[tel]

Having done this both with Clojure and Erlang, I find the unversioned, dynamic mode of achieving this is fraught with trouble. It's really easy to end up in complex atypical states during upgrade which can at best be hard to debug and worst corrupt other, longer term state.

[jamii]

Exactly. I think what a lot of people miss is that erlang makes live upgrades possible but certainly not easy. Trying to upgrade a complex application one component at a time (the way erlang/otp promotes) is still insanely hard and not worth the pain unless you're working in a domain where you have absolutely no choice (which is of course why Ericsson developed erlang in the first place). For the vast majority of applications the right choice is to restart the server and accept a few seconds downtime.

[tel]

I think with better documentation I'd actually call Erlang's model "easy" in that its really easy to do things correctly with absolutely minimum chance of corruption/failure/non-repeatability. OTP gives you the tooling—releases, versions thereof, and simultaneous multiple module tenancy being key—to actually do a good job. That's a rare thing.