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This should be a simple API call to notify a Hub of changes to topics. Again, this should provide real-time notifications either synchronously or asynchronously. Importantly for Asynchronous notifications, they should block subsequent change notifications pending a notification to the Hub.

References: 

https://github.com/nlothian/RTUpdates/blob/master/src/com/nicklothian/pubsubhub/PubSubHubSubscriptionServlet.java

http://grack.com/blog/2009/09/09/parsing-the-pubsubhubbub-notifications/