[jzwinck]

I thought the exact same thing, but then I realized that we don’t need Redis to actually support a choice of two passwords for a single account. Rather, clients can be configured with a list of credentials to try. When rolling credentials over, simply add the new ones to the clients’ lists, update the service, then remove the old ones from the clients. Then you can wait hours or days between steps for safety, and there is no time when system reliability is degraded by a service instance being inaccessible.

[andrewvc]

That's pushing complexity back onto app devs. Functionality like that is best solved once in the database for everyone.