Mahout recommendations with categories

Question

I am a newbie of Mahout so please apologize me if I am asking something trivial. I would like to implement recommendations but the problem I am trying to address is not an easy one. I have items with ratings that belong to different categories (let's call them CATA, CATB...). Users can receive recommendations just for the category they are in. There are thousands of categories but each category has just one hundred items in it.

so if User1 gave a 2 stars rate on an item in CATA and 3 stars in CATB, if User2 gave a 3 stars on the same item in CATA then when he is in CATB I want to obtain the recommendation. Is it possible to achieve this easily? Can it run real time or should it be a batch processing?