Web cache - Wikipedia
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System for optimizing the Web
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
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Forward and reverse
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A forward cache is a cache outside the
web server
's network, e.g. in the client's
web browser
, in an
Internet service provider
, or within a corporate network. A network-aware forward cache only caches heavily accessed items. 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.
HTTP options
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The Hypertext Transfer Protocol (HTTP) defines three basic mechanisms for controlling caches: freshness, validation, and invalidation. 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.
Legality
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In 1998, the
Digital Millennium Copyright Act
added rules to the United States Code (17 U.S.C. §: 512) that exempts system operators from copyright liability for the purposes of caching.
Server-side software
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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
Apache 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
See also
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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
Temporary Internet Files
References
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Fountis, Yorgos (4 May 2017).
"How does the browser cache work?"
Messaoud, S.; Youssef, H. (2009).
"An analytical model for the performance evaluation of stack-based web cache replacement algorithms"
International Journal of Communication Systems
23
1–
22.
doi
10.1002/dac.1036
S2CID
46507769
Further reading
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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
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Cache (computing)
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Web caching protocol
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