PURL

The Persistent - or Permanent - Uniform Resource Locator (PURL) was first developed by the US Online Computer Library Center (OCLC), developers of the Dublin Core metadata suite. It is intended as a naming and resolution service for web publications, pending successful implementation of the URN scheme described on a separate page of this briefing. 

    URLs

Finding information on the web is based on URLs - an address for each document with a format that's similar to the URL for this page:

metadataprofile5.htm

URLs identify documents according to their location. This document, for example is located in the identification folder of the briefings component of the Caslon domain within the dot com domain space.

URLs are familiar to most users of the web, who take them for granted as a mechanism for identifying online documents and describing their location for future retrieval. However, they have been criticised by some as unsatisfactory. 

Critics note that each URL simply points to the current location of a document, rather than uniquely identifying it independent of its location in cyberspace. If a resource is moved to a new location (renamed, placed in a new folder on the same site, moved to a new site), the URL is no longer useful because it points to a location that no longer exists. It's not unique and it's not persistent.

     PURLs

A PURL looks just like a URL, except it points to a 'resolver service' instead of the actual location of the digital publication. 

The resolver service then redirects the user to the appropriate URL. The PURL is essentially a link to the current location of the particular document to which it is assigned. The intention is that when a document's location changes - eg it moves from one part of a site to another or to another site altogether - the metadata in the PURL resolver service would be updated, so that people could still find the publication using the same PURL.

While the results of initial work on PURLs are promising, global implementation requires significant development of the Uniform Resource Identifier (URI) architecture - essentially a major feature of the next generation of the internet.

    reading

Cliff Lynch's 1997 article on Identifiers & Their Role In Networked Information Applications is an excellent introduction to the nature of identifiers in the networked environment, criticisms of URLs, URNs, PURLs and other identifiers such as SICI or USIN (discussed in the ECMS page of our Intellectual Property guide).

PURLs: Persistent Uniform Resource Locators, a 1998 paper by Stuart Weibel & Keith Shafer, offers a succinct introduction. Persistent Identifiers on the Digital Terrain, a paper by Sandra Payette, and Ian Peacock's article What is...a URI? are other useful introductions.

There's a more comprehensive introduction to PURLs and broader information architecture questions in Tita van der Werf-Davelaar's paper on Identification, Location & Versioning of Web Resources. 

The US Library of Congress article on The Relationship between URNs, Handles & PURLs outlines the interrelationship between the three schemes.



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