Post on 22-Dec-2015
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CS 502: Computing Methods for Digital Libraries
Lecture 4
Identifiers and Reference Links
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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
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Syntax<naming_authority>/<locally_unique_string>
orhdl:<naming_authority>/<locally_unique_string>
Examples10.1234/1995.02.12.16.42.21;9 (date-time stamp)cornell.cs/cstr-94.45 (mnemonic name)loc/a43v-8940cgr (random string)
Syntax of Handles
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Examples of DOIs
10.156 / catalog-96
Publisher ID
assigned byDOI Agency
Item IDassigned byPublisher
10.1048 / 872
10.1532 / PII
10.18698 / SICI
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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
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Hierarchy of Naming Authorities
loc 10 cornell
loc.cords
10.1234
cornell.temp
cornell.cs
cornell.cs.d
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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.
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Handle Record for a Digital Object
Adm Admin Data
URL
cnri.dlib/arms-09 Adm Admin Data
http://www.cnri/xyz
RAP merlin.dlib.orgNEWorb:#cornell[]norb
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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
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Resolving a Handle Without CachesHandle cnri.dlib/wya in Global G
GlobalClient
? cnri.dlib/wya ?
handle data
G
cnri.dlib/wya
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Resolving a Handle Without CachesHandle 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 datacnri.dlib/wya
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Caching Handle Service
Client Caching Server Handle Servers
Hash
Cache
Hash table
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Replication
All data is replicated at several sitesfor performance and reliability
Washington, DCLos Angeles, CA
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Applications of Identifiers
The challenges:Persistent, unique identifiersEliminate broken linksControl duplicates
Applications:On-line publicationRegistrationCitation (reference links)Collection managementArchives
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User
HandleSystem
DOIs and URNs in Action
Publisher
DOI
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Flexibility for Publisher
Warehouse
Database
Repository
Every publisher can have adifferent system.
DOIDOI
DOI
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Reorganization by Publisher
Database
Repositories
The publisher can create a newsystem.
DOI
DOI
DOI
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Change of Publisher
Halfmoon
Millenium
User
DOI
HandleSystem
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HandleSystem
Citation
Publisher
User 1
DOI
User 2
DOI
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User
HandleSystem
Publisher
Search System
DOI
Catalogs and Indexes
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Copyright Registration
Copyright Registry
User
Handle System
Halfmoon
DOI
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Multiple Copies
Halfmoon Europe
User
DOI
HandleSystem
Halfmoon USA
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User
HandleSystem
Archives
Archive
DOI
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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, howdoes a user get from the citation to an appropriate copy of the article?
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The General Model
Referencedatabase
Locationdatabase
Content
Publisher
Client
Publisher places information in databases
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The General Model
Referencedatabase
Locationdatabase
Content
Publisher
ClientCitation
Identifiers
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The General Model
Referencedatabase
Locationdatabase
Content
Publisher
Client
URLsIdentifier
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The General Model
Referencedatabase
Locationdatabase
Content
Publisher
Client
URLContent
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The General Model
Referencedatabase
Locationdatabase
Content
Publisher
Client
CitationIdentifiers
URLsIdentifierURL
Content
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Target of Citations
• Work
• Expression
• Manifestation
• Item
IFLA model
Citations can refer to any specific creation but for journals usually refer to the work.
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Identifiers
• Are identifiers necessary?
– Persistence
– Flexible targets
• Examples:
– PubMed ID, BibCode, DOI, etc.
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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.
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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.
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Metadata in Reference Database
• Existing schemes
– Considerable agreement on minimal elements
– Considerable differences in details and syntax
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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")
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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
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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?