I've seen a number of posts talking about how PubSub's system is fundamentally different because it does prospective searching, unlike the stodgy old backwards facing search that Technorati, BlogPulse and others provide. But how is prospective search any different from having a subscription to a search feed? Technorati and BlogPulse (not to mention everyone else) has this as part of a standard feature set. Cindy Sullivan says:
PubSub is a prospective matching engine, meaning it's only concerned with information you want to be alerted to from this very moment forward. I'm by no means the expert here just really fascinated with what PubSub is doing.
Isn't that a description of a search feed? I have to be missing something here.
PubSub describes a system that would require a representation of all queries in a form that can be efficiently matched against each new post (or rather the content of an RSS feed):
The heart of the PubSub service is a powerful, proprietary Matching Engine that makes it possible, for the first time, to match millions of search queries against thousands of new pieces of information every second.
A naive implementation of this would simply place the new data in the index and then run a time restricted search. A more interesting implementation (theoretically interesting) would be one which keeps no object data state, just query state. The criterion that drives this is the time taken from a post being published to notification of the search subscriber. This time becomes more and more critical given the importance of the issue and/or the prominence of the poster. PubSub have worked on the later issue with their linkRank system. I don't believe any of the free public services allows for a request like 'let me know if someone says anything libelous about my product.'