Redis is an advanced key-value store.  It is similar to memcached but the 
dataset is not volatile, and values can be strings, exactly like in memcached, 
but also lists, sets, and ordered sets.  These data types can be manipulated 
with atomic operations to push/pop elements, add/remove elements, perform 
server side union, intersection, difference between sets, and so forth. 
Redis supports different kinds of sorting abilities.
