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Competitive Evaluation of PURLs

Larry Stone, 22-Mar-00

This document is a statement of my evaluation of the PURL system developed by the OCLC, as a technology to support MIT's Persistent Naming discovery project. Most of my information about PURLs comes from close reading of material from the www.purl.org website itself and explication of PURLs in various published papers. I have firsthand experience administering Handle System servers and namespaces for the NCSTRL and Theses Online projects. I will compare PURLs to "Handle System" Handles to demonstrate why Handles are the better choice for truly archival naming and experimentation with other uses.

On the surface, PURLs and Handles have a lot in common. Both purport to implement the URN concept. At the design and implementation level, however, I believe PURL design is more simplistic and tied to current technology standards that limit its longevity. It presently has some operational and convenience advantages but sacrifices the archival quality of its names.

The following text assumes some familarity with PURLs and Handles. Please consult the relevant websites for background information if you are not already familiar with them:





Since the goal of this project is to attach truly archival, long-lived names to network-accessible resources, I think PURLs should not be considered. My primary objection is that PURLs rely on DNS for labeling namespaces, which has at least two problems in the long run: DNS names are controlled by outside agencies at many levels (i.e. not just local administrators, but our ".EDU" parent domain is subject to the Internet governing bodies). Also, I believe the entire DNS naming system will be revised within the next 100 years, which is probably shorter than the range MIT Archives routinely anticipates.

Although the Handle System currently needs the crutch of HTTP proxies which have the same DNS naming problem, it is inherently free of the domain name system and even the current Internet implementation. The Handle namespace is not connected to any other protocol or standard, because it was properly designed to persist as a meaningful, resolvable naming system well into the forseeable future.

The comparison of PURLs to Handles is a worthwhile exercise, since it illuminates some weaknesses of the present unfortunate need to use HTTP redirection to resolve any type of "URN". It also inspires more rational thinking of what an absolutely ideal persistent naming system might be like, outside of the influence of the Handle System.


  1. The PURL website, http://www.purl.org
  2. W3C and Digital Libraries, by James S. Miller, D-Lib magazine nov. 1996 www.dlib.org/dlib/november96/11miller.html
  3. Best Practices for Digital Archiving An Information Life Cycle Approach by Gail M. Hodge, D-Lib January 2000 http://www.dlib.org/dlib/january00/01hodge.html
  4. Naming and Addressing: URIs, URLs, ... by the W3 Consortium (also includes pointers to appropriate RFCs for URN, etc.) http://www.w3.org/Addressing/
  5. The Myth of Names and Addresses, by Tim Berners-Lee, design note, at http://www.w3.org/DesignIssues/NameMyth.html