Of what you listed, only the telephone network is one where I don't recall experiencing downtime; probably because it degrades gracefully to reduced functionali

Of what you listed, only the telephone network is one where I don't recall experiencing downtime; probably because it degrades gracefully to reduced functionality if something fails. Can't say much about air traffic control, but I imagine they have failures too and just have backup protocols in place when primary systems fail. A payment system is probably the closest to a computer system where you really don't want to drop any incoming requests, though after the initial payment event has been recorded, the behind-the-scenes processing can tolerate quite a lot of delay in the worst case. Networks certainly fail all the time, and power delivery issues aren't uncommon either; the downtime just tends to be localized and if you depend on your internet connection or power, you have backup links and UPS systems that reduce or avoid the impact of downtime. I never said you shouldn't strive for high availability, especially if you're moving vast amounts of customer traffic, I specifically made the argument against "no downtime", because people often seem to think that if your system doesn't have five nines of uptime it's unsuitable for handling "real" traffic, and that to achieve enough high availability you somehow need a highly complex system.