In a recent interview a friend was asked to design Facebook\'s/Snapchat\'s "Nearby Friends" feature. His design included a Poller in the client app that intermittently polls the server for nearby friend locations in a particular radius. The client would also send its location in the background  to the server intermittently (only if you\'re moving). My friend discussed using geohashes instead of latitude/longitudes. \n\nThe interviewer wasn\'t impressed.\n\nI think the interviewer was looking for:\n* If each user has an avg of 500 friends and we\'re working at Facebook scale, would polling be a right fit?\n* How would you filter out friends outside your given radius? Would u keep doing this at every poll at Facebook scale?\n* How about pushing nearby friends location to client app?\n\nNote there are YouTube videos about Yelp/Uber system design that talk about persistent connections etc., but I don\'t think any video solves the problem at scale. Thoughts?

In a recent interview a friend was asked to design Facebook\'s/Snapchat\'s "Nearby Friends" feature. His design included a Poller in the client app that intermi