pattern.ex

defmodule Features do
  use GenServer
  
  @features [chat: true]
  
  def enable(feature), do: GenServer.call(__MODULE__, {:toggle, feature, true})
  def disable(feature), do: GenServer.call(__MODULE__, {:toggle, feature, false})
  
  def enabled?(feature) do
    case :ets.lookup(__MODULE__, feature) do
      [] -> Keyword.get(@features, feature, false)
      [{_, enabled}] -> enabled
    end
  catch
    :error, :badarg ->
      # Table is missing
      Keyword.get(@features, feature, false)
  end
  
  def start_link(_), do: GenServer.start_link(__MODULE__, [], name: __MODULE__)
  def init(_) do
    :ets.new(__MODULE__, [:set, :public, :named_table, read_concurrency: true)
    for {feature, enabled} <- @features, do: :ets.insert(__MODULE__, {feature, enabled})
    {:ok, nil}
  end
  
  def handle_call({:toggle, feature, enabled}, _from, state) do
    :ets.update_element(__MODULE__, feature, {2, enabled})
    {:reply, :ok, state}
  end
end