While Ehcache concerns itself with Java objects, caching is used throughout computing, from CPU caches to the Internet Domain Name System (DNS) system. Why? Because many computer systems exhibit “locality of reference.” Data that is near other data or has recently been used is more likely to be used again.

Chris Anderson, of Wired Magazine, coined the term “the long tail” to refer to cases in e-commerce systems where a small number of items can make up the bulk of sales (or a small number of blogs can get the most hits) and there is a long “tail” of less popular items.

The long tail is an example of a power -law probability distribution, such as the Pareto distribution or 80:20 rule. If 20% of objects are used 80% of the time and a way can be found to reduce the cost of obtaining that 20%, system performance will improve.