1 CS 502: Computing Methods for Digital Libraries Guest Lecture William Y. Arms Identifiers: URNs, Handles, PURLs, DOIs and more

2 Uniform Resource Names (URNs) K. Sollins and L. Masinter, Functional Requirements for Uniform Resource Names. RFC 1737, December Uniform Resource Locator (URL) - identifies by location Uniform Resource Name (URN) - identifies by name URN resolves to a Uniform Resource Characteristics (URC) Kahn and Wilensky (1995) Proposed a universal, high-performance Handle System

3 Desirable Properties of URNs Location independent name Globally unique Persistent across time Choice of human generated or automatic generation Fast resolution with huge numbers of identifiers Decentralized administration Supported from standard user interfaces

4 The CNRI Handle System

5 Handles: Names for Internet Resources A Handle is a name. The resource named by a Handle can be: A library item A collection of library items A catalog record A computer An address A public key for encryption etc., etc., etc.....

6 Syntax / or hdl: / Examples / ;9 (date-time stamp) cornell.cs/cstr (mnemonic name) loc/a43v-8940cgr(random string) Syntax of Handles

7 Example of a Handle and its Data Used to Identify Two Locations URL loc.ndlp.amrlp/ Handle Data typeHandle data RAPloc/repository-1r4589

8 Use of Handles in a Digital Library Repository Handle System Search System User interface

9 Scalability and Caching Client Caching Server Handle Servers Hash Cache Hash table

10 Replication for Performance and Reliability Example: the Global Handle System Washington, DC Los Angeles, CA

11 Global and Local Handle Servers Global Local Handle Servers

12 Ways to Resolve Handles I. Resolution by Program Any program can resolve Handles by sending standard format messages to the Handle System. A set of procedures, with Java and C versions, is available to link into applications programs. They are known as the Handle Client Library.

13 Ways to Resolve Handles II. Web Browsers Browsers modified to recognize Handles. This requires installation of a Handle Extension. 1. Whenever the browser expects a URL, it will recognize "hdl:". 2. The Handle is passed to the Handle System, where it is resolved and a data item of type "URL" is returned. Handle Extensions for Netscape and Internet Explorer are available for most versions of Windows.

14 Ways to Resolve Handles III. Proxies Any Web browser can resolve Handles, even with no extension, via a proxy. For example, the following URL can be used to resolve the Handle loc.ndlp.amrlp/3a16616:

15 Proxy Resolution WWW browser HTTP server URL to Proxy URL Resource Handle System hdl.handle.net Proxy server

16 OCLC's PURL System

17 OCLC's Persistent URL (PURL) A PURL is a URL -> Is fully compatible with today's Internet browsers -> Users need no special software Has some of the desirable features of URNs Lacks some desirable features of URNs -> Resolves only to a URL -> Does not support multiple resolution Developed by OCLC Software openly available

18 PURL Syntax A PURL is a URL. PURL resolvers use standard http redirects to return the actual URL. protocolresolver addressname

19 PURL Namespaces A PURL provides a local (not-global namespace) is different from

20 OCLC PURL Resolution WWW browser PURL server HTTP server PURL database PURL URL Resource

21 Digital Object Identifiers (DOIs)

22 DOIs Digital Object Identifiers (DOIs) A URN scheme used by publishers International DOI Foundation provides administrative framework established guidelines develops services (e.g., reference linking) Uses Handle System DOIs are the only URN scheme that has become reasonably widely accepted

23 Examples of DOIs / catalog-96 Publisher ID assigned by International DOI Foundation Item ID assigned by Publisher / / PII / SICI

24 User Handle System DOIs and URNs in Action Publisher DOI

25 Flexibility for Publisher Warehouse Database Repository Every publisher can have a different system. DOI

26 Reorganization by Publisher Database Repositories The publisher can create a new system. DOI

27 Change of Publisher Halfmoon Millenium User DOI Handle System

28 Handle System Citation Publisher User 1 DOI User 2 DOI

29 User Handle System Publisher Search System DOI Catalogs and Indexes

30 Copyright Registration Copyright Registry User Handle System Halfmoon DOI

31 Multiple Copies Halfmoon Europe User DOI Handle System Halfmoon USA

32 User Handle System Archives Archive DOI

33 DOIs: Target of Citations Work Expression Manifestation Item What does the DOI refer to? Citations can refer to any specific creation but for journals usually refer to the work. -> DOIs refer to work

34 Why are URNs not used more? No browser support -- need proxy or software One size does not fit all -- special purpose URN schemes have been successful, e.g., PubMed ID, Astrophysics BibCode Handle system is overly complex and poorly supported Persistence of identifiers needs well regulated administrative system, e.g., DOIs Lack of compelling applications to compensate for extra complexity -- reference linking is emerging