Web cache

A Web cache (or HTTP cache) is a system for optimizing the World Wide Web. It is implemented both client-side and server-side. The caching of images and other files can result in less overall delay when browsing the Web.^[1]

Parts of the system[]

Forward and reverse[]

A forward cache is a cache outside the web server's network, e.g. in the client's web browser, in an ISP, or within a corporate network.^[2] A network-aware forward cache only caches heavily accessed items.^[3] A proxy server sitting between the client and web server can evaluate HTTP headers and choose whether to store web content.

A reverse cache sits in front of one or more web servers, accelerating requests from the Internet and reducing peak server load. This is usually a content delivery network (CDN) that retains copies of web content at various points throughout a network.

HTTP options[]

The Hypertext Transfer Protocol (HTTP) defines three basic mechanisms for controlling caches: freshness, validation, and invalidation.^[4] This is specified in the header of HTTP response messages from the server.

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. Many CDNs and manufacturers of network equipment have replaced this standard HTTP cache control with dynamic caching.

Legality[]

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

Server-side software[]

This is a list of server-side web caching software.

See also[]

• Cache Discovery Protocol
• Cache manifest in HTML5
• Content delivery network
• Harvest project
• Proxy server
• Web accelerator
• Search engine cache

References[]

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[]

• RFC 7234 Hypertext Transfer Protocol (HTTP/1.1): Caching