defmodule MemcachedDiscovery.DNS do
  @moduledoc """
  Discovers memcached nodes via a DNS query against
  an A-record (for example against a kubernetes headless
  service)
  """
  @behaviour Herd.Discovery
  require Logger

  def nodes() do
    service = service_name()
    case :inet_res.getbyname(service, :a) do
      {:ok, {:hostent, _fqdn, [], :inet, _value, addresses}} ->
        addresses
        |> Enum.map(&:inet_parse.ntoa(&1))
        |> Memcachir.Util.parse_hostname()

      {:error, reason} ->
        Logger.error("lookup against #{service} failed: #{inspect(reason)}")
        []
    end
  end

  def service_name(),
    do: Application.get_env(:memcachir, __MODULE__)[:dns]
end