1. Identifiers and Types CS431 – Architecture of Web Information Systems
Carl Lagoze – Cornell University – Feb. 09, 2004
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2. Identity Change Persistence
Paradox: reality contains things that persist and change over time
Heraclitus and Plato: can you step into the same river twice?
Ship of Theseus: over the years, the Athenians replaced each plank in the original ship of Theseus as it decayed, thereby keeping it in good repair. Eventually, there was not a single plank left of the original ship. So, did the Athenians still have one and the same ship that used to belong to Theseus
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3. Identity Change Persistence
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4. Provide us with a necessary figment of persistence
Identifiers
Provide a key or handle linking abstract concepts to physical or perceptible entities
Provide us with a necessary figment of persistence
They are perhaps the one essential and common form of metadata
Why bother?
Finding things
Referring to things
Asserting ownership over things
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5. I have lots of identifiers
Carl Jay Lagoze, Dad, Hey you
(SSN)
(Visa Card)
FZBMLH (US Airways locator on March 21 flight to San Diego)
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6. Location independence Global uniqueness Persistent across time
Identifier Issues
Object granularity
Identifier Context
Object atomicity
Part/whole relationships
Location independence
Global uniqueness
Persistent across time
Human vs. machine generation
Machine resolution
Administration (centralized vs. decentralized)
Intrinsic semantics
Type specificity
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7. Two common pre-digital identifiers
ISBN (International Standard Book Number)
Uniquely identifies every monograph (book)
One ISBN for each format
HP & SS hardback
HP & SS softcover X
Number is semantically meaningful (components)
International administration (>150 countries)
ISSN (International Standard Serial Number)
Uniquely identifies every serial (not issue or volume)
Semantically meaningless
International administration
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8. URI: Universal Resource Identifier
Generic syntax for identifiers of resources
Defined by RFC 2396
Syntax: <scheme>://<authority><path>?<query>
Scheme
Defines semantics of remainder of URI
ftp, gopher, http, mailto, news, telnet
Authority
Authority governing namespace for remainder of URI
Typically Internet-based server
Path
Identification of data within scope of authority
Query
String of information to be interpreted by authority
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9. Partial and relative URIs
Why is RFC 2396 so big?
Character encodings
Partial and relative URIs
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10. URL: Universal Resource Locator
String representation of the location for a resource that is available via the Internet
Use URI syntax
Scheme has function of defining the access (protocol) method. Used by client to determine the protocol to “speak”.
- open socket to an.org on port 80 and issue a GET for index.html
ftp://an.org/index.html - open socket to an.org on port 21, open ftp session, issue ftp get for index.html….
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11. Valid only at the item level Multiple resolution
URL Issues
Persistence
Location dependence
Valid only at the item level
What about works, expressions, manifestations
Multiple resolution
“get the one that is cheapest, most reliable, most recent, most appropriate for my hardware, etc.”
Non-digital resources?
Disconnection from the entity
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12. URC – Uniform Resource Characteristic (Catalog)
Failed but interesting effort
Multiple resolution
Describe resource by its characteristics
Provide adequate bundled information about a resource (metadata) to create identification block for any given resource (including locations)
Exactly what are the common set of characteristics for describing different types of resources?
Where are these characteristics stored?
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13. Can be used to locate document (via search engine) after it is moved
Robust Hyperlinks
Characteristic of document (metadata) is computed automatically via fingerprint of its content.
“Lexical” signatures: The top n words of a document chosen for rarity, subject to heuristic filters to aid robustness.
“a TF-IDF-like” measure
Five or so words are sufficient
Can be used to locate document (via search engine) after it is moved
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14. Robust Hyperlinks – Why does this work?
Number of terms on Web is reportedly close to 10,000,000.
If terms were distributed independently, the probability of 5 even moderately common terms occurring in more than one document is very small.
In fact, picking 3 terms restricted to those occurring in 100,000 documents works pretty well.
Many documents contain very infrequently used words.
There is lots of room for independence to be off, and to play with term selection for robustness, etc..
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15. URN – Universal Resource Name
“globally unique, persistent names”
Independence from location and location methods
<URN> ::= "urn:" <NID> ":" <NSS>
NID: namespace identifier
NSS: namespace-specific string
examples:
urn:ISSN:
urn:isbn:
urn:doi: /140
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16. Why isn’t DNS sufficient (parenthetical comment)
Issue of semantic vs. non-semantic names
Changing ownership
Hierarchical legacy of DNS is sometimes inappropriate
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17. Handles: Names for Internet Resources
Naming system for location-independent, persistent names
One name, multiple resolutions
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. ....
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18. <naming_authority>/<locally_unique_string> or
Syntax of Handles
<naming_authority>/<locally_unique_string> or
hdl:<naming_authority>/<locally_unique_string>
Examples
/ ; (date-time stamp)
cornell.cs/cstr (mnemonic name)
loc/a43v-8940cgr (random string)
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19. Example of a Handle and its Data Used to Identify Two Locations
Data type
Handle data
loc.ndlp.amrlp/123456
URL
RAP
loc/repository-1r4589
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20. Use of Handles in a Digital Library
Repository
User
interface
Search System
Handle System
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21. Replication for Performance and Reliability
Example: the Global Handle System
Los Angeles, CA
Washington, DC
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22. Proxies to Resolve Handles
A Web browser can resolve Handles via a proxy. For example, the following URL can be used to resolve the Handle loc.ndlp.amrlp/3a16616:
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23. Proxy Resolution URL to Proxy Proxy server WWW browser URL
hdl.handle.net
Handle System
URL
HTTP
server
Resource
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24. DOI – Digital Object Identifier
Technology and social infrastructure for naming
Established by publishers for persistent naming of entities (articles, journals, conference proceedings)
Cognizant of FRBR elements
Underlying technology is handle system
“persistent” names
Persistence is fortified by social underpinnings
Rules for establishing registration agencies
Multiple resolution
Registration/mechanism has metadata associated with it
doi: /186
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25. 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
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26. • PURL resolvers use standard http redirects to return the actual URL.
PURL Syntax
• A PURL is a URL.
• PURL resolvers use standard http redirects to return the actual URL.
protocol
resolver address
name
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27. A PURL provides a local (not-global namespace)
PURL Namespaces
A PURL provides a local (not-global namespace)
is different from
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28. OCLC PURL Resolution PURL PURL PURL server database URL WWW browser
HTTP
server
Resource
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29. Making links context sensitive
Why?
“Appropriate item” differs for each user
Licensing locality
Some users may want a choice (abstract, full text, etc.)
Conceptualize link as service rather than object targeted.
OpenURL
Transports metadata about the work to…
A localized service that interprets the metadata and provides contextualized choices to the user.
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30. . OpenURL linking link source destination link context-sensitive
transportation of
metadata & identifiers
user-specific
link source .
link
destination
linking
server
OpenURL
reference
link
destination
OpenURL
context-sensitive
link
destination
link
destination
resolution of
metadata & identifiers into services
provision of OpenURL
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31. OpenURL 0.1 syntax http://www.mysrv.org/menu? id=doi:10.111/12345&
genre=article&
aulast=Weibel&aufirst=Stu&ISSN=
&year=2001&volume=14&issue=3&spage=44&
pid= &
sid=OCLC:Inspec
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32. Why haven’t URNs caught on beyond certain communities?
Complexity of systems
One size does not fit all - special purpose URN schemes have been successful, e.g., PubMed ID, Astrophysics BibCode
No guarantee of persistence – longevity is an organizational not technical issue
Requires well-regulated administrative systems
Absence of “killer” applications – although reference linking is emerging
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33. Types: Not all data and content is the same
Format or Genre
How you sense it
What you can do with it
E.G. – audio, video, map, book
Type
What you need to process it
What is its bit layout
Compression or encoding
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34. Multipurpose Internet Mail Extensions
RFC 822 – define textual format of messages
RFC – Extend textual to allow
Character sets other than US-ASCII
Extensible set of non-ASCII types for message bodies
Definition of multi-part mail (attachments)
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35. Two part type hierarchy
MIME Types
Two part type hierarchy
Top level type
text
audio
video
image
application
multipart
Examples
text/plain image/gif application/postscript
Extensions are handled by IANA
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36. MIME in HTTP (Content Negotiation)
Accept in request-header
Accept: text/plain; q=0.5, text/html, text/x-dvi; q=0.8, text/xml
text/plain and text/xml are preferred, then text/x-dvi, then text/html
Content-Type in response-header
Content-Type: text/html
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37. Two-level type depth is simplistic Multi-media documents
MIME is too limited
Two-level type depth is simplistic
Multi-media documents
“Documents” that have many types or views
FEDORA
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