Elixir 1.9 provides runtime config. When set it up by providing a config/releases.exs, it will copy this file to _build/prod/rel/APP/releases/VSN/release.exs and when start the release via bin/App start_iex, it will first start beam, then restart beam within the same os process, then starting again.

I’m not quite sure what’s the reason behind restarting beam? Here’s my guess:

1. When starting beam, kernel application will start application_controller process, who will load all env’s in sys.config, which is generated via the config/config.exs(this is the compile time config). All the application’s env are stored in an ets table called ac_tab.
2. After all application started, some code will load the configs in config/releases.exs, and update ac_tab with runtime config
3. Then a restart beam command is issued, maybe via :init.restart(), all the application is shutdown
4. Then all the application is starting again, but this time with runtime config inserted in ac_tab, so the application will have access to the runtime config when they starts

Is that why beam needs to restarts when runtime config is provided?