1 CS 502: Computing Methods for Digital Libraries Lecture 4 Identifiers and Reference Links

2 Desirable Properties of Identifiers Location independent name Globally unique Persistent across time Choice of human generated or automatic generation Fast resolution Decentralized administration Supported from standard user interfaces

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

4 Examples of DOIs / catalog-96 Publisher ID assigned by DOI Agency Item ID assigned by Publisher / / PII / SICI

5 Elements of the Handle System Handle services: global handle service local handle services caching services Clients: client libraries browser extension WWW proxy servers Handle administration System utilities

6 Hierarchy of Naming Authorities loc 10cornell loc.cords cornell.temp cornell.cs cornell.cs.d

7 Handle Servers and Handle Service The Global Handle Service provides central coordination for all handle services. Each naming authority has a home handle service (which may be Global) where its handles are maintained. Each handle service may be implemented as several handle servers. A hashing algorithm determines the server used to store a given handle.

8 Handle Record for a Digital Object Adm Admin Data URL cnri.dlib/arms-09 Adm Admin Data RAP merlin.dlib.org NEW orb:#cornell[]norb

9 Address Rules The Global Handle Service stores: a record for each naming authority a record for each local handle service The record for each naming authority includes: the home handle service for that naming authority For each handle, the home handle service stores: the handle record

10 Resolving a Handle Without Caches Handle cnri.dlib/wya in Global G Global Client ? cnri.dlib/wya ? handle data G cnri.dlib/wya

11 Resolving a Handle Without Caches Handle cnri.dlib/wya in Home Service abc Global Home HS for cnri.dlib Client ? cnri.dlib/wya ? G abc pointer to abc ? cnri.dlib/wya ? handle data cnri.dlib/wya

12 Caching Handle Service Client Caching Server Handle Servers Hash Cache Hash table

13 Replication All data is replicated at several sites for performance and reliability Washington, DC Los Angeles, CA

14 Applications of Identifiers The challenges: Persistent, unique identifiers Eliminate broken links Control duplicates Applications: On-line publication Registration Citation (reference links) Collection management Archives

15 User Handle System DOIs and URNs in Action Publisher DOI

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

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

18 Change of Publisher Halfmoon Millenium User DOI Handle System

19 Handle System Citation Publisher User 1 DOI User 2 DOI

20 User Handle System Publisher Search System DOI Catalogs and Indexes

21 Copyright Registration Copyright Registry User Handle System Halfmoon DOI

22 Multiple Copies Halfmoon Europe User DOI Handle System Halfmoon USA

23 User Handle System Archives Archive DOI

24 Reference Linking: The Problem Generic Given the information in a standard citation, how does one get to the thing to which the citation refers? Specific Given the information in a citation to a journal article, how does a user get from the citation to an appropriate copy of the article?

25 The General Model Reference database Location database Content Publisher Client Publisher places information in databases

26 The General Model Reference database Location database Content Publisher Client Citation Identifier s

27 The General Model Reference database Location database Content Publisher Client URL s Identifier

28 The General Model Reference database Location database Content Publisher Client URL Content

29 The General Model Reference database Location database Content Publisher Client Citation Identifiers URLs Identifier URL Content

30 Target of Citations Work Expression Manifestation Item IFLA model Citations can refer to any specific creation but for journals usually refer to the work.

31 Identifiers Are identifiers necessary? –Persistence –Flexible targets Examples: –PubMed ID, BibCode, DOI, etc.

32 How are Identifiers Obtained? Often the client knows the citation, but not the identifier. In the general model identifiers are obtained by searching the reference database. In limited domains, identifiers can be calculated from metadata. The identifier may be embedded in the citation.

33 Reference Database Lookup Static: Reference links are established once for all time. –Current model in journal publishing –Not suitable for general user queries Dynamic: Reference links are established on demand. –Provides link based on most recent information –Success can not be guaranteed Quality of metadata in reference database(s) is crucial.

34 Metadata in Reference Database Existing schemes –Considerable agreement on minimal elements –Considerable differences in details and syntax

35 Minimal Metadata Elements for Journal Article Title of journal article Creator(s) Journal title Date of publication Enumeration (e.g., volume and issue) Location (e.g., page or article number) Type (e.g., "journal article")

36 Resolution of Identifier Choice of resolver (distributed resolution) –Simple model: identifier determines resolver Selection from multiple copies (selective resolution) –Performance criteria –Economic and related criteria –User requirements

37 Interoperability Several reference linking services under development: PubMed Astrophysics Data Center DOI reference service Los Alamos National Laboratory internal reference service What levels of agreement and tools are needed for cross- linking?