Problem with Google App Engine Channel API (Python/Django)

Question

Hey, everybody. I'm trying to use the App Engine Channel API (documented here) to enable push updates for my application. However, I'm running into some problems setting up the mechanism for initializing a communication channel.

The problem is that, when I load the page in question, I get debug prints for the onError and onClose handlers, but I don't get a debug print for the onOpen handler. Nothing else happens. Below is the javascript console output using Google Chrome Developer Tools:

Resource interpreted as script but transferred with MIME type text/html. ..... jsapi:-1

onError ..... 443088:88

onClose ..... 443088:80

And here is the relevant section of (mostly) javascript code from my Django template:

<script type="text/javascript">
onOpen = function() {
   console.debug('onOpen');

   var xhrArgs = {
      url: '/channel/connect/',
      headers: { 'Content-Type': 'application/json' },
      postData: dojo.toJson({ 'channel_token': '{{ channel_token }}' }),
      handleAs: 'text',
      load: function(response) {
         alert('success');
      },
      error: function(error) {
         alert('failure: ' + error);
      }
   };
   var deferred = dojo.xhrPost(xhrArgs);
 };

 onClose = function() {
  console.debug('onClose');
 };

 onMessage = function(msg) {
  console.debug('onMessage');
 };

 onError = function() {
  console.debug('onError');
 };

 openChannel = function() {
  var channel = new goog.appengine.Channel('{{  channel_token }}');
  var handler = {
   'onopen': onOpen,
   'onclose': onClose,
   'onmessage': onMessage,
   'onerror': onError
  };
  var socket = channel.open(handler);
  socket.onopen = onOpen;
  socket.onmessage = onMessage;
 };

 setTimeout(openChannel, 100);
 </script>

If I understand this correctly, then the server-side code isn't relevant at this point, since onOpen doesn't appear to even be called yet. I know I must be missing something simple, but if anybody could help out with this, I would greatly appreciate it!

Answer 1

It looks to me like you're using an unitialized channel token. The server needs to call channel.create_channel to retrieve a token for this client, that you'll then pass to the Channel object's constructor.