Example of elixir runtime configuration

application.ex

defmodule Huddle.Application do
  @moduledoc false

  use Application

  def start(_type, _args) do
    children = [
      Endpoint,
      Repo,
      {RedisClient, redis_config()}
    ]

    opts = [strategy: :one_for_one, name: Huddle.Supervisor]
    Supervisor.start_link(children, opts)
  end
  
  def redis_config() do
    env = System.get_env()
    
    [
      pool_size: env["APP_REDIS_POOL_SIZE"] || 20,
      host: env["APP_REDIS_HOST"] || "localhost",
      port: env["APP_REDIS_PORT"] || "5432",
      database: env["APP_REDIS_DATABASE"] || 1,
    ]
  end
end

endpoint.ex

defmodule Endpoint do
  use Phoenix.Endpoint, otp_app: :app_name

  def init(_, config) do
    http_config =
      config[:http]
      |> Enum.map(&update_port/1)

    config = put_in(config, [:http], http_config)

    {:ok, config}
  end

  defp update_port({:port, _}), do: {:port, port()}
  defp update_port(config), do: config

  defp port do
    name =
      Node.self()
      |> Atom.to_string()
      |> String.replace(~r/@.*$/, "")
      |> String.upcase()

    env = System.get_env()

    String.to_integer(env["#{name}_PORT"] || env["PORT"] || "4000")
  end
end

repo.ex

defmodule Repo do
  @moduledoc false

  use Ecto.Repo,
    otp_app: :app_name,
    adapter: Ecto.Adapters.Postgres

  def init(_type, config) do
    config = Keyword.merge(config, runtime_config(), &update_config/3)

    {:ok, config}
  end

  defp runtime_config do
    env = System.get_env()

    [
      hostname: env["APP_PG_REPLICA_HOST"],
      database: env["APP_PG_REPLICA_DB_NAME"],
      username: env["APP_PG_REPLICA_DB_USER"],
      password: env["APP_PG_REPLICA_DB_PASSWORD"],
      pool_size: String.to_integer(env["APP_PG_REPLICA_POOL_SIZE"] || "30")
    ]
  end
end