Web caching is the caching of web documents (e.g., HTML pages, images) to reduce bandwidth usage, server load, and perceived lag. A web cache stores copies of documents passing through it; subsequent requests may be satisfied from the cache if certain conditions are met.^[1]

It should not to be confused with a web archive, a site that keeps old versions of web pages.

Contents 1 Controlling Web caches 2 Legal Issues 3 Web cache software 4 See also 5 References 6 External links

[edit] Controlling Web caches

HTTP defines three basic mechanisms for controlling caches: freshness, validation and invalidation.

• Freshness allows a response to be used without re-checking it on the origin server, and can be controlled by both the server and the client. For example, the Expires response header gives a date when the document becomes stale, and the Cache-Control: max-age directive tells the cache how many seconds the response is fresh for.
• Validation can be used to check whether a cached response is still good after it becomes stale. For example, if the response has a Last-Modified header, a cache can make a conditional request using the If-Modified-Since header to see if it has changed.
• Invalidation is usually a side effect of another request that passes through the cache. For example, if URL associated with a cached response subsequently gets a POST, PUT or DELETE request, the cached response will be invalidated.

[edit] Legal Issues

In 1998 the DMCA added rules to the United States Code (17 U.S.C. § 512) that relieves system operators from copyright liability for the purposes of caching.

[edit] Web cache software

• ApplianSys CACHEbox (appliance)
• Blue Coat ProxySG (appliance)
• Squid (software)
• WinGate (software)
• [CCC]

[edit] See also

• Proxy server

[edit] References

• Ari Luotonen, Web Proxy Servers (Prentice Hall, 1997) ISBN 0-13-680612-0
• Duane Wessels, Web Caching (O'Reilly and Associates, 2001). ISBN 1-56592-536-X
• Michael Rabinovich and Oliver Spatschak, Web Caching and Replication (Addison Wesley, 2001). ISBN 0-201-61570-3

[edit] External links

• Is the Internet Heading for a Cache Crunch?
• Caching Tutorial for Web Authors and Webmasters
• Web Caching and Content Delivery Resources
• Web Caching, Web caching in general with some references to SQUID