Mirror (computing)
A mirror in computing is a direct copy of a data set. On the Internet, a mirror site is an exact copy of another Internet site (often a web site). Mirror sites are most commonly used to provide multiple sources of the same information, and are of particular value as a way of providing reliable access to large downloads. Mirroring is a one-way operation whereas file synchronization is two-way.
Rationale
Mirroring of sites occurs for a variety of reasons:
- To protect data from a failure, usually in hardware. See disk mirror.
- To allow faster downloads for users at a specific geographical location. For example, a US server could be mirrored in Japan, allowing Japanese Internet users to download content faster from the local Japanese server than from the original American one. This may be viewed as caching on a worldwide scale.
- To counteract censorship and promote freedom of information. For example, an activist might post pictures on a website of a company conducting illegal activities or make available information on secret government activity and be litigated for such. Other internet users will make the content in question available on other servers when the legal action results in the cancellation of ISP or DNS services for the original activist.
- To provide access to otherwise unavailable information. For example, when the popular Google search engine was banned in 2002 by the People's Republic of China, the mirror elgooG was set up as a way of effectively circumventing the ban.
- To preserve historic content. Financial constraints may prevent the maintainers of a server from keeping older and unsupported content available to users who still may desire them - a mirror may be made to prevent this content from disappearing.
- To balance load. If one server is extremely popular a mirror may help relieve this load: for example if a Linux distribution is released as an ISO image onto the distribution developer's own server, this server may become overloaded with demand. Alternative download points allow the total number of download requests to be spread among several servers, maintaining the availability of the distribution.
- As a temporary measure to counterbalance a sudden, temporary increase in traffic. For example, Slashdotted websites will often be mirrored by a few slashdot posters until the article is pushed off the front page.
- To increase a site's ranking in a search engine by placing hyperlinks from each mirror to every other mirror (a technique known as link farming). This is viewed as unethical by most search engine administrators and websurfers.
- Rarely, as a form of plagiarism. Usually pointless since a website popular enough to be worth plagiarizing will quickly discover the copy as soon as one of their many readers stumbles onto the plagiarized site.
- As a form of raising advertising revenue. Wikipedia is probably the best example of material released under the GNU Free Documentation License which is then duplicated by other companies which, unlike wikipedia, then attempt to generate money from advertising, etc. See Wikipedia:Mirrors and forks. An example of this is the television article, which is mirrored at
http://omniknow.com/common/wiki.php?in=en&term=Television
http://freeencyclopedia.net/index.php?title=Television
http://www.digeraticafe.com/reference/Television
http://www.startlearningnow.com/TV.htm
http://factsandlinks.com/common/wiki.php?in=en&term=Chile%252FCommunications
http://www.baghdadmuseum.org/ref/index.php?title=Television
http://link-ads.com/resource.php?title=Television
http://www.yourart.com/research/encyclopedia.cgi?subject=/television
http://www.apawn.com/search.php?title=Television
http://www.internet-encyclopedia.org/wiki.php?title=Television
http://recipes.paellaman.com/encyclopedia.php?title=Television
http://www.mygnu.net/articles/Television
http://www.everybase.com/Television
Examples
A good example of mirroring is the well-known SourceForge.net website. The basis of the Sourceforge concept is, primarily, the hosting of open source software projects, but secondarily the use of many different locations to achieve one goal: to maintain download availablity to the user. Many innovative computer projects host their sites and software on SourceForge, which provides mirrors in several states and countries, from Dublin, Ireland to Tokyo, Japan.
Examples of even larger mirrored networks include those of the Debian and FreeBSD software projects. The encyclopedia Wikipedia is mirrored at numerous locations.
Programs
There are numerous computer programs that provide automated mirroring of entire sites. Some are oriented towards personal use, which allows browsing from a local copy — this means an initial waiting time but much improved load time for those pages once they're mirrored. Others are intended to be used by public mirror maintainers.
Examples include: