Hello.

I made an online status indicator function with elixir.
It manages online status of users with database and channels.

Codes and a table and json examples

online_channel.ex

defmodule PappapWeb.OnlineChannel do
  use Phoenix.Channel

  require Logger

  alias Pappap.Accounts

  def join("online", _payload, socket) do
    {:ok, socket}
  end

  def handle_in("online", payload, socket) do
    %{"sender" => sender} = payload

    case Accounts.get_user_by_user_id(sender) do
      [] ->
        Logger.info("Unknown user")
      user ->
        user
        |> hd()
        |> Accounts.update_user(%{is_online: true})
    end

    broadcast!(socket, "online", %{user: sender})
    {:noreply, socket}
  end

  def handle_in("offline", payload, socket) do
    %{"sender" => sender} = payload

    case Accounts.get_user_by_user_id(sender) do
      [] ->
        Logger.info("Unknown user")
      user ->
        user
        |> hd()
        |> Accounts.update_user(%{is_online: false})
    end

    broadcast!(socket, "offline", %{user: sender})
    {:noreply, socket}
  end
end

database

# select * from users;
 id | user_id | is_online |     inserted_at     |     updated_at
----+---------+-----------+---------------------+---------------------
  1 | 1       | t         | 2020-08-05 04:25:55 | 2020-08-05 11:03:36
  2 | 2       | f         | 2020-08-05 04:25:55 | 2020-08-06 14:51:06
(2 rows)

json

{"topic":"online", "ref":1, "payload": {"sender": "2"}, "event":"phx_join"}
or
{"topic":"online", "ref":1, "payload": {"sender": "2"}, "event":"online"}
or
{"topic":"online", "ref":1, "payload": {"sender": "2"}, "event":"offline"}

The server changes is_online by a json sent from a user through websocket. When user 2 joins a topic online, he need to send 2 json data.

{"topic":"online", "ref":1, "payload": {"sender": "2"}, "event":"phx_join"}
{"topic":"online", "ref":1, "payload": {"sender": "2"}, "event":"online"}

The former one just means “user 2 joined topic online”, and the latter one means like “user 2 is online”.
And then, is_online turns true by the latter json. The former one does not have meanings more than “user 2 joined topic online”.

It was about how to turn online. Then, the next is about how to turn offline.

To be offline in the system, a user has to send json before leaving a channel and disconnecting from the server. This is the json.

{"topic":"online", "ref":1, "payload": {"sender": "2"}, "event":"offline"}

It means like “user 2 is offline”.
handle_in("offline", payload, socket) works when the json is sent, and is_online of user 2 turns false.
With those processes, the system can manage online status of users.

But I think the system has a problem.

It cannot manage online status without handle_in/3. If a client get disconnected from the server without sending json. The online status does not turn offline. So I want to have the system send json at the disconnecting event. But I don’t think phoenix can get message at disconnecting.

If there’s a better way of managing online status, teach me how!
(Let me know my English feels weird, because I’m from Japan)