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?

Answer 1

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.