For a project I was working on, I needed a no-nonsense way to run simple code when my application, or the application of a user of this library, was starting. This is the simple method I came up with to run a function on application start.

This particular case imports certificates into the cacert store, but it can be extended to do whatever your application requires.

defmodule ApplicationStart do
    use Application
    
    @cacert_path Path.join(:code.priv_dir(:myapp), "cacert.pem")
    
    # {:error, reason} is returned if there is an issue loading CA Certs, which also matches
    # the return type of the Application `start` callback
    def start(_type, _args) do
        with :ok <- :public_key.cacerts_load(@cacert_path) do   # <-- The work being done
            Supervisor.start_link([], strategy: :one_for_one)   # <-- Applications starts
        end
    end
end

The solution above takes advantage of the Application.start/2 callback. In this case, I needed to run :public_key.cacerts_load(@cacert_path). It happens that the error tuple from this operation also matches the expected error tuple of the start callback, but if your startup logic is more complicated, ensure that you conform to the format expected by the callback contract.

Finally, register your module in the application function in mix.exs, as so:

  def application do
    [
      extra_applications: [:logger],
      mod: {ApplicationStart, []}    # <-- Module name where Application was implented, see above
    ]
  end

And that’s it!