I’m trying to figure out the best way to handle an env var that MUST be present at runtime but not at compile time. The following is used to generate this env var:

  defp vault_key(_) do
    vault_key = :base64.encode(:crypto.strong_rand_bytes(16))
    IO.puts("#{vault_key}")
  end

Effectively I have an env var called VAULT_KEY which must be present at runtime to encrypt and decrypt values in mnesia, but I don’t want to require it at compile time, as if it’s unset, you can’t fire off this mix task to generate and export the key in the first place. Using System.get_env("VAULT_KEY") allows it to compile, but of course sets it to nil if the env var wasn’t set prior to compiling, which is not the desired behavior. Using System.fetch_env!("VAULT_KEY") requires the key to be present, but won’t allow for compilation if not present. The behavior I’m trying to accomplish here is that if the key isn’t present at compile time, thats’s fine, but if it’s not present at runtime then I the application should bail and fail to start. And I wan’t to apply that to all env’s (dev, test, staging, prod). Any thought’s or pointers would be greatly appreciated.