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Identifiers and Types

Identifiers and Types. CS 502 – 20020205 Carl Lagoze – Cornell University. Identity Change Persistence. Paradox: reality contains things that persist and change over time Heraclitus and Plato: can you step into the same river twice?

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Identifiers and Types

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  1. Identifiers and Types CS 502 – 20020205 Carl Lagoze – Cornell University

  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

  3. Identity Change Persistence

  4. 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

  5. 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 Jan 31 flight to San Diego)

  6. Identifier Issues • Location independence • Global uniqueness • Persistent across time • Human vs. machine generation • Machine resolution • Administration (centralized vs. decentralized) • Intrinsic semantics • Type specific

  7. 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 (>150 countries) • ISSN (International Standard Serial Number) • Uniquely identifies every serial (not issue or volume) • Semantically meaningless • International administration

  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

  9. Why is RFC 2396 so big? • Character encodings • Partial and relative URIs

  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”. • 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….

  11. 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

  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? • Robust URLs – Berkeley • Characteristic of document or metadata is computed automatically via fingerprint of its content.

  13. 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:1234-5678 • urn:isbn:9044107642 • urn:doi:10.1000/140

  14. 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. ....

  15. Syntax of Handles <naming_authority>/<locally_unique_string> or hdl:<naming_authority>/<locally_unique_string> Examples 10.1234/1995.02.12.16.42.21;9 (date-time stamp) cornell.cs/cstr-94.45 (mnemonic name) loc/a43v-8940cgr (random string)

  16. 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

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

  18. Hash table Cache Scalability and Caching Client Caching Server Handle Servers Hash

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

  20. Global and Local Handle Servers Global Local Handle Servers

  21. 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.

  22. 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.

  23. 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

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

  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 http://www.purl.org

  26. http://purl.oclc.org/keith/home protocol resolver address name PURL Syntax • A PURL is a URL. • PURL resolvers use standard http redirects to return the actual URL.

  27. 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

  28. OCLC PURL Resolution WWW browser PURL PURL server PURL database URL URL HTTP server Resource

  29. Why haven’t 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 “killing” applications – although reference linking is emerging

  30. 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

  31. Multipurpose Internet Mail Extensions • RFC 822 – define textualformat 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)

  32. 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

  33. 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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