depends who the customer is, I'm a customer of AWS and I expect 100% availability, mostly because my customers are everywhere in the world and there's no available window for downtime
If you have this 100% availability expectation you're going to have to face the reality that DBMS versions fall out of support, you will have to upgrade or AWS will force-upgrade you their way, the AWS-provided default mechanism has significant DB-size dependent downtime (in order to maintain consistency, and you really don't want to lose that), and that the only alternative is to go through the pain of sifting through your database estate and logically replicating table by table with verification as shown in this article, with care especially for large tables and reindexing, and you can't avoid that if you have the (IMO mostly unreasonable) expectation of 100% availability.
Change the wheel mid-journey or take a pitstop.
The article is entirely about tooling for safely changing wheels mid-journey. In that context, it's not weird to expect the database to remain available during updates.
Yes, it will require more work and time than just taking the database down and performing the update while it is offline. But as long as the database remains available it doesn't really matter if the update takes 5 minutes or 2 days, just that you can do changes faster than they appear. Since DBMS updates happen at most every few months, that should hopefully not be a problem.
At one of my previous workplaces we had a multi-TB table that could take several days to migrate with the online tooling, and would take 12+ hours to migrate even offline. Nobody wanted to take 12+ hours of downtime (for a busy customer-oriented website) but as long as the db stayed up nobody much cared how long it took.
If you expect 100% and you’re making business decisions based on that expectation, I encourage you to increase your sophistication about reliability before it costs you a great deal of money.
