This is a demo showing how to send updates to multiple connected clients using the Google App Engine Channel API. The messages are used to display a product on a Google Map.

The weakness of the implementation is that there is a race condition with adding or removing clients to the JSON object in memcache. However, it works as a simple demo.

This code was extracted from the BBYScan map which displays live QR code scans from Best Buy stores. For more details about the BBYScan project, read our blog post.

For the demo, scans are simulated from a few sample products and stores.

This demo requires the Google App Engine SDK.

1. Check out the code and cd into that directory
2. Start your local server: dev_appserver.py .
3. Visit the map page at http://localhost:8080/
4. In another tab, launch the mock scan submission service at http://localhost:8080/scan
5. Watch the simulated scans come in.

You can open the map in multiple web browsers to see the channel API push the same updates to multiple clients.

Since channels can only stay open for 2 hours, it is necessary to periodically re-establish a new channel connection. This is accomplished by sending a 'refresh' JSON message to the client. In this demo, that causes the page to reload, which is the simplest way to re-establish a channel. This is accomplished with a cron.yaml entry that runs once an hour.

To demo this behavior, the channel refresh timeout has been set very low, to one minute. Since cron does not run in the SDK, you must trigger it manually. Do this by visiting http://localhost:8080/cleanup in another window. If your web browser has been on the map page for more than one minute, it will refresh.

This code is copyright Best Buy and licensed under the AGPL. See LICENSE.txt for details.