Web cache

A web cache is a mechanism for the temporary storage (caching) of web documents, such as HTML pages and 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] Google's cache link in its search results provides a way of retrieving information from websites that have recently gone down and a way of retrieving data more quickly than by clicking the direct link.

Systems

Web caches can be used in various systems.

• A search engine may cache a website.
• A forward cache is a cache outside the webserver's network, e.g. on the client software's ISP or company network.^[2]
• A network-aware forward cache is just like a forward cache but only caches heavily accessed items.^[3]
• A reverse cache sits in front of one or more Web servers and web applications, accelerating requests from the Internet.^[4]
• A client, such as a web browser, can store web content for reuse. For example, if the back button is pressed, the local cached version of a page may be displayed instead of a new request being sent to the web server.
• A web proxy sitting between the client and the server can evaluate HTTP headers and choose to store web content.
• A content delivery network can retain copies of web content at various points throughout a network.

Cache control

HTTP defines three basic mechanisms for controlling caches: freshness, validation, and invalidation.^[5]

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. The ETag (entity tag) mechanism also allows for both strong and weak validation.

Invalidation 

is usually a side effect of another request that passes through the cache. For example, if a URL associated with a cached response subsequently gets a POST, PUT or DELETE request, the cached response will be invalidated.

Legal issues

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

Comparison of web caches

See also

• Harvest project
• Proxy server
• Web accelerator
• Cache manifest in HTML5

Notes

Further reading

• 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

External links

• Caching Tutorial for Web Authors and Webmasters
• Web Caching and Content Delivery Resources
• Web Caching, Web caching in general with some references to SQUID
• Cache control directives demystified Explanations, do's and don't