I would highly recommend using X-Analytics header for this, and establishing a "well known" key name(s). X-Analytics gets parsed into key-value pairs (object field) by our varnish/hadoop infrastructure, whereas the user agent is basically a semi-free form text string. Also, user agent cannot be set for by any javascript client, so we will constantly have to perform two types of analysis - those that came from the "backend" and those that were made by the browser.