never ever ever let your app servers and your DB be in different regions
it's so, so much worse than you think
"but i wanted app servers more central to users, and my DB was hard to move" does not hold up. 1 app <> DB roundtrip, you're back to breakeven; 2 and you're behind.
you'd think it should be rare, right, but i have somehow seen it 3 times in my career!
seemingly a far more common mixup than i would have ever expected
cannot believe how many times i've seen app servers making 60+ms roundtrips to the DB
app servers closer to users only helps if there's no talking to the DB. even for only 1 DB roundtrip, pythagorean theorem says put them next to each other!
your DB is the center of the universe; put your app servers next to it, and make everyone travel to that place, even if it's less central
it's faster to make requests travel a little further once, than to have app servers travel the same extra distance (or more) multiple times!
