Identifiers and Types

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Identifiers and Types

• CS502 Architecture of Web Information Systems
• Carl Lagoze Cornell University Feb. 03, 2003

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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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Identity Change Persistence
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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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I have lots of identifiers

• Carl Jay Lagoze, Dad, Hey you
• 123-456-7890 (SSN)
• 1234-5678-1234-1234 (Visa Card)
• FZBMLH (US Airways locator on March 21 flight to
  San Diego)

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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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Two common pre-digital identifiers

• ISBN (International Standard Book Number)
• Uniquely identifies every monograph (book)
• One ISBN for each format
• HP SS hardback 0590353403
• HP SS softcover 059035342X
• Number is semantically meaningful (components)
• International administration (gt150 countries)
• ISSN (International Standard Serial Number)
• Uniquely identifies every serial (not issue or
  volume)
• Semantically meaningless
• International administration

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URI Universal Resource Identifier

• Generic syntax for identifiers of resources
• Defined by RFC 2396
• Syntax ltschemegt//ltauthoritygtltpathgt?ltquerygt
• 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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Why is RFC 2396 so big?

• Character encodings
• Partial and relative URIs

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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.
• http//an.org/index.html - 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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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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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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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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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..
• http//www.cs.berkeley.edu/phelps/Robust/

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URN Universal Resource Name

• globally unique, persistent names
• Independence from location and location methods
• ltURNgt "urn" ltNIDgt "" ltNSSgt
• NID namespace identifier
• NSS namespace-specific string
• examples
• urnISSN1234-5678
• urnisbn9044107642
• urndoi10.1000/140

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Handles Names for Internet Resources

• Naming system for location-independent,
  persistent names
• http//www.handle.net

The resource named by a Handle can be A
library item A collection of library items
A catalog record A computer An e-mail
address A public key for encryption etc.,
etc., etc. ....
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Syntax of Handles
ltnaming_authoritygt/ltlocally_unique_stringgt or hd
lltnaming_authoritygt/ltlocally_unique_stringgt Exam
ples 10.1234/1995.02.12.16.42.219
(date-time stamp) cornell.cs/cstr-94.45
(mnemonic name) loc/a43v-8940c
gr (random string)
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Example of a Handle and its DataUsed to Identify
Two Locations
Data type
Handle data
Handle
loc.ndlp.amrlp/123456
URL
http//www.loc.gov/.....
RAP
loc/repository-1r4589
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Use of Handles in a Digital Library
Repository
User interface
Search System
Handle System
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Scalability and Caching
Client
Caching Server
Handle Servers
Hash
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Replication for Performance and Reliability
Example the Global Handle System
Los Angeles, CA
Washington, DC
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Global and Local Handle Servers
Global
Local Handle Servers
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Ways to Resolve HandlesI. 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.
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Ways to Resolve HandlesII. 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.
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Ways to Resolve HandlesIII. 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 http//hdl.handle.net/loc
.ndlp.amrlp/3a16616
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Proxy Resolution
URL to Proxy
WWW browser
Proxy server
URL
hdl.handle.net
Handle System
URL
HTTP server
Resource
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OCLC's Persistent URL (PURL)

• A PURL is a URL
• -gt Is fully compatible with today's Internet
  browsers
• -gt Users need no special software
• Has some of the desirable features of URNs
• Lacks some desirable features of URNs
• -gt Resolves only to a URL
• -gt Does not support multiple resolution
• Developed by OCLC
• Software openly available
• http//www.purl.org

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PURL Syntax

• A PURL is a URL.
• PURL resolvers use standard http redirects to
  return the actual URL.

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PURL Namespaces
A PURL provides a local (not-global namespace)
http//purl.oclc.org/keith/home is different
from http//purl.stanford.edu/keith/home
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OCLC PURL Resolution
WWW browser
PURL
PURL server
PURL database
URL
URL
HTTP server
Resource
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Why havent URNs caught on?

• 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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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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Multipurpose Internet Mail Extensions

• RFC 822 define textual format of email messages
• RFC 2045-2049 Extend textual email 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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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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MIME in HTTP (Content Negotiation)

• Accept in request-header
• Accept text/plain q0.5, text/html, text/x-dvi
  q0.8, 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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MIME is too limited

• Two-level type depth is simplistic
• Multi-media documents
• Documents that have many types or views
• FEDORA