Best way to perform application level high availability authentication

I believe that the problem you will end up having, if you were to fallback on sqlite, is that for most applications you have so many MySQL joins and complex queries that you'd end up with a website that doesn't work unless you were to replicate the entire database, not just the user authentication. So you just want to let users submit trouble tickets, but if the entire website is down then it's pretty obvious you're going to be busy getting it back up, and having users submitting trouble tickets is going to be a side issue in that case. I'd be more worried about getting the whole website up, not dealing with the less important trouble ticket.

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