Thanks everyone for the useful answers. I'll go too: We have 20M customers with a history 1-1000 activities per customer, updated once daily. We do nightly run

Thanks everyone for the useful answers. I'll go too: We have 20M customers with a history 1-1000 activities per customer, updated once daily. We do nightly runs of a collaborative filtering algo to produce per-customer suggested activities, and push this to a sql database. Business users can load recommendations for each customer from this db...