Combining Parse API with PubNub Realtime Messaging - By combining the Parse API with PubNub Data Streams you will have a better starting point rather than building a custom backend from scratch.

parse-cloud-publish-to-pubnub.js

var pubnub = { 
    'publish_key'   : 'demo', 
    'subscribe_key' : 'demo'
};

var bob_channel   = "channel-bob";
var sally_channel = "channel-sally";

var message = {
    "from"    : "Sally",
    "to"      : "Bob",
    "message" : "Hi there Bob!"
};

Parse.Cloud.httpRequest({
    url: 'http://pubsub.pubnub.com/publish/' + 
         pubnub.publish_key   +   '/' + 
         pubnub.subscribe_key + '/0/' + 
         bob_channel          + '/0/' + 
         encodeURIComponent(JSON.stringify(message)),

    // SUCCESS CALLBACK
    success: function(httpResponse) {
        console.log(httpResponse.text);
        // httpResponse.text -> [1,"Sent","14090206970886734"]
    },

    // You should consider retrying here when things misfire
    error: function(httpResponse) {
        console.error('Request failed ' + httpResponse.status);
    }
});

parse-pfuser-pubnub-channel.m

PFUser *user       = [PFUser currentUser];
PNChannel *channel = [PNChannel channelWithName:user.objectId];

pubnub-objective-c-app.m

// Define a channel
PNChannel *myChannel = [PNChannel channelWithName:@"channel-bob"];
PNChannel *friendChannel = [PNChannel channelWithName:@"channel-sally"];

// Receive Messages Sent to Me!
[PubNub subscribeOnChannel:myChannel];

// Send a Message to Sally
[PubNub sendMessage:@"Hello from PubNub iOS!" toChannel:friendChannel];

//(In AppDelegate.m, define didReceiveMessage delegate method:)
- (void)pubnubClient:(PubNub *)client didReceiveMessage:(PNMessage *)message {
    NSLog(@"Received: %@", message.message);
}