Tuesday, August 11, 2009

Link rot

Link rot (or linkrot) is the process by which links on a website gradually become irrelevant or broken as time goes on, because websites that they link to disappear, change their content, or move to new locations.

The phrase also describes the effects of failing to update web pages so that they become out-of-date, containing information that is old and useless, and that clutters up search engine results.

The 404 "not found" response is familiar to even the occasional Web user. A number of studies have examined the prevalence of link rot on the Web, in academic literature, and in digital libraries. In a 2003 experiment, Fetterly et al. (2003) discovered that about one link out of every 200 disappeared each week from the internet. McCown et al. (2005) discovered that half of the URLs cited in D-Lib Magazine articles were no longer accessible 10 years after publication, and other studies have shown link rot in academic literature to be even worse (Spinellis, 2003, Lawrence et al., 2001). Nelson and Allen (2002) examined link rot in digital libraries and found that about 3% of the objects were no longer accessible after one year.

Some news sites contribute to the link rot problem by keeping only recent news articles online where they are freely accessible at their original URLs, then removing them or moving them to a paid subscription area. This causes a heavy loss of supporting links in sites discussing newsworthy events and using news sites as references.


Detecting link rot for a given URL is difficult using automated methods. If a URL is accessed and returns back an HTTP 200 (OK) response, it may be considered accessible, but the contents of the page may have changed and may no longer be relevant. Some web servers also return a soft 404, a page returned with a 200 (OK) response (instead of a 404) that indicates the URL is no longer accessible. Bar-Yossef et al. (2004) developed a heuristic for automatically discovering soft 404s.

There are several tools that have been developed to help combat link rot.

* WordPress guards against link rot by replacing non-canonical URLs with their canonical versions.[1]
* When a user receives a 404 response, the Google Toolbar for Internet Explorer attempts to assist the user in finding the missing page.[2]
* IBM's Peridot attempts to automatically fix broken links.

Web archiving
To combat link rot, web archivists are actively engaged in collecting the Web or particular portions of the Web and ensuring the collection is preserved in an archive, such as an archive site, for future researchers, historians, and the public. The largest web archiving organization is the Internet Archive Wayback Machine, which strives to maintain an archive of the entire Web, taking periodic snapshots of pages that can then be accessed for free and without registration many years later simply by typing in the URL. National libraries, national archives and various consortia of organizations are also involved in archiving culturally important Web content.

Individuals may also use a number of tools that allow them to archive web resources that may go missing in the future:

* WebCite, a tool specifically for scholarly authors, journal editors and publishers to permanently archive "on-demand" and retrieve cited Internet references (Eysenbach and Trudel, 2005).
* Archive-It, a subscription service that allows institutions to build, manage and search their own web archive
* Some social bookmarking websites, such as Furl, make private copies of web pages bookmarked by their users.

Webmasters have developed a number of best practices for combating link rot:

* Avoiding unmanaged hyperlink collections
* Avoiding links to pages deep in a website ("deep linking")
* Using hyperlink checking software or a Content Management System (CMS) that automatically checks links
* Using permalinks
* Using redirection mechanisms (e.g. "301: Moved Permanently") to automatically refer browsers and crawlers to the new location of a URL

Authors citing URLs
A number of studies have shown how widespread link rot is in academic literature (see below). Authors of scholarly publications have also developed best-practices for combating link rot in their work:

* Avoiding URL citations that point to resources on a researcher's personal home page (McCown et al., 2005)
* Using Persistent Uniform Resource Locators (PURLs) and digital object identifiers (DOIs) whenever possible
* Using web archiving services (e.g. WebCite) to permanently archive and retrieve cited Internet references (Eysenbach and Trudel, 2005).

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