(Cool) URIs

1 Introduction

1.1 Today’s Core Questions

• What are URI, URL, URN?
• How are URIs used in the context of Linked Data?

1.2 Learning Objectives

• Explain how URIs can identify “things”
• Explain how hash URIs and URIs with redirection help to resolve ambiguity

2 URIs

2.1 Uniform Resource Identifier (URI)

• Character string to identify entities
• RFC 3986
• Examples from RFC 3986 (some containing DNS names)
  • ftp : // ftp.is.co.za /rfc/rfc1808.txt
  • http : // www.ietf.org /rfc/rfc2396.txt
  • ldap : //[2001:db8::7]/c=GB?objectClass?one
  • mailto : John.Doe@ example.com
  • news : comp.infosystems.www.servers.unix
  • tel : +1-816-555-1212
  • telnet : //192.0.2.16:80/
  • urn : oasis:names:specification:docbook:dtd:xml:4.1.2

2.1.1 URI Structure

• (Somewhat) Uniform, specified in RFC 3986
  • Different types of IDs under consistent format
  • Green parts on previous slide specify schemes for types
    • Our focus is on http(s)
• Syntax for absolute URIs depends on given scheme
  • <scheme>:<scheme-specific-part>
  • Scheme-specific part often structured
    • <scheme>://<authority><path>?<query>#fragment
    • E.g., https://oer.gitlab.io/oer-courses/vm-neuland/URIs.html?default-navigation#slide-uri-structure
• URIs subsume URLs and URNs

2.1.2 IRIs

• URIs are limited to ASCII characters
• Internationalized Resource Identifiers (IRIs) allow Unicode
  • Defined in RFC 3987
• Translations from IRI to encoded URI and back
• Examples
  • Visit https://de.wikipedia.org/wiki/Dom%C3%A4ne
    • Notice how browser translates encoded URI
  • Visit Wikipedia on red-black trees
    • Non-ASCII? Use copy&paste from address bar…

2.2 URLs, URNs

• Clarification in RFC 3305
  • Uniform Resource Locator (URL)
    • “URL is a useful but informal concept”
    • Identification of web resources via primary access mechanism
      • Network location, address of access point
    • Scalable
    • Address, thus potentially invalid
  • Uniform Resource Name (URN)
    • Permanent, location independent name of web resource
    • Registration of URN and URL for resource with URN-service
    • urn:… (RFC 8141)
    • E.g., urn:nbn:de:1111-200606299
      • Deutsche Nationalbibliothek
      • https://nbn-resolving.org/

3 Linked Data and Cool URIs

3.1 Linked Data

• “Linked Data” coined by Tim Berners-Lee, 2006; four rules

  1. Use URIs as names for things
  2. Use HTTP URIs so that people can look up those names.
  3. When someone looks up a URI, provide useful information, using the standards (RDF*, SPARQL)
  4. Include links to other URIs. so that they can discover more things.

3.2 Cool URIs

• “Cool URI” defined in context of Semantic Web
  • Semantic Web: Use standard vocabularies/ontologies to make Web contents machine-readable and -processable
    • E.g., “+1-816-555-1212” is the telephone number of some person Bob
  • Cool URIs come with
    • Two requirements
      1. Be on the Web
      2. Be unambiguous
    • And three properties
      1. Simplicity
      2. Stability
      3. Manageability

3.2.1 Requirements in Detail

1. Be on the Web
   • Obvious

2. Ambiguity is mainly for URIs that do not describe Web pages

   • Example: Person vs her homepage

     <URI-of-alice> a foaf:Person;
       foaf:name "Alice";
       foaf:mbox <mailto:alice@example.com>;
       foaf:homepage <http://www.example.com/people/alice> .
     

   • What should <URI-of-alice> be?
     • Two solutions: Hash URIs and 303 URIs

3.2.2 Properties in Detail

• Simplicity
  • Short, mnemonic URIs
• Stability
  • URI for resource should persist
  • No implementation-specific bits and pieces such as .php and .asp
• Manageability, e.g.:
  • Current year in URI path
    • Allows change of URI-schema each year without breaking older URIs
  • Keeping 303 URIs on dedicated subdomain, e.g., http://id.example.com/alice

3.3 Hash URIs

• Hash URI = URI with fragment after hash sign
  • E.g., https://ruben.verborgh.org/profile/#me
    • URI to identify a person (as abstract concept)
  • Browser strips off fragment before GET request
    • E.g., GET https://ruben.verborgh.org/profile/
    • Thus, https://ruben.verborgh.org/profile/#me does not identify the returned document
  • Returned document contains descriptions for fragment identifiers, e.g.:
    • :me a foaf:Person
    • :me foaf:name "Ruben Verborgh"@en
    • :me foaf:img <https://ruben.verborgh.org/images/ruben.jpg>

3.3.1 Hash URIs with Content Negotiation

• Client sends Accept header in HTTP request, e.g.:

  “Hash URI with content negotiation” Copyright © 2008 W3C® (MIT, ERCIM, Keio) under W3C Document License; from W3C

  • Human interpretation: Accept: text/html
  • Machine: Accept: application/rdf+xml
    • Or Accept: */* (server preferences determine content type)
• Server response may include Content-Location

3.4 303 URIs

• 303 = Redirection
  • If “thing” is requested, server does not respond with HTTP code 200 OK but with 303 See other
  • Two options

4 Sample Solid URIs

4.1 URIs for People

• Solid WebIDs
  • E.g., https://lechten.solidcommunity.net/profile/card#me
• Your tasks
  1. Use Web Developer Tools of your browser to see HTTP requests and responses for above WebID.
  2. Verify that hash fragment is not part of GET request for WebID.
     • How does the request look like, what content is returned?
     • Beware, Firefox may mislead you
       • Use right-click on request, then copy headers, paste somewhere
  3. Note how content negotiation (different Accept header) is used later on with same URI to retrieve Turtle document.

4.2 A Suggestion

• Try out curl as command line tool for data transfer
  • curl -H "Accept: text/html" https://lechten.solidcommunity.net/profile/card
  • curl -H "Accept: text/turtle" https://lechten.solidcommunity.net/profile/card
  • curl https://lechten.solidcommunity.net/profile/card

5 Conclusions

5.1 Summary

• URIs identify things
  • URIs are names (at least, they can be)
• URIs are basis of Linked Data
  • Hash URIs and redirection remove ambiguities
  • “Understandable” links based on standardized vocabularies

License Information

This document is part of a larger course. Source code and source files are available on GitLab under free licenses.