Tripadvisor Senior Software Developer interview questions

Design an alert system. You have 500,000 hotels you represent, and about 250,000 are visited each day. You want to track hotel page visits such that, if more than 10 visitors per a) 5 minutes, b) 1 hour, and c) 24 hours occur, then a (different for each time period) "alert" appears when someone visits one of those "hot" hotel pages. Don't worry about the UI part, just design the underlying alert system. Then optimize your solution for space.