ha-handler/lib/ha_handler/pgsql/watcher.ex

defmodule HAHandler.PGSQL.Watcher do
  use GenServer
  require Logger

  def start_link(opts) do
    GenServer.start_link(__MODULE__, opts)
  end

  @impl true
  def init(opts) do
    # Starts a Postgrex child but does not means the connection was
    # successful.
    # TODO: set dbconnections backoff and connect hooks
    # See https://github.com/elixir-ecto/db_connection/blob/master/lib/db_connection.ex#L343
    case Postgrex.start_link(opts) do
      {:ok, pid} ->
        state = %{
          backend: pid,
          hostname: Keyword.get(opts, :hostname)
        }

        {:ok, state}
      {:error, err} ->
        # Will be catched by the supervisor if anything happen. It should not
        # be triggered even if a PGSQL node down, since Postgrex has its own
        # surpervision tree.
        {:error, err}
  end

  @impl true
  def handle_call({:execute, query, params}, _from, %{backend: backend} = state) do
    {:reply, Postgrex.query(backend, query, params), state}
  end
end