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Information about Web Cache

Web caching is the caching of web documents (e.g., HTML pages, images) in order 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.

Types of Web caches

Web caches can be deployed in a variety of ways. User agent caches, such as those in web browsers, are private caches, operating on behalf of a single user. Intermediaries can also implement shared caches that serve more than one person.

Proxy caches, also known as forward proxy caches, are usually deployed by internet service providers, schools and corporations to save bandwidth. Interception proxy caches (sometimes called "transparent caches") are a variant that doesn't require clients to be explicitly configured to use them.

Gateway caches, sometimes known as reverse proxy caches, surrogate caches, or web accelerators, operate on behalf of the origin server, and to clients are indistinguisable from it. A number of gateway caches can work together to implement a Content Delivery Network.

Intermediaries that cache often perform other duties, such as user authentication and content filtering. Multiple caches can also be coordinated using peering protocols like Internet Cache Protocol and HTCP.

Controlling Web caches

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.
  • 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.
In 1998 the DMCA added rules to the United States Code (17 U.S.C. § 512) that largely relieves system operators from copyright liability for the purposes of caching.

See also

References

  • Ari Luotonen, Web Proxy Servers (Prentice Hall, 1997) ISBN 0-13-680612-0
  • Duane Wessels, Web Caching (O'Reilly and Associates, 2001). ISBN 1-56592-536-X
  • Michael Rabinovich and Oliver Spatschak, Web Caching and Replication (Addison Wesley, 2001). ISBN 0-201-61570-3

External links

cache (IPA:/kæʃ/, like "catch" [1]) is a collection of data duplicating original values stored elsewhere or computed earlier, where the original data is expensive to fetch (due to longer access time) or to
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World Wide Web (commonly shortened to the Web) is a system of interlinked, hypertext documents accessed via the Internet. With a web browser, a user views web pages that may contain text, images, videos, and other multimedia and navigates between them using hyperlinks.
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An electronic document is any electronic media content (other than computer programs or system files) that are intended to be used in their electronic form, without being printed (although printing is usually possible).
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A Web page or webpage is a resource of information that is suitable for the World Wide Web and can be accessed through a web browser. This information is usually in HTML or XHTML format, and may provide navigation to other web pages via hypertext links.
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A digital image is a representation of a two-dimensional image as a finite set of digital values, called picture elements or pixels. The digital image contains a fixed number of rows and columns of pixels.
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Bandwidth is the difference between the upper and lower cutoff frequencies of, for example, a filter, a communication channel, or a signal spectrum, and is typically measured in hertz.
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web server can mean one of two things:
  1. A computer program that is responsible for accepting HTTP requests from clients, which are known as web browsers, and serving them HTTP responses along with optional data contents, which usually are web pages such as HTML documents and

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In computing and especially computer networks, a lag is a symptom where result of an action appears later than expected. While different kinds of latency are well defined technical terms, lag is the symptom, not the cause.
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A user agent is the client application used with a particular network protocol; the phrase is most commonly used in reference to those which access the World Wide Web, but other systems such as SIP uses the term user agent to refer to the user's phone.
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A web browser is a software application that enables a user to display and interact with text, images, videos, music and other information typically located on a Web page at a website on the World Wide Web or a local area network.
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In computer networks, a proxy server is a server (a computer system or an application program) which services the requests of its clients by forwarding requests to other servers.
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Internet service provider (abbr. ISP, also called Internet access provider or IAP) is a business or organization that provides consumers or businesses access to the Internet and related services. In the past, most ISPs were run by the phone companies.
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In telecommunications, the term gateway has the following meanings:
  • In a communications network, a network node equipped for interfacing with another network that uses different protocols.

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A reverse proxy is a proxy server that is installed within the neighborhood of one or more servers. Typically, reverse proxies are utilized in front of webservers. All connections coming from the Internet addressed to one of the webservers are routed through the proxy server, which
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A web accelerator is a proxy server that reduces web site access times. They can be a self-contained hardware appliance or installable software.

Techniques

Web accelerators may use several techniques to achieve this reduction:

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In telecommunications, the term gateway has the following meanings:
  • In a communications network, a network node equipped for interfacing with another network that uses different protocols.

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A content delivery network (CDN) is a system of computers networked together across the Internet that cooperate transparently to deliver content (especially large media content) to end users.
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Authentication (from Greek αυθεντικός; real or genuine, from authentes; author) is the act of establishing or confirming something (or someone) as authentic, that is, that claims made by or about the thing are true.
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Content-control software, also known as censorware or web filtering software, is a term for software designed and optimized for controlling what content is permitted to a reader, especially when it is used to restrict material delivered over the Web.
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The Internet Cache Protocol (ICP) is a protocol used for coordinating web caches. Its purpose is to find out the most appropriate location to retrieve a requested object from in the situation where multiple caches are in use at a single site.
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Hyper Text Caching Protocol (HTCP) is a protocol published in 2000 for querying, managing and discovering HTTP cache servers. It is defined in RFC 2756, and was designated as an "experimental" protocol (that is, not on the standards track).
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Hypertext Transfer Protocol (HTTP) is a communications protocol used to transfer or convey information on the World Wide Web. Its original purpose was to provide a way to publish and retrieve HTML hypertext pages.
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Introduced in the House of Representatives as H.R. 2281 by Rep. Howard Coble (R-NC) on July 29, 1997
  • Committee consideration by: House Judiciary Committee (Subcommittee on Courts and Intellectual Property); House Commerce Committee (Subcommittee on
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    • The United States Code (U.S.C.) is a compilation and codification of the general and permanent federal law of the United States.

    Codification process

    The official text of an Act of Congress is that of the "enrolled bill" (traditionally printed on parchment)
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    In computer networks, a proxy server is a server (a computer system or an application program) which services the requests of its clients by forwarding requests to other servers.
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