overview
on the web
Dublin Core
RDF
PICS
PURLs
URNs
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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: metadata5.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 (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.
next page (URNs)
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