How does FBConnect actually work?

Question

Basically, how does it work technologically?

I was under the impression that SiteA could not read the cookies that SiteB set. So what exactly is going on under the engine? Basically, what would the flowchart look like when a user goes to a page to make it that the page knows who that user is?

Answer 1

You are signed in to Facebook, so all request from you will be authenticated. The document on which FBConnect is running on executes such a request, and by means of cross-domain communication it in turn gains the knowledge of 'who' you are as related to facebook.

My article at Script Junkie, Cross-Document Messaging and RPC explains how easyXDM does the XDM (and its implementation is far superior to FB's) (Disclaimer: I'm the main developer), and it's mainly about using one of several different features/tricks for relaying data in string form between the two domains, using documents hosted in iframes to transport data from the server to the client.

Btw, many large services like Twitter, Disqus, Scribd and VKontakte are using easyXDM for it's XDM, so it has some credibility ;)