آئینه رایانه

در علوم رایانه، آینه (به انگلیسی: Mirror) به نسخه رونوشت عینی یک مجموعه داده گفته می‌شود. در شبکهٔ اینترنت یک وب‌گاه آینه، رونوشتی عینی (کپی دقیق و کامل) از وب‌گاه دیگر می‌باشد.

مورد استفاده آینه‌ها در اینترنت ایجاد نمودن منابع متعدد برای داده‌های یکسان است. آینه‌ها امکان دسترسی همیشگی و قابل اطمینان کاربران را به وبگاه‌هایی با حجم درخواست و بارگذاری بالا، فراهم می‌کنند. در این صورت در هنگام از کار افتادن یک سرویس دهنده به دلیل ترافیک بالا، نسخه پشتیبان دیگری قادر به سرویس دهی خواهد بود.

یک آینه زنده به صورت خودکار و به محض به روز شدن منبع، خود را به روز می‌کند.

دلایل استفاده از آینه‌ها [ویرایش]

حفظ و نگهداری از اطلاعات از دسترس خارج شده یا در شرف خروج (صفحات، وب‌گاه‌ها و یا داده‌های قدیمی که نگهداری آنها هزینه بر است)
فراهم نمودن بارگذاری سریعتر برای کاربرانی که در موقعیت‌های جغرافیایی مختلف بسر می‌برند
مقابله با سانسور اطلاعاتی و ترویج آزادی بیان
ایجاد تعادل میان بار سرویس دهنده‌ها
تدبیر موقت برای مقابله با افزایش ناگهانی ترافیک در مواقع اضطراری
بالا بردن رتبه وبگاه در موتورهای جستجوگر با قرار دادن پیوند به آینه‌های مشابه.(این عمل در نظر بسیاری از مدیران موتورهای جستجو و نیز کاربران، امری غیر اخلاقی در حیطه رایانه به شمار می‌رود)
بهره برداری مالی ناشی از قرار دادن تبلیغات در وبگاه‌های فاقد تبلیغات و گاه بر خلاف سیاست‌های آنان.(مانند ویکی‌پدیا[۱])
به عنوان روشی برای دور زدن دیواره‌های آتش
پانویس

↑ http://en.wikipedia.org/wiki/Wikipedia:Mirrors_and_forks
منبع

مشارکت‌کنندگان ویکی‌پدیا، «(Mirror (computing»، ویکی‌پدیای انگلیسی، دانشنامهٔ آزاد (بازیابی در ۲۸ فوریه ۲۰۱۰).
از ویکی پدیا

همچنین:
In computing, a mirror is an exact copy of a data set. On the Internet, 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.
A live mirror is automatically updated as soon as the original is changed.
Contents [hide]
1 Reasons
2 Examples
3 Software
4 See also
5 References
[edit]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. 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 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 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. Metalink is frequently used for automatic load balancing by listing all mirrors.
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; 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.[citation needed]
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.
To serve as a method of circumventing firewalls.
[edit]Examples

A good example of mirroring is the well-known Sourceforge 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 availability to the user. Many innovative computer projects host their sites and software on SourceForge, which provides mirrors in several countries, from Dublin, Ireland to Tokyo, Japan.
Examples of even larger mirrored networks include those of the Debian,[1] Ubuntu,[2] and FreeBSD software projects. The encyclopedia Wikipedia is mirrored at numerous locations.
Wikileaks also applies this technique in cooperation with volunteer servers worldwide in response to its account termination at Amazon.com as well as heavy cyber attack upon its server after its controversial publication of secret United States diplomatic cables around the world in order to make it "impossible to ever fully remove Wikileaks from the Internet".[3]
[edit]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
rsync
wget
Free file mirroring software includes MMup by MassMirror and wget, mirror and mirrordir available as add ons for many Linux distributions.
[edit]See also

Coral CDN
Disk mirroring
Offline browsing
Website mirroring software
Wikipedia:Mirrors and forks
[edit]References

^ "Debian mirrors". Debian. Retrieved 2010-02-19.
^ "Mirrors - Ubuntu Wiki". Ubuntu. Retrieved 2010-02-19.
^ "Wikileaks - Mirrors of our website". Wikileaks. Retrieved 2010-12-07.
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