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")

Beware because the Channel API only allows up to 100 channels if I am not mistaken so it's not a good approach to replace AJAX. Also has limitations per week etc. — Jimmy Kane, Jan 07 '13 at 11:45
thanks @JimmyKane, I was hoping not to use a client side poll, The 100 channels should be OK for what I am doing, for now anyway — , Jan 07 '13 at 11:52

score 2 · Accepted Answer · answered Jan 07 '13 at 13:06

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

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

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.

Do you know why both onerror() and onclose() would be invoked in the same case? Is there a difference between the two? Basically if I need to insert the token refresh logic in one of those methods which one would cover all the possibilities? — Karra, Sep 20 '16 at 15:28

python Channel API expiry and usage in google app engine

1 Answers1