If you use OTP (gen_server, etc), your code is a set of callbacks that return promptly, so it never stays "old" until the next upgrade. Your callbacks produce

If you use OTP (gen_server, etc), your code is a set of callbacks that return promptly, so it never stays "old" until the next upgrade. Your callbacks produce and use a "state" object (opaque to OTP) that OTP passes around[1], and OTP calls a code_change method whenever your module changes, so you can change the format of this object between versions. The gen_server itself passes its module name to sys:handle_system_msg[2] when it receives a system message like a code-change notification, which ends up being the Module:Function type of call that always refers to the new version of the module, so it always ends up running the latest version when you upgrade gen_server. [1] http://www.erlang.org/doc/man/gen_server.html#Module:init-1 http://www.erlang.org/doc/man/gen_server.html#Module:init-1 [2]file:///C:/Program%20Files/erl5.9.1/lib/stdlib-1.18.1/doc/html/sys.html#handle_system_msg-6