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(Cool) URIs

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Neuland im Internet 2019
Dr. Jens Lechtenbörger (License Information)

DBIS Group
Prof. Dr. Gottfried Vossen
Chair for Computer Science
Dept. of Information Systems
WWU Münster, Germany

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 : // /rfc/rfc1808.txt
    • http : // /rfc/rfc2396.txt
    • ldap : //[2001:db8::7]/c=GB?objectClass?one
    • mailto : John.Doe@
    • news : comp.infosystems.www.servers.unix
    • tel : +1-816-555-1212
    • telnet : //
    • urn : oasis:names:specification:docbook:dtd:xml:4.1.2

2.1.1 URIs

  • (Somewhat) Uniform
    • Different types of IDs under consistent format
    • Green parts on previous slide specify schemes for types
  • Syntax for absolute URIs
  • URIs subsume URLs and URNs

2.1.2 IRIs

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

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
  1. 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 <>;
        foaf:homepage <> .
    • What should <URI-of-alice> be?

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.,

3.3 Hash URIs

  • Hash URI = URI with fragment after hash sign
    • E.g.,
    • Browser strips off fragment before GET request
      • E.g., GET
      • Thus, 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 <>

3.3.1 Hash URIs with Content Negotiation

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

    Hash URI with content negotiation

    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
  • Server response includes Content-Location

3.4 303 URIs

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

303 URI with redirect to generic document

303 URI with redirect to generic document

Copyright © 2008 W3C® (MIT, ERCIM, Keio) under W3C Document License; from W3C

303 URI with redirect with content negotiation

303 URI with redirect with content negotiation

Copyright © 2008 W3C® (MIT, ERCIM, Keio) under W3C Document License; from W3C

4 Sample Solid URIs

4.1 URIs for People

  • E.g.,
    • So-called WebID for some individual bob
  • Your tasks
    1. Web browsers (Firefox and other reasonable ones) allow you to inspect their network interactions. Find out how to see HTTP requests and responses with your browser.
    2. Verify that the hash fragment is not part of the GET request when you visit the above URI. What content is returned?
    3. Note how content negotiation is used later on with the same URI to retrieve a Turtle document.

4.2 Dokieli

  • Visit
  • Note how human-readable information is annotated
    • Notifications Inbox with rel="ldp:inbox"
    • Annotation Service with rel="oa:annotationService"
  • Note GET request for Turtle document describing Inbox
    • And subsequent GET requests for Inbox items

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.

Except where otherwise noted, this work, “(Cool) URIs”, is © 2018, 2019 by Jens Lechtenbörger, published under the Creative Commons license CC BY-SA 4.0.

No warranties are given. The license may not give you all of the permissions necessary for your intended use.

In particular, trademark rights are not licensed under this license. Thus, rights concerning third party logos (e.g., on the title slide) and other (trade-) marks (e.g., “Creative Commons” itself) remain with their respective holders.