View source for Web cache

{{short description|System for optimizing the Web}} 
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 [[Software:Web browser|browsing]] the Web.<ref>{{Cite web |last=Fountis |first=Yorgos |date=4 May 2017 |title=How does the browser cache work? |url=https://pressidium.com/blog/2017/browser-cache-work/}}</ref><ref>{{Cite journal |last1=Messaoud |first1=S. |last2=Youssef |first2=H. |date=2009 |title=An analytical model for the performance evaluation of stack-based Web cache replacement algorithms |url=https://onlinelibrary.wiley.com/doi/10.1002/dac.1036 |journal=International Journal of Communication Systems |language=en |volume=23 |pages=1–22 |doi=10.1002/dac.1036|s2cid=46507769 }}</ref>

==Parts of the system==

===Forward and reverse===
A forward cache is a cache outside the [[Web server|web server]]'s network, e.g. in the client's [[Software:Web browser|web browser]], in an ISP, or within a corporate network.<ref>{{cite web |url=http://www.isaserver.org/tutorials/Understanding-Web-Caching-Concepts-ISA-Firewall.html |title=Understanding Web Caching Concepts for the ISA Firewall |first=Thomas |last=Shinder |access-date=27 February 2011 |date=2 September 2008 |work=ISA Server |archive-url=https://web.archive.org/web/20110723104247/http://www.isaserver.org/tutorials/Understanding-Web-Caching-Concepts-ISA-Firewall.html |archive-date=23 July 2011 |publisher=TechGenix Ltd.}}</ref> A network-aware forward cache only caches heavily accessed items.<ref name=":0">{{cite journal |url=http://www.research.att.com/export/sites/att_labs/techdocs/TD_7LAUDD.pdf |citeseerx=10.1.1.159.1786 |title=Network-Aware Forward Caching |first1=Jeffrey |last1=Erman |first2=Alexandre |last2=Gerber |first3=Mohammad T. |last3=Hajiaghayi |first4=Dan |last4=Pei |first5=Oliver |last5=Spatscheck |journal=AT&T Labs |year=2008 |access-date=11 March 2019 |archive-url=https://web.archive.org/web/20110401145549/http://www.research.att.com/export/sites/att_labs/techdocs/TD_7LAUDD.pdf |archive-date=1 April 2011 |pages=291–300}}</ref> A [[Proxy server|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|content delivery network]] (CDN) that retains copies of web content at various points throughout a network.

===HTTP options===
The Hypertext Transfer Protocol (HTTP) defines three basic mechanisms for controlling caches: freshness, validation, and invalidation.<ref>{{cite web |url=http://ws-rest.org/files/03-Link%20Header-based%20Invalidation%20of%20Caches.pdf |title=Using HTTP Link: Header for Gateway Cache Invalidation |access-date=14 June 2013 |first1=Mike |last1=Kelly |first2=Michael |last2=Hausenblas |pages=20 |work=WS-REST |archive-date=10 July 2010 |archive-url=https://web.archive.org/web/20100710072021/http://www.ws-rest.org/files/03-Link%20Header-based%20Invalidation%20of%20Caches.pdf |url-status=dead }}</ref> 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 [[HTTP ETag|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==
In 1998, the DMCA added rules to the United States Code (17 U.S.C. §: 512) that exempts system operators from [[Copyright|copyright]] liability for the purposes of caching.

==Server-side software==
This is a list of server-side web caching software.
{| style="font-size: 85%; text-align: center; width: auto;" class="sortable wikitable"
|-
!rowspan="2"|Name
!colspan="3"|Operating system
!rowspan="2"|Forward<br />mode
!rowspan="2"|Reverse<br />mode
!rowspan="2"|License
|-
![[Software:Microsoft Windows|Windows]]
![[Unix-like]]
!Other
|-
! scope="row" |[[Software:Apache HTTP Server|Apache HTTP Server]]
| {{yes}}
| {{yes|OS X, Linux, Unix, FreeBSD, Solaris, Novell NetWare}}
| {{yes|OS/2, TPF, OpenVMS, eComStation}}
|{{Yes}}
|
| {{open source|Apache 2.0}}
|-
! scope="row" | [[Company:AiScaler|aiScaler]] Dynamic Cache Control
| {{no}}
| {{yes|[[Software:Linux|Linux]]}}
| {{no}}
|
|
| {{Proprietary}}
|-
! scope="row" | [[Company:ApplianSys#CACHEbox|ApplianSys CACHEbox]]
| {{no}}
| {{yes|[[Software:Linux|Linux]]}}
| {{no}}
|
|
| {{Proprietary}}
|-
! scope="row" | [[Company:Blue Coat Systems|Blue Coat]] ProxySG
| {{no}}
| {{no}}
| {{yes2|SGOS}}
| {{Yes}}
| {{Yes}}
| {{Proprietary}}
|-
! scope="row" | [[Software:Nginx|Nginx]]
| {{yes}}
| {{yes|[[Software:Linux|Linux]], BSD, [[Software:OS X|OS X]], [[Software:Solaris (operating system)|Solaris]], [[Software:AIX|AIX]], [[Software:HP-UX|HP-UX]]}}
| {{yes}}
| {{Yes}}
| {{Yes}}
| {{open source|2-clause [[Software:BSD licenses|BSD]]-like}}
|-
! scope="row" | [[Software:Microsoft Forefront Threat Management Gateway|Microsoft Forefront Threat Management Gateway]]
| {{yes}}
| {{no}}
| {{no}}
| {{Yes}}
| {{Yes}}
| {{Proprietary}}
|-
! scope="row" | [[Software:Polipo|Polipo]]
| {{yes}}
| {{yes|[[Software:OS X|OS X]], [[Software:Linux|Linux]], [[Software:OpenWrt|OpenWrt]], [[Software:FreeBSD|FreeBSD]]}}
| {{dunno}}
| {{Yes}}
| {{Yes}}
| {{open source|[[Software:MIT License|MIT License]]}}
|-
! scope="row" | [[Software:Squid|Squid]]
| {{yes}}
| {{yes|[[Software:Linux|Linux]]}}
| {{dunno}}
| {{Yes}}
| {{Yes}}
| {{open source|[[GPL]]}}
|-
! scope="row" | Traffic Server
| {{dunno}}
| {{yes|[[Software:Linux|Linux]]}}
| {{dunno}}
| {{Yes}}
| {{Yes}}
| {{open source|Apache 2.0}}
|-
! scope="row" | Untangle
| {{no}}
| {{yes|[[Software:Linux|Linux]]}}
| {{no}}
| {{Yes}}
| {{Yes}}
| {{Proprietary}}
|-
! scope="row" | [[Software:Varnish|Varnish]]
| {{no}}
| {{yes|[[Software:Linux|Linux]]}}
| {{no}}
| {{Needs}} a VMOD
| {{Yes}}
| {{open source|[[Software:BSD licenses|BSD]]}}
|-
! scope="row" | [[Software:WinGate|WinGate]]
| {{yes}}
| {{no}}
| {{no}}
| {{Yes}}
| {{Yes}}
| {{Proprietary}} (Free for 8 users)
|-
! scope="row" |Nuster
| {{no}}
| {{yes|[[Software:Linux|Linux]]}}
| {{no}}
| {{Yes}}
| {{Yes}}
| {{open source|[[GPL]]}}
|-
![[Company:McAfee|McAfee]] Web Gateway
| {{no}}
| {{yes2|McAfee Linux Operating System}}
| {{no}}
| {{Yes}}
| {{Yes}}
| {{Proprietary}}
|}

== See also ==
* [[Software:InterPlanetary File System|InterPlanetary File System]] - makes web caches redundant
* [[Cache Discovery Protocol]]
* [[Cache manifest in HTML5]]
* [[Content delivery network]]
* [[Harvest project]]
* [[Proxy server]]
* [[Software:Web accelerator|Web accelerator]]
*[[Software:Search engine cache|Search engine cache]]

== References ==
{{Reflist}}

==Further reading==
* 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}}

{{DEFAULTSORT:Web Cache}}
[[Category:Hypertext Transfer Protocol]]
[[Category:Cache (computing)]]
[[Category:Proxy servers|*]]
[[Category:Web caching protocol]]

{{Sourceattribution|Web cache}}