@callback handshake(transport :: term(), socket :: term()) ::
  {:ok, auth :: Kvasir.PubSub.credentials(), state :: term()} | {:error, atom()}

@callback port() :: pos_integer()

@callback run(
  transport :: term(),
  socket :: term(),
  state :: term(),
  server_state :: Kvasir.PubSub.server_state()
) :: :ok

@callback secure_port() :: pos_integer()