Web cache

For a site that keeps old versions of web pages, see web archive.

A web cache (or HTTP cache) is an information technology 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 system stores copies of documents passing through it; subsequent requests may be satisfied from the cache if certain conditions are met.[1] A web cache system can refer either to an appliance, or to a computer program.

Systems

Web caches can be used in various systems (as viewed from direction of delivery of web content):

Forward position system (recipient or client side)

A forward cache is a cache outside the web server's network, e.g. on the client computer, in an ISP or within a corporate network.[2] A network-aware forward cache is just like a forward cache but only caches heavily accessed items.[3] A client, such as a web browser, can also 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 whether to store web content.

Reverse position system (content provider or web-server side)

A reverse cache sits in front of one or more web servers and web applications, accelerating requests from the Internet,[4] reducing peak web server load. A content delivery network (CDN) can retain copies of web content at various points throughout a network. A search engine may also cache a website; it provides a way of retrieving information from websites that have recently gone down or a way of retrieving data more quickly than by clicking the direct link. Google, for instance, does so. Links to cached contents may be found in Google search results.

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.

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

See also: No-cache tag

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.

Web caching software

The following is a list of dedicated web caching server software:

Name Operating system Forward
mode		 Reverse
mode		 License
Apache HTTP Server Windows, OS X, Linux, Unix, FreeBSD, Solaris, Novell NetWare, OS/2, TPF, OpenVMS and eComStation No Yes Apache License 2.0
aiScaler Dynamic Cache Control Linux Yes Yes Proprietary
ApplianSys CACHEbox Linux Yes Yes Proprietary
Blue Coat ProxySG SGOS Yes Yes Proprietary
Nginx Linux, BSD variants, OS X, Solaris, AIX, HP-UX, other *nix flavors No Yes 2-clause BSD-like
Microsoft Forefront Threat Management Gateway Windows Yes Yes Proprietary
Polipo Windows, OS X, Linux, OpenWrt, FreeBSD No Yes MIT License
Squid Linux, Unix, Windows Yes Yes GNU General Public License
Traffic Server Linux, Unix Yes Yes Apache License 2.0
Untangle Linux Yes Yes Proprietary
Varnish Linux, Unix No (possible with a VMOD) Yes BSD
WinGate Windows Yes Yes Proprietary / Free for 3 users

See also

Notes

  1. ↑ Geoff Huston. "Web Caching". Cisco. The Internet Protocol Journal - Volume 2, No. 3. Retrieved 2009-09-10.
  2. ↑ Thomas Shinder. "Understanding Web Caching Concepts for the ISA Firewall". Retrieved 2011-02-27.
  3. ↑ Erman, Jeffrey; Gerber, Alexandre; Hajiaghayi, Mohammad T.; Pei, Dan; Spatscheck, Oliver (2008). "Network-Aware Forward Caching" (PDF). AT&T Labs. CiteSeerX: 10.1.1.159.1786.
  4. ↑ Multiple (wiki). "Web application/Caching". Docforge. Retrieved 2010-03-06.
  5. ↑ "PowerPoint Presentation" (PDF). Retrieved 2013-06-14.

Further reading

External links

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