Precomputation

[Young]

Hi all,
I am currently working on building a recommendation engine for one business 
website. I am holding 3 tables, --"favorite items table", "rated items table" 
and "purchased items tables".
I want to give recommendations based on browsing history. I am new to this 
field and I do not know which should I pre-compute offline and which should be 
computed online. 
Could someone give me some suggestions or any papers?
 
Thanks very much.
--Young