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copied!<p>After discussing this question in the <a href="https://groups.google.com/forum/?fromgroups#!topic/redis-db/OsUylXWDxWU" rel="nofollow">Redis DB Google Groups</a>, my proposed solution (inspired by <a href="http://www.lukemelia.com/blog/archives/2010/01/17/redis-in-practice-whos-online/" rel="nofollow">this article</a>) is to <code>SADD</code> all my online users into a single set, and for each of my users, I create a <code>user:#{user_id}:friends_list</code> and store their friends list as another set. Whenever a user logs in, I would <code>SINTER</code> the user's friends list and the current online users set. Since we're read heavy and not write heavy, we would use a single master node for writes. To make it scale, we'd have a cluster of slave nodes replicating from the master, and our application will use a simple round-robin algorithm to do the <code>SINTER</code></p> <p><a href="http://twitter.com/#!/dr_josiah" rel="nofollow">Josiah Carlson</a> suggested a more refined approach:</p> <blockquote> <ol> <li>When a user X logs on, you intersect their friends set with the online users to get their initial online set, and you keep it with a Y-minute TTL (any time they do anything on the site, you could update the expire time to be Y more minutes into the future) </li> <li>For every user in that 'online' set, you find their similar 'initial set', and you add X to the set </li> <li>Any time a user Z logs out, you scan their friends set, and remove Z from them all (whether they exist or not) </li> </ol> </blockquote>

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