Mirror (computing) - AbsoluteAstronomy.com

Computing

Computing is usually defined as the activity of using and improving computer hardware and software. It is the computer-specific part of information technology...

, a mirror is an exact copy of a data

Data

The term data refers to qualitative or quantitative attributes of a variable or set of variables. Data are typically the results of measurements and can be the basis of graphs, images, or observations of a set of variables. Data are often viewed as the lowest level of abstraction from which...

set. On the Internet

Internet

The Internet is a global system of interconnected computer networks that use the standard Internet protocol suite to serve billions of users worldwide...

, a mirror site is an exact copy of another Internet 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 type of file synchronization

File synchronization

File synchronization in computing is the process of ensuring that computer files in two or more locations are updated via certain rules....

.

A live mirror is automatically updated as soon as the original is changed.

Reasons

Mirroring of sites occurs for a variety of reasons.

To preserve a website or page, especially when it is closed or is about to be closed.
To allow faster downloads for users at a specific geographical location. For example, a U.S.
United States
The United States of America is a federal constitutional republic comprising fifty states and a federal district...

server could be mirrored in Japan
Japan
Japan is an island nation in East Asia. Located in the Pacific Ocean, it lies to the east of the Sea of Japan, China, North Korea, South Korea and Russia, stretching from the Sea of Okhotsk in the north to the East China Sea and Taiwan in the south...

, 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
Cache
In computer engineering, a cache is a component that transparently stores data so that future requests for that data can be served faster. The data that is stored within a cache might be values that have been computed earlier or duplicates of original values that are stored elsewhere...

on a worldwide scale.
To counteract censorship
Censorship
thumb|[[Book burning]] following the [[1973 Chilean coup d'état|1973 coup]] that installed the [[Military government of Chile |Pinochet regime]] in Chile...

and promote freedom of information
Freedom of information
Freedom of information refers to the protection of the right to freedom of expression with regards to the Internet and information technology . Freedom of information may also concern censorship in an information technology context, i.e...

. 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
Internet service provider
An Internet service provider is a company that provides access to the Internet. Access ISPs directly connect customers to the Internet using copper wires, wireless or fiber-optic connections. Hosting ISPs lease server space for smaller businesses and host other people servers...

or DNS
Domain name system
The Domain Name System is a hierarchical distributed naming system for computers, services, or any resource connected to the Internet or a private network. It associates various information with domain names assigned to each of the participating entities...

services for the original activist.
To provide access to otherwise unavailable information. For example, when the popular Google
Google
Google Inc. is an American multinational public corporation invested in Internet search, cloud computing, and advertising technologies. Google hosts and develops a number of Internet-based services and products, and generates profit primarily from advertising through its AdWords program...

search engine was banned in 2002 by the People's Republic of China
People's Republic of China
China , officially the People's Republic of China , is the most populous country in the world, with over 1.3 billion citizens. Located in East Asia, the country covers approximately 9.6 million square kilometres...

, the mirror elgooG
ElgooG
elgooG is the literal mirror image of the engine searcher; not only is all of its content a reversal of Google, the search terms must also be written in reverse in order to yield the desired results...

was used as a way of effectively circumventing the ban.
To preserve historic content. Financial constraints and/or bandwidth 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
Linux distribution
A Linux distribution is a member of the family of Unix-like operating systems built on top of the Linux kernel. Such distributions are operating systems including a large collection of software applications such as word processors, spreadsheets, media players, and database applications...

is released as an ISO image
ISO image
An ISO image is an archive file of an optical disc, composed of the data contents of every written sector of an optical disc, including the optical disc file system...

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. Metalink
Metalink
Metalink is a cross-platform and cross-application Internet standard/framework/file format for programs that download, including download managers, BitTorrent clients, Web browsers, FTP clients, and P2P programs...

is frequently used for automatic load balancing by listing all mirrors.
As a temporary measure to counterbalance a sudden, temporary increase in traffic
Web traffic
Web traffic is the amount of data sent and received by visitors to a web site. It is a large portion of Internet traffic. This is determined by the number of visitors and the number of pages they visit...

. For example, Slashdotted
Slashdot effect
The Slashdot effect, also known as slashdotting, occurs when a popular website links to a smaller site, causing a massive increase in traffic. This overloads the smaller site, causing it to slow down or even temporarily close. The name stems from the huge influx of web traffic that results from...

websites will often be mirrored by a few slashdot
Slashdot
Slashdot is a technology-related news website owned by Geeknet, Inc. The site, which bills itself as "News for Nerds. Stuff that Matters", features user-submitted and ‑evaluated current affairs news stories about science- and technology-related topics. Each story has a comments section...

posters until the article is pushed off the front page.
To increase a site's ranking in a search engine
Search engine
A search engine is an information retrieval system designed to help find information stored on a computer system. The search results are usually presented in a list and are commonly called hits. Search engines help to minimize the time required to find information and the amount of information...

by placing hyperlinks from each mirror to every other mirror (a technique known as link farm
Link farm
On the World Wide Web, a link farm is any group of web sites that all hyperlink to every other site in the group. Although some link farms can be created by hand, most are created through automated programs and services. A link farm is a form of spamming the index of a search engine...

ing). This is viewed as unethical
Ethics
Ethics, also known as moral philosophy, is a branch of philosophy that addresses questions about morality—that is, concepts such as good and evil, right and wrong, virtue and vice, justice and crime, etc.Major branches of ethics include:...

by most search engine administrators and websurfers.
Rarely, as a form of plagiarism
Plagiarism
Plagiarism is defined in dictionaries as the "wrongful appropriation," "close imitation," or "purloining and publication" of another author's "language, thoughts, ideas, or expressions," and the representation of them as one's own original work, but the notion remains problematic with nebulous...

; this is, however, usually pointless, as 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
Wikipedia
Wikipedia is a free, web-based, collaborative, multilingual encyclopedia project supported by the non-profit Wikimedia Foundation. Its 20 million articles have been written collaboratively by volunteers around the world. Almost all of its articles can be edited by anyone with access to the site,...

is probably the best example of material released under the GNU Free Documentation License
GNU Free Documentation License
The GNU Free Documentation License is a copyleft license for free documentation, designed by the Free Software Foundation for the GNU Project. It is similar to the GNU General Public License, giving readers the rights to copy, redistribute, and modify a work and requires all copies and...

which is then duplicated by other companies which, unlike Wikipedia, then attempt to generate money from advertising, etc. See Wikipedia:Mirrors and forks.
To serve as a method of circumventing firewalls.

Examples

A good example of mirroring is the well-known Sourceforge website. The basis of the Sourceforge

SourceForge

SourceForge Enterprise Edition is a collaborative revision control and software development management system. It provides a front-end to a range of software development lifecycle services and integrates with a number of free software / open source software applications .While originally itself...

concept is, primarily, the hosting of open-source software

Open-source software

Open-source software is computer software that is available in source code form: the source code and certain other rights normally reserved for copyright holders are provided under a software license that permits users to study, change, improve and at times also to distribute the software.Open...

projects, but secondarily the use of many different locations to achieve one goal: to maintain download availability to the user. Many innovative computer

Computer

A computer is a programmable machine designed to sequentially and automatically carry out a sequence of arithmetic or logical operations. The particular sequence of operations can be changed readily, allowing the computer to solve more than one kind of problem...

projects host their sites and software on SourceForge, which provides mirrors in several countries, from Dublin, Ireland

Ireland

Ireland is an island to the northwest of continental Europe. It is the third-largest island in Europe and the twentieth-largest island on Earth...

to Tokyo

Tokyo

, ; officially , is one of the 47 prefectures of Japan. Tokyo is the capital of Japan, the center of the Greater Tokyo Area, and the largest metropolitan area of Japan. It is the seat of the Japanese government and the Imperial Palace, and the home of the Japanese Imperial Family...

, Japan

Japan

Japan is an island nation in East Asia. Located in the Pacific Ocean, it lies to the east of the Sea of Japan, China, North Korea, South Korea and Russia, stretching from the Sea of Okhotsk in the north to the East China Sea and Taiwan in the south...

.

Examples of even larger mirrored networks include those of the Debian

Debian

Debian is a computer operating system composed of software packages released as free and open source software primarily under the GNU General Public License along with other free software licenses. Debian GNU/Linux, which includes the GNU OS tools and Linux kernel, is a popular and influential...

, Ubuntu

Ubuntu (operating system)

Ubuntu is a computer operating system based on the Debian Linux distribution and distributed as free and open source software. It is named after the Southern African philosophy of Ubuntu...

, and FreeBSD

FreeBSD

FreeBSD is a free Unix-like operating system descended from AT&T UNIX via BSD UNIX. Although for legal reasons FreeBSD cannot be called “UNIX”, as the direct descendant of BSD UNIX , FreeBSD’s internals and system APIs are UNIX-compliant...

software projects. The encyclopedia Wikipedia

Wikipedia

Wikipedia is a free, web-based, collaborative, multilingual encyclopedia project supported by the non-profit Wikimedia Foundation. Its 20 million articles have been written collaboratively by volunteers around the world. Almost all of its articles can be edited by anyone with access to the site,...

is mirrored at numerous locations.

Wikileaks

Wikileaks

WikiLeaks is an international self-described not-for-profit organisation that publishes submissions of private, secret, and classified media from anonymous news sources, news leaks, and whistleblowers. Its website, launched in 2006 under The Sunshine Press organisation, claimed a database of more...

also applies this technique in cooperation with volunteer servers worldwide in response to its account termination at Amazon.com

Amazon.com

Amazon.com, Inc. is a multinational electronic commerce company headquartered in Seattle, Washington, United States. It is the world's largest online retailer. Amazon has separate websites for the following countries: United States, Canada, United Kingdom, Germany, France, Italy, Spain, Japan, and...

as well as heavy cyber attack upon its server after its controversial publication of secret United States diplomatic cables around the world

United States diplomatic cables leak

The United States diplomatic cables leak, widely known as Cablegate, began in February 2010 when WikiLeaks—a non-profit organization that publishes submissions from anonymous whistleblowers—began releasing classified cables that had been sent to the U.S. State Department by 274 of its consulates,...

in order to make it "impossible to ever fully remove Wikileaks from the Internet".

Software

There are numerous offline browsers 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.

Other programs are intended to be used by public mirror maintainers.

CVSup
CVSup
CVSup is a computer program written for Unix/Linux based systems that synchronizes files and directories from one location to another while minimizing data transfer using file-type specific delta encoding when appropriate...
rsync
Rsync
rsync is a software application and network protocol for Unix-like and Windows systems which synchronizes files and directories from one location to another while minimizing data transfer using delta encoding when appropriate. An important feature of rsync not found in most similar...

Free file mirroring software includes MMup by MassMirror and wget

Wget

GNU Wget is a computer program that retrieves content from web servers, and is part of the GNU Project. Its name is derived from World Wide Web and get...

, mirror and mirrordir available as add ons for many Linux distribution

Linux distribution

A Linux distribution is a member of the family of Unix-like operating systems built on top of the Linux kernel. Such distributions are operating systems including a large collection of software applications such as word processors, spreadsheets, media players, and database applications...

Reasons

Examples

Software

See also