We might want a map from questionIds to longs for dates. I can envision a scenario where the front-end might want to get the time the user last saw a question, so as not to show them questions they have seen in the past 10 minutes, for example. At the very least, this should be an ordered set, where the most recently viewed questions are appended to the end of the set (or the front, however you want to do it). Maybe that was already your idea for this.