Phoenix channels: send push to a particular client

Question

Hi I am trying to reimplement whatsapp functionality using elixir phoenix. I am having a problem figuring out the following: if all people in the chat room has received the message, I want to send the owner of the message status "received" so that he can show double tick sign. However how do you broadcast to one particular client?

Check up this question: http://stackoverflow.com/questions/37373766/phoenix-return-ecto-query-results-to-a-specific-client/37375057#37375057 — simo, Jun 08 '16 at 10:58
@simo The question is a bit off, because there client A sends the message and receives the message as well. What I need is: client A sends the message and only client B receives some feedback — almeynman, Jun 08 '16 at 11:51
I think its a good idea to dedicate a topic for the client B only, then you can broadcast to that topic, otherwise I don't know how to exclude a specific user, have a look here: http://stackoverflow.com/questions/37279779/how-to-exclude-some-users-when-broadcast — simo, Jun 08 '16 at 12:05
@simo I see. Dedicating seems quite complicated, especially in a group chat, where each person need his separate topic. I was just thinking that each socket has an id. and If I can contruct socket from id I can push messages at it. However I do not know how to do that — almeynman, Jun 08 '16 at 12:21
Dedicating a topic for each user is easy, use the pattern matching for the topic name, as `def join("users:" <> user_id, _params, socket) do` — simo, Jun 08 '16 at 12:34
@simo Actually I might go with your solution. I was just thinking that having a separate topic just for that small use case would be overkill. But I might need it anyways for other stuff. Thanks for help — almeynman, Jun 08 '16 at 12:51
I don't know if having a topic per each user is would be overkill or not? I am not sure, although I am applying this in my project, after all, its a process for each user.. erlang will do the heavy lifting, but I didn't test that, I will test it using TSung tool if you know it, its very good to test with phoenix.. — simo, Jun 08 '16 at 14:00
@simo never heard of it, but will give it a try :). You can write an answer about user specific topic and I will accept it. I actually needed user specific topics anyways, so it is natural fit in my case — almeynman, Jun 09 '16 at 07:29

score 13 · Accepted Answer · edited Jan 04 '21 at 12:54

You can solve this via topic per user, this can be easily implemented by pattern matching, notice the security validation also:

def join("users:" <> user_id, _params, socket) do
    {user_id, _} = Integer.parse(user_id)

    %{id: id} = socket.assigns[:user]

    #prevent connection to solo channel of other users, but allow in development
    case id == user_id || Mix.env == :dev do
      true ->
        {:ok, socket}
      false ->
        {:error, "This is not your solo channel!"}
      end
  end

As you would have stored the user from Repo.get when the user connect to the socket at:

defmodule MyApp.UserSocket do
  use Phoenix.Socket

  def connect(%{"token" => token}, socket) do
    case Phoenix.Token.verify(socket, "user", token, max_age: 1209600) do
      {:ok, user_id} ->
        socket = assign(socket, :user, Repo.get!(User, user_id))
        {:ok, socket}
      {:error, _} -> #...
    end
  end
end

And finally, you can send messages to a specific user out of socket context as:

YourAll.Endpoint.broadcast user_topic, "message", %{details: "etc"}

To test the performance, this is a very informative session in which Gary Rennie shows how to benchmark WebSockets using Tsung tool.

Phoenix channels: send push to a particular client

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