Web caching is the caching of web documents (HTML pages, images, etc.) in order to reduce bandwidth usage and web site access times. A web cache stores copies of documents requested by users. Subsequent requests may be satisfied from the cache if certain conditions are met. Web caches generally achieve hit rates around 30%-50% and become more effective as the user population increases.

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 ISPs, 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.

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

• memcached
• Akamai
• Squid cache

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.

See also

• Ari Luotonen, Web Proxy Servers (Prentice Hall, 1997) ISBN 0136806120
• Duane Wessels, Web Caching (O'Reilly and Associates, 2001). ISBN 156592536X
• Michael Rabinovich and Oliver Spatschak, Web Caching and Replication (Addison Wesley, 2001). ISBN 0201615703

External links and references

• Caching Tutorial for Web Authors and Webmasters
• Web Caching and Content Delivery Resources
• www.web-cache.com