Use SCAN for redis-server ~> 2.8.0

The SCAN command and the closely related commands SSCAN, HSCAN and ZSCAN are used in order to incrementally iterate over a collection of elements.

Since these commands allow for incremental iteration, returning only a small number of elements per call, they can be used in production without the downside of commands like KEYS or SMEMBERS that may block the server for a long time (even several seconds) when called against big collections of keys or elements.