How to configure Ecto at runtime?

Question

Following the setup instructions, I have the following Ecto configuration in my config/config.exs file :

config :my_app, MyApp.Repo,
  adapter: Ecto.Adapters.Postgres,
  url: "postgresql://postgres@localhost/myrepo",
  size: 20

If my understanding is correct, the config.exs is evaluated at compile-time.

Is there a way to do this configuration step at runtime ?

This is for an app which will be distributed as a compiled binary (via exrm). The end-user should be able to customize the database url and pool size via flags or environment variables, not by editing sys.config

Answer 1

Loading from the system is possible by using {:system, "KEY" } e.g.:

config :my_app Repo
   url: {:system, "DATABASE_URL" },
   size: {:system, "DATABASE_POOL_SIZE" }

instead

config :my_app, Repo,
   url: "ecto://postgres:postgres@localhost/ecto_simple",
   size: 20

In this case you set up Ecto to use the system properties. Of course, a user has to configure it.

Answer 2

Using {:system, "KEY"} has been deprecated in Ecto v3.

Instead, you are advised to define an init/2 callback function within your Repo module to set runtime config:

def init(_type, config) do
  config = Keyword.put(config, :url, System.get_env("DATABASE_URL"))

  {:ok, config}
end

Using a runtime init/2 function allows configuration to be read from more than just environment variables.