python Channel API expiry and usage in google app engine
Question: I want to use the channel api to push updates to open pages, What I have done
so far is to store the page client ids in ndb - I have included a code summary

My question is: How do I manage closed pages and expired tokens?

and is this the best way to push updates to many open pages?

open page code:

    
    
    import webapp2
    import uuid
    from google.appengine.api import channel
    from google.appengine.ext import ndb
    
    class Frame(ndb.Model):
      clientID = ndb.StringProperty()
      date = ndb.DateTimeProperty(auto_now_add=True)
    
    class MainHandler(BaseHandler):
        def get(self):
            client_id = str(uuid.uuid4())
            channel_token = channel.create_channel(client_id)
            frame = Frame(clientID = client_id)
            frame.put()
    
            self.render_response('home.html',** "token":channel_token,"client_id":client_id)
    

send message code:

    
    
    from google.appengine.api import channel
    from google.appengine.ext import ndb
    
    class Frame(ndb.Model):
      clientID = ndb.StringProperty()
      date = ndb.DateTimeProperty(auto_now_add=True)
    
    frames = Frame.query().fetch(10)
    
    for i in frames:
       channel.send_message(i.clientID, "some message to update")
    


Answer: When you enable channel_presence, your application receives POSTs to the
following URL paths:

    
    
    POSTs to /_ah/channel/connected/ 
    POSTs to /_ah/channel/disconnected/ 
    

These signal that the client has connected to the channel and can receive
messages or has dicsonnected.

[Tracking_Client_Connections_and_Disconnections](https://developers.google.com/appengine/docs/python/channel/overview#Tracking_Client_Connections_and_Disconnections)

Dealing with expired tokens:

> By default, tokens expire in two hours, unless you explicitly set an
> expiration time by providing the duration_minutes argument to the
> create_channel() function when generating the token. If a client remains
> connected to a channel for longer than the token duration, the socket’s
> onerror() and onclose() callbacks are called. At this point, the client can
> make an XHR request to the application to request a new token and open a new
> channel.

So on your `onerror` function you basically do it all over again just like the
original connection.

[Tokens and
security](https://developers.google.com/appengine/docs/python/channel/overview#Tokens_and_Security)

To send updates to many open pages simply iterate round your list of connected
users and send them the message individually. There is no "transmit to all"
functionality.

You may also want to build in a "heartbeat" that sends messages to supposedly
connected clients and remove them if no reply. This is because sometimes
(apparently) the disconnected messages are not sent (power failure, whatever)
when the browser window is closed.

