We want to count the number of users, including guests, who are currently viewing a resource. To count guests, we plan to count unique IPs, so each IP should be counted at most once even if it connects to several different servers as a result of load balancing.<p>The approach we&#x27;re moving towards is an augmented bloom filter where each &quot;bit&quot; is actually an unsigned integer counting the number of items hashed to the position, allowing us to add and remove IPs to the filter as users come and go (just rehashing and &quot;subtracting&quot; the IP when it leaves). This lets us ensure that IPs are not counted more than once. SETBIT and GETBIT would work with a normal filter, but they can&#x27;t increment multiple bits, so we&#x27;d end up needing to lock.