HTTP has a relatively complicated set of features that user agents and origin servers can use to control whether or not documents are stored in a cache, and when cached copies may be reused. Some web sites are cache-friendly, and some are not.

Web caches come in two flavors: client-side and server-side. Client-side caches, also sometimes called forward caches, exist to serve a local user population. These are often used by internet service providers, schools, and corporations for their users. Server-side caches, also known as reverse-caches and web accelerators, are placed in front of origin servers to reduce their load.

All major websites which routinely receive millions of queries per day require some form of web caching. If multiple cache servers are used together, these may coordinate using protocols like the Internet Cache Protocol and HTCP.

Modern web browsers include internal web caches. Examples of external web caches are:

• memcached


• Akamai


• Squid cache


• Microsoft Internet Security and Acceleration Server

Web caches also perform related tasks such as user authentication and content filtering.

Some people worry that web caching may be an act of copyright infringement. In 1998 the DMCA added rules to the United States Code (17 Sec. 512) that largely relieves system operators from copyright liability for the purposes of caching.

Viewing caches

On the Firefox, Camino, and SeaMonkey browsers, it is possible to view the contents of the browser's cache by typing "about:cache" in the url field (no quotes.)