Private messages in a chat application using WAMP

[att...@gmail.com]

I am trying to create a hip-chat alone... Currently figuring out what would be the best approach for enabling private messages in a chat application. Actual questions at listed at the end, but here is the general background.

Channels/Topics:

This is working fine and every thing is understood. 

1. User views a list of channels to join.

2. ON JOIN user is subscribed to the topic (using onSubscribe)

3. User sends a message to channel and it is broad-casted to all.

4. ON PART user is unsubscripted from the topic

5. and so on..

Private Message (which is what the question is about):

I have couple of ideas on how to achieve this and looking for feedback:

A) RPC Approach

1. User sends an RPC to server with something like

session.call("private", {to: session/user, from: session/user, message: "My message for TO user", timestamp, etc..}).then(function (result) {
   // do stuff with the result
}, function(error) {
   // handle the error
});

2. Server gets the command and sends the callRequest to the receiver. This is perhaps not possible, as $id (from onCall()) is generated.  

B) Using a global/internal topic

1. When user connects subscribe to a topic "system:all" (or something like that). Basically every user is subscribed to this topic.

ab.connect(
   // The WebSocket URI of the WAMP server
   wsuri,
 
   function (session)
   {
        session.subscribe("system:all", function (topic, event){
          handleSystemAll(topic, event);

        })
   }
   ...
}

2. User publishes a message to "system:all" , with something like the following:

var myTopic = "system:all";
var myEvent = {eventname: "private", to: user/session, from: user/session, message: "my message"...};
sess.publish(myTopic, myEvent);

3. Then on server side do something like this:

onPublish(ConnectionInterface $conn, Topic $topic, string $event)
{
    if ($topic == "system:all")
    {
        if ($event['eventname'] == 'private')
        {
            $toUserConn = $this->getUserConnFromInternalStorage($event['user']);
            $message = array(
                'eventname' => $event['eventname'],
                'message' => $event['message'],
                'from' => $event['from']
            );

            $toUserConn->event($topic, $message);
        }
    }
}

4. And then finally some JS to display message to the user

function handleSystemAll(topic, event)
{
    if (topic == 'system:all' && event.eventname == 'private')
        console.log(event.from + ': ' + event.message);
}

Questions:

1. Is it possible for the server to initiate a callRequest() to a client, without $id?

2. Is it possible to for the server to send event() back to a client? In my testing it has not been the case (using autobahnjs). The user must be subscribed to a topic to receive message. 

3. There are advantages to having the global topic, but using that for sending private message seems a bit hacky?

4. Any other ideas/feedback on how to achieve this?

Thanks for your feedback.

[Grim Lodaf]

How about each user, on connecting, subscribes to their own unique channel. All personal updates, private chats/messages and all, get pushed to this
channel.This could be in addition to a global topic.

[cboden]

1. RPC's (callRequest) is client to server only.

2. events() must be sent through a topic but you can choose to isolate them to a single connection

3. It's a little bit hacky in that WAMP is a messaging protocol and that is an anti-pattern - BUT it will work and gives you control. If it suits your needs go for it. 

4. See Grim's suggestion, it's a good messaging pattern.