[mxwhnhbf] PubSubHubbub with predicates

A site posts a lot, but a subscriber is only interested in a small subset of posts.  We would like to avoid the subscriber from having to download the entire new updated RSS feed to check if the interested change occurred.

One simple first implementation is for the origin to be able dynamically create RSS feeds of search results, then ping the pubsubhubbub server for only the searches that people are subscribed to.

Need some way of quickly computing which subscribed search feeds are affected by a small change in the site.

Need to avoid denial of service attack of a subscriber requesting a very high number of search subscriptions.

Could the search feed service be provided by a third party?  The origin merely publishes diffs, and the computation of what is affected becomes the third party's responsibility.

Need a good way of the subscriber notifying, no longer interested in a search feed, to avoid the constant recomputation.