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Web cache
From Wikipedia, the free encyclopedia Jump to: navigation, searchA web cache is a mechanism 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 stores copies of documents passing through it; subsequent requests may be satisfied from the cache if certain conditions are met.
It should not be confused with a web archive, a site that keeps old versions of web pages.
Contents
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Systems
Web caches exist in various systems.
- A search engine may cache a website.
- A forward cache is a cache outside the webserver's network, e.g. on the client software's ISP or company network.
- A network-aware forward cache is just like a forward cache but only caches heavily accessed items.
- A reverse cache sits in front of one or more Web servers and web applications, accelerating requests from the Internet.
- A client, such as a web browser, can 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 to store web content.
- A content delivery network can retain copies of web content at various points throughout a network.
Cache control
HTTP defines three basic mechanisms for controlling caches: freshness, validation, and invalidation.
- 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 URL associated with a cached response subsequently gets a POST, PUT or DELETE request, the cached response will be invalidated.
Browser cache
See also: Temporary Internet FilesWeb browsers cache content on the client machine, in memory and on disk.
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.
Comparison of web caches
| Name | Type | Operating System | Forward Mode | Reverse Mode | License |
|---|---|---|---|---|---|
| ApplianSys CACHEbox | Appliance | Linux | Yes | Yes | Commercial |
| Ara Networks Jaguar5000 | Software/Appliance | Linux | Yes | Yes | Commercial |
| Blue Coat ProxySG | Appliance | SGOS | Yes | Yes | Commercial |
| Nginx | Software | Linux, Unix | No | Yes | 2-clause BSD-like |
| Microsoft ISA | Software | Windows | Yes | Yes | Commercial |
| Squid | Software | Linux, Unix, Windows | Yes | Yes | GNU GPL |
| Traffic Server | Software | Linux, Unix | Yes | Yes | Apache License 2.0 |
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