100% reliability needs to also be measured by usage. It's easy to get that if you have 1 or 10 customers vs a hundred customers. How many unique customers & transactions were you seeing?

Even the ridiculously-conservative telecoms measure their uptime in "nines". Four-nines, five-nines, whichever. "100%" is not a meaningful target.

> That doesn't mean it doesn't suffer from failures in one or multiple AZs or that it is perfect

If "100% reliability" doesn't mean 100% uptime for all users, then what does it mean?

"Several years" of uptime, sample size of one. Most people here probably have systems like that, but would never have the chutzpah to believe that they'd totally engineered away the potential for downtime.

How often are changes made to the system?

My laptop's hard drive had 100% reliability for upwards of 6-8 years.

I'm still making backups.