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]
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.
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.
This is a list of server-side web caching software.
| Name | Operating system | Forward mode |
Reverse mode |
License | ||
|---|---|---|---|---|---|---|
| Windows | Unix-like | Other | ||||
| Apache HTTP Server | Yes | OS X, Linux, Unix, FreeBSD, Solaris, Novell NetWare | OS/2, TPF, OpenVMS, eComStation | Yes | Apache 2.0 | |
| aiScaler Dynamic Cache Control | No | Linux | No | Proprietary | ||
| ApplianSys CACHEbox | No | Linux | No | Proprietary | ||
| Blue Coat ProxySG | No | No | SGOS | Yes | Yes | Proprietary |
| Nginx | Yes | Linux, BSD, OS X, Solaris, AIX, HP-UX | Yes | Yes | Yes | 2-clause BSD-like |
| Microsoft Forefront Threat Management Gateway | Yes | No | No | Yes | Yes | Proprietary |
| Polipo | Yes | OS X, Linux, OpenWrt, FreeBSD | ? | Yes | Yes | MIT License |
| Squid | Yes | Linux | ? | Yes | Yes | GPL |
| Traffic Server | ? | Linux | ? | Yes | Yes | Apache 2.0 |
| Untangle | No | Linux | No | Yes | Yes | Proprietary |
| Varnish | No | Linux | No | Needs a VMOD | Yes | BSD |
| WinGate | Yes | No | No | Yes | Yes | Proprietary (Free for 8 users) |
| Nuster | No | Linux | No | Yes | Yes | GPL |
| McAfee Web Gateway | No | McAfee Linux Operating System | No | Yes | Yes | Proprietary |