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 multimedia and other files can result in less overall delay when browsing the Web.[1][2]

Distributed web applications should consider distributed caching.

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.[3] A network-aware forward cache only caches heavily accessed items.[4] 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.[5] 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.


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.

Apache HTTP Server Yes
aiScaler Dynamic Cache Control No Linux No
ApplianSys CACHEbox No Linux No
Blue Coat ProxySG No SGOS Yes
Nginx Yes Linux, BSD, OS X, Solaris, AIX, HP-UX Yes Yes
Microsoft Forefront Threat Management Gateway Yes No No Yes
Polipo Yes OS X, Linux, OpenWrt, FreeBSD ? Yes
Squid Yes Linux ? Yes
Traffic Server ? Linux ? Yes
Untangle No Linux No Yes
Varnish No Linux No Yes
WinGate Yes No No Yes
Nuster No Linux No Yes
McAfee Web Gateway No McAfee Linux Operating System No Yes

See also


  1. ^ Fountis, Yorgos (4 May 2017). "How does the browser cache work?".
  2. ^ Messaoud, S.; Youssef, H. (2009). "An analytical model for the performance evaluation of stack-based Web cache replacement algorithms". International Journal of Communication Systems. 23: 1–22. doi:10.1002/dac.1036. S2CID 46507769.
  3. ^ Shinder, Thomas (2 September 2008). "Understanding Web Caching Concepts for the ISA Firewall". ISA Server. TechGenix Ltd. Archived from the original on 23 July 2011. Retrieved 27 February 2011.
  4. ^ Erman, Jeffrey; Gerber, Alexandre; Hajiaghayi, Mohammad T.; Pei, Dan; Spatscheck, Oliver (2008). "Network-Aware Forward Caching" (PDF). AT&T Labs: 291–300. CiteSeerX Archived from the original (PDF) on 1 April 2011. Retrieved 11 March 2019.
  5. ^ Kelly, Mike; Hausenblas, Michael. "Using HTTP Link: Header for Gateway Cache Invalidation" (PDF). WS-REST. p. 20. Archived from the original (PDF) on 10 July 2010. Retrieved 14 June 2013.

Further reading