An alternative registry implementation in Elixir

simple_registry.ex

defmodule DriverRegistry do
  @moduledoc """
  Most key-value registries in Elixir can and perhaps should be created with a
  `GenServer`. This module illustrates an alternative implementation of a
  registry using macros that eliminates the need for a `GenServer`. This removes
  OTP dependencies like a supervision tree and also reduces unnecessary
  "process noise" in your application.

  The use-case for such a registry is a driver registry where consumers opt-in
  to drivers by declaring which drivers they would like to use in the
  `config.exs` file under the key, `:additional_drivers`.

  Note: each driver module implements a `behaviour` with a `callback` of
  `supported_schemes/0`, which returns a `[String.t]`. These are used to
  generate function heads at compile-time against which inputs are
  pattern-matched. This simulates an in-memory KV store.

  The key takeaway here is that `GenServer` processes are not always necessary.
  When a registry is populated on application initialization and remains fixed
  throughout the application's lifetime, a KV registry can be created with
  functions defined at compile time.

  """

  @default_drivers [Driver.Local, Driver.HTTP]
  @additional_drivers Application.get_env(:driver_registry, :additional_drivers)

  for driver <- @default_drivers ++ @additional_drivers do
    for scheme <- driver.supported_schemes() do
      def get_driver(unquote(scheme)), do: unquote(driver)
    end
  end

  def get_driver(scheme), do: raise("No driver registered that supports the scheme: \"#{scheme}\"")
end