How to create a Google Reader?


I need to create a web tool like Google Reader for my college project.

I have 2 question about it:

1) How Google Reader track the read and unread posts ?

2) Google Reader save every post in the db or load the feeds at the moment ?

  1. assign a hash to a single feed post (ie. date+url+??? = hash to identify a single post)
  2. loads them on the fly would be my guess, maybe caches a limited number per user.