Note: Many topics at this site are reduced versions of the text in "The Encyclopedia of Networking and Telecommunications." Search results will not be as extensive as a search of the book's CD-ROM.

A cache is a memory area that holds information so that it may be quickly accessed by the next person that needs it. A cache normally resides between a slow device and a fast device. It may be RAM memory, a disk storage area, or a combination of both. A cache may be a very small amount of memory used by a microprocessor for "shuffling" information during its processing operations, or a cache may be very large-that is, an entire server or cluster of servers that caches frequently accessed Web pages.

This topic covers caching in general. Web-related caching is covered under Web Caching. The topic "Content Distribution" discusses techniques like that used by Akamai in which content is replicated from Web servers to special edge server at ISPs and automatically maintained and updated.

The primary purpose of caching is to keep information readily available for later access. When a process needs information, it first checks the cache to see if the information can be more quickly accessed there, rather than retrieving it from disk or in the case of a network, another server. Caches have hit rates, which is a measure of how often the cache information has been accessed. Information in a cache ages, meaning that at some point the information in the cache is no longer reliable or no longer needed, so caches are usually flushed on an ongoing basis by removing old information or updating it on a continuous basis.

The algorithms for discarding and updating cache information can get quite complex. An algorithm can evaluate all cache entries to decide which entries should be flushed based on how often those entries are used. For example, if a lot of people in your organization access the "Dilbert" Web site to view the latest cartoons, the caching server connected to the Internet may keep that information in the cache and constantly update it from the host site.