[toast0]

It really depends on how much effort is involved in setting up the connections, and how much effort is involved in directing connections to a new host.

If I'm running 1M sessions per machine, and each machine can handle 10k new connections / second, rolling restart is really expensive. If I'm running 10k sessions per machine, and can handle 10k new connections / second, rolling restart isn't too bad. This generalizes to really anything that gather essentially ephemeral state, but that state is costly to gather (tcp flows in this case, data caches in others, etc).

BEAM excels at applications with huge numbers of sessions per machine, which is why some users really value hot reloading.

Editted to add -- the OTP application update sequence doesn't necessarily need to be used. Where I work, we certainly don't do that. Just a little bit of logic around code:soft_purge/1 and code:load_file/1