Persistent uniform resource locator

A persistent uniform resource locator (PURL) is a uniform resource locator (URL) (i.e., location-based uniform resource identifier or URI) that is used to redirect to the location of the requested web resource. PURLs redirect HTTP clients using HTTP status codes. PURLs are used to curate the URL resolution process, thus solving the problem of transitory URIs in location-based URI schemes like HTTP. Technically the string resolution on PURL is like SEF URL resolution.

The PURL concept was developed at OCLC (the Online Computer Library Center) in 1995 and implemented using a forked pre-1.0 release of Apache HTTP Server. The software was modernized and extended in 2007 by Zepheira under contract to OCLC and the official website moved to http://purlz.org (the 'Z' came from the Zepheira name and was used to differentiate the PURL open-source software site from the PURL resolver operated by OCLC).

PURL version numbers may be considered confusing. OCLC released versions 1 and 2 of the Apache-based source tree, initially in 1999 under the OCLC Research Public License 1.0 License and later under the OCLC Research Public License 2.0 License (http://opensource.org/licenses/oclc2). Zepheira released PURLz 1.0 in 2007 under the Apache License, Version 2.0. PURLz 2.0 was released in Beta testing in 2010 but the release was never finalized. The Callimachus Project implemented PURLs as of its 1.0 release in 2012.

The oldest PURL HTTP resolver was operated by OCLC from 1995 to September 2016 and was reached as purl.oclc.org as well as purl.org, purl.net, and purl.com.

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