Web cache
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Web caching is the caching of web documents (e.g., HTML pages, images) in order 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.
[edit] Types of Web caches
Web caches can be deployed in a variety of ways. User agent caches, such as those in web browsers, are private caches, operating on behalf of a single user.
Intermediaries can also implement shared caches that serve more than one person. Proxy caches, also known as forward proxy caches, are usually deployed by internet service providers, schools and corporations to save bandwidth. Interception proxy caches are a variant that doesn't require clients to be explicitly configured to use them. Packages that can be used as a proxy cache include Squid cache, Microsoft Internet Security and Acceleration Server, ARA Networks Jaguar 3000 and Blue Coat.
Gateway caches, sometimes known as reverse proxy caches, surrogate caches or web accelerators operate on behalf of the origin server, and to clients are indistinguisable from it. A number of gateway caches can work together to implement a Content Delivery Network, such as Akamai.
Intermediaries that cache often perform other duties, such as user authentication and content filtering. Multiple caches can also be coordinated using peering protocols like Internet Cache Protocol and HTCP.
[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.
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 U.S.C. § 512) that largely relieves system operators from copyright liability for the purposes of caching.
[edit] See Also
[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
