elixir - how to send message to different user in phoenix framework chat app -

i working phoenix framework create different type chat app. in case, have chat rooms not functioning normal chat room.

every user has own room can join room using different devices (mobile, pc, other sources).

user has own room , user b has own room, these 2 members not connect single room in normal scenario in real world.

now user wants send message user b

message data eg:

from :   :b   message : test message    

this snippet app.js used connect user's specific room :

let room = socket.channel("room:"+  user, {}) room.on("presence_state", state => {   presences = presence.syncstate(presences, status)   console.log(presences)   render(presences) }) 

this snippet back-end join room function
/web/channel/roomchannel.ex

  def join("room:" <> _user, _, socket)     send self(), :after_join     {:ok, socket}   end 

but stuck in middle because cannot find way send messages between users. eg: cannot identify way deliver user a's message user b using specific scenario

this basic architect of chat app :

this rest of code in roomchannel file

def handle_info(:after_join, socket)     presence.track(socket, socket.assigns.user, %{       online_at: :os.system_time(:millisecond)      })     push socket, "presence_state", presence.list(socket)     {:noreply, socket}   end    def handle_in("message:new", message, socket)     broadcast! socket, "message:new", %{       user: socket.assigns.user,       body: message,       timestamp: :os.system_time(:millisecond)     }     milisecondstime = :os.system_time(:millisecond)     [room, user] = string.split(map.get(socket, :topic), ":")     data = poison.encode!(%{"created_at" => :os.system_time(:millisecond), "updated_at" => :os.system_time(:millisecond), "uuid" => uuid.uuid1(), "date" => :os.system_time(:millisecond), "from" => user, "to" => user, "message" => message})     redix.command(:redix, ["set", uuid.uuid1(), data])      {:noreply, socket}   end 

can show me way can pass messages between user's chat rooms?

i'm using redis store data , this basic chat app following developer.

look phoenix.endpoint.broadcast can send message given topic using broadcast/3 function

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another way can go subscribe each user unique private topic when user joins channel join function

def join("room:" <> _user, _, socket)     send self(), :after_join     myapp.endpoint.subscribe("private-room:" <> user) # subscribe connecting client private room     {:ok, socket} end 

then in handle_in("message:new",message, socket) extract receipient user id message payload , call

endpoint.broadcast

def handle_in("message:new", message, socket)      myapp.endpoint.broadcast!("private-room:" <> to_user , "message:new",%{         user: socket.assigns.user,         body: message,         timestamp: :os.system_time(:millisecond)    }) #sends message connected clients to_user      {:noreply, socket} end