Ah, my apologies, because when you said "The holy grail of code reloading is to upgrade the code of a HTTP server while it is running and without disturbing any requests being processed." I thought you were referring to the ability of the BEAM VM to also have multiple versions of the same gen_* running, so that old requests can still be running on the original code while the new requests come in on the new code. This is in contrast to the things that came to mind like Python, which technically can replace a function reference live, but there's no such ability to partition who's in the old vs. new space like that, so it's too dangerous to be practically used (or at least not without a lot more supporting code).

BEAM's really got it all on this front.