title: Redis, session expiration, and reverse lookup
body: <p>I'm currently bulding a web app and would like to use Redis to store sessions. At login, the session is inserted into Redis with a corresponding user id, and expiration set at 15 minutes. I would now like to implement reverse look-up for sessions (get sessions with a certain user id). The problem here is, since I can't search the Redis keyspace, how to implement this. One way would be to have a redis set for each userId, containing all session ids. But since Redis doesn't allow expiration of an item in a set, and sessions are set to expire, there would be a ton of inexistent session ids in the sets.</p>
  
  <p>What would be the best way to remove ids from sets on key expiration? Or, is there a better way of accomplishing what I want (reverse look-up)?</p>