The changes happen on two different levels.
Internally: it means that Ecto will internally expect configurations to be set at runtime rather than compile time.
Externally: it means that Ecto will provide a proper API to do runtime configuration. The only way to do runtime configuration in Ecto 2.0 was by using url: {:system, "DATABASE_URL"} which has two big problems: it only works for the :url parameter and it only allows the configuration to be read from the system environment. If you need to read it from elsewhere, you are out of luck.
Ecto 2.1 now provides the init/2 callback inside your repo for runtime configuration:
def init(_, config) do
{:ok, Keyword.put(config, :url, System.get_env("DATABASE_URL"))}
end
Now you have the flexibility to do whatever you want. Also note Ecto allows the configuration to be given at the moment you call start_link on the Repo.
