How google doc sync two document at the same time?

I am making a cooperation application like google docs, but I found this is different to do on the web. The problem is, when the user typing, another user should see the update at the same time. But what actually behind the screen? Is that when the user have an action, it sent a http request, and write into database. At the same time, another user get the action from database, and rendering the result that the user just type. If use this way t开发者_开发问答o implement, the database need to keep read and write.....apart from this solution, how can I sync two people work on the fly? Thank you.

---

Check out something like this http://pusherapp.com/

Or http://www.tornadoweb.org/

Both are good at real time pushes without constant AJAX requests that will put a lot of strain on your server