.
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