Mirroring

Mirroring is the production of an exact copy of a set of computer data, to create a backup or for some other purpose. For instance, mirror sites on the Internet are exact copies (with functioning internal and external links and all data, images, and media) of Web sites. Mirror sites may be created to protect data in the event that something happens to the server on which the original is stored—for instance, to protect the data from damage to the server as the result of disaster—or to preserve a copy of the data in the event that the original is deliberately tampered with. Some sites now automatically mirror the Twitter feeds of all politicians, so if a controversial tweet is later deleted, a copy of it exists so that its issue cannot be denied. In other instances, government, employer, or court-ordered censorship may result in a site being taken down; creating mirror sites (typically not under the control of the original site's owner) makes it more difficult to constrain that information. For this reason, for example, there are mirrors of Wikileaks and its files, which allowed it to survive the termination of its account with Amazon. Mirror sites also provide access to information when access to an original site is restricted, a method that has been used to get around the “great firewall of China” that restricts Chinese computer users’ access to many sites.

Mirror sites may also be created in order to spread out the bandwidth usage of site access; this is often done with sites hosting data to be downloaded, so that when the original site is overloaded, users may download from mirror sites instead. Such mirror sites also allow users to download from the server closest to them, enabling faster access. (Less commonly, mirror sites are created in order to “link farm”—manipulating search engine results by using the mirrors to link to each other. This is contrary to the goal of search engines, and is considered unethical.) Sites are sometimes temporarily mirrored when traffic spikes as a result of some specific event, such as recent news coverage. And there is increasing interest, as the Web steadily grows older and much of the material produced for it exists in no other form, in mirroring sites for archival purposes, just as historians produce and preserve audio and video recordings.

Sites that have been mirrored may be browsed off-line, which may be seen as an additional benefit. Some browsers are designed specifically for off-line browsing, but most popular Web browsers can be used for the purpose. Sites can also be automatically downloaded and mirrored locally (on the user's computer, not on a server) for offline browsing later, and software has existed for this purpose nearly as long as Web browsers have; today, the software is often offered as extensions for Web browsers. Use of the Kiwix off-line Wikipedia reader allows, in essence, access to a large copyright-free encyclopedia without requiring Internet access; Kiwix has been used for schools and libraries in developing countries.

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