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A web cache (or HTTP cache) is an information technology 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 system stores copies of documents passing through it; subsequent requests may be satisfied from the cache if certain conditions are met. A web cache system can refer either to an appliance, or to a computer program.
Web caches can be used in various systems (as viewed from direction of delivery of web content):
Forward position system (recipient or client side)
A forward cache is a cache outside the web server's network, e.g. on the client computer, in an ISP or within a corporate network. A network-aware forward cache is just like a forward cache but only caches heavily accessed items. A client, such as a web browser, can also 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 whether to store web content.
Reverse position system (content provider or web-server side)
A reverse cache sits in front of one or more web servers and web applications, accelerating requests from the Internet, reducing peak web server load. A content delivery network (CDN) can retain copies of web content at various points throughout a network. A search engine may also cache a website; it provides a way of retrieving information from websites that have recently gone down or a way of retrieving data more quickly than by clicking the direct link. Google, for instance, does so. Links to cached contents may be found in Google search results.
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 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.
See also: No-cache tag