Fair enough I guess. I don’t think you need two servers to do zero downtime upgrades. And the other issues are, imo, beyond the 0.99 uptime threshold that most services realistically have when you add in breakage due to upgrades.
I like your statelessness point. I suppose in your view it’s better to have the concentrated stateful core with stateless servers as opposed to just one stateful instance. Two instances mean you can’t easily store foo in memory and hope the server doesn’t die until it’s not needed there anymore. Counterpoint is that the extra layer of indirection is 10x slower and horizontal scaling won’t be needed as much if you don’t pay that price in the first place, but you are right, the temptation to store foo in memory would still be in its prime. The thing is, if one machine can scale, putting foo in memory isn’t actually bad. It’s only when things don’t scale that it’s bad.
> I don’t think you need two servers to do zero downtime upgrades
Absolutely not and I can't understand why I keep hearing this argument. Doing zero downtime upgrades on a single server have been simple since basically forever, run another process on another port, change config, restart front balancer gracefully and there you go.
I like your statelessness point. I suppose in your view it’s better to have the concentrated stateful core with stateless servers as opposed to just one stateful instance. Two instances mean you can’t easily store foo in memory and hope the server doesn’t die until it’s not needed there anymore. Counterpoint is that the extra layer of indirection is 10x slower and horizontal scaling won’t be needed as much if you don’t pay that price in the first place, but you are right, the temptation to store foo in memory would still be in its prime. The thing is, if one machine can scale, putting foo in memory isn’t actually bad. It’s only when things don’t scale that it’s bad.