During last BBW class Martin explained us two main concepts of internet information tracking. Since the terms were new for me, I decided toput a note on this.
There are two main concepts of information tracking - pinging and crawling.
Crawling is what Google does - indexation the change in Internet. To provide personalized advertizing, forexample, Google datacenter machines register the changes made evey second. If you ned thoroughness, you take this method, which has two main disadvantages: expensive (computer consumes a lot of energy) and perception issue (they are spying).
Pinging idea was used by David Sifry for Technocraty - blog searching engine. The source of change reports to a server automatically if he made a change and want it to appear in search results of the server's engine. You go for it if you need speed.