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.
Parts of the system
Forward and reverse
A forward cache is a cache outside the
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.
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
This is a list of server-side web caching software.
|Apache HTTP Server||Yes||OS X, Linux, Unix, FreeBSD, Solaris, Novell NetWare||OS/2, TPF, OpenVMS, eComStation||Yes|
|aiScaler Dynamic Cache Control||No||Linux||No||Proprietary|
|Blue Coat ProxySG||No||No||SGOS||Yes||Yes||Proprietary|
|Microsoft Forefront Threat Management Gateway||Yes||No||No||Yes||Yes||Proprietary|
|Varnish||No||Linux||No||Needs a VMOD||Yes||BSD|
|WinGate||Yes||No||No||Yes||Yes||Proprietary (Free for 8 users)|
|McAfee Web Gateway||No||McAfee Linux Operating System||No||Yes||Yes||Proprietary|
- InterPlanetary File System - makes web caches redundant
- Cache Discovery Protocol
- Cache manifest in HTML5
- Content delivery network
- Harvest project
- Proxy server
- Web accelerator
- Search engine cache
- Fountis, Yorgos (4 May 2017). "How does the browser cache work?".
- 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.