URIs and the architecture of the World Wide Web

1. Acknowledgements

• I'm fortunate to work with and learn from some of the proprietors of the Web
  • Most notably Tim Berners-Lee and Dan Connolly
• They're not in any way responsible for the opinions presented here

2. URIs unpacked

• 'URI' stands for

  Uniform

  There's a standard generic syntax (specified by IETF RFC 3986) and (partial) semantics

  Identifier

  URIs are identifiers, that is, names (not addresses)

  Resource

  What it is they identify -- anything at all

3. An aside about standards (bodies)

• The IEEE (Ethernet and WiFi), IETF (RFC 3986) and the W3C (WebArch) have no legal authority
  • The IEEE is a semi-public membership organisation with individual members
  • The IETF is an crypto-anarchist commune open to anyone who turns up (not quite true anymore, but close)
  • The W3C is a consortium of two private (MIT, Keio) and one semi-public (ERCIM) bodies, with membership open to companies who pay the fee.
• So their standards only work if conformance is in everyone's interests

4. More about syntax

• Organised hierarchically
  • Left-to-right dominance
  • scheme ":" hier-part [ "?" query ] [ "#" fragment ]
• Federated (each scheme defines its own syntactic details)
• Absolute or relative

5. The nature of resources

• At first a vacuous label
  • If anything at all can be a resource
  • Knowing something is a resource tells you nothing
• But consider the word referent
  • Anything can be a referent
  • When/if someone/something refers to it!
• So, being a resource is not about some intrinsic property of something
• It just means, being identified by a URI

6. More about resources

• Who gets to say what the resource is that a URI identifies?
  • The owner of the URI
    • Just like the coiner of a new word
• Who is the owner (sometimes also called the 'minter')?
  • For URI schemes (such as http and ftp) whose hier-part starts with a domain name, the owner of that domain name.
  • For others, should (!) be specified by the scheme specification.

7. More about identification

• In principle, all URIs are opaque
  • That is, no conclusions about the shape of the space of things identified should be drawn from the shape of the set of their identifiers.
• In practice, e.g. for http URIs, this is false
  • It's entirely up to minters to manage this, and document it (or not :-(
• As for (some) naming elements of ordinary language, URIs can be deiectic:
  • http://localhost/
  • file:///etc/hosts (and indeed, all file: URIs
  • http://www.guardian.co.uk/

8. What about web pages?

• Isn't that what URIs are really for?
• Short answer
  • No!
• It's perfectly OK to mint URIs with no intention to ever produce a web page at all
  • http://www.ltg.ed.ac.uk/~ht/Concepts/liberty
  • There, I just did it.
• Longer answer
  • 3 billion web page authors can't be wrong
  • But what they've authored are not the resources identified by the URIs
    • They're representations of those resources.
    • (from WebArch)

9. Resources vs. representations

• Why make this distinction? That is, why isn't the web page itself the resource?
  • Some resources don't have web pages associated with them in any way
  • Many resources have more than one web page associated with them
  • Many URIs produce representations without there being any static web pages involved

10. A digression about media types and encodings

• [Assume all URIs are http: URIs and all retrieval uses HTTP]
• A message, i.e. the response to an HTTP GET request, includes information about the character encoding and media type of the byte-string which follows.
• The character encoding tells you how to interpret the byte-string as a character string, e.g. iso-8859-1, shift-jis, utf-8.
• The media type tells you how to interpret the resulting character string, e.g. application/jpg, application/pdf, text/html
• You can also signal in an HTTP GET request what media type you want back using an Accept header

11. Multiple representations

• So a server might have
  • utf-8 or iso-8859-1 encodings of
  • application/xml or text/html representations of
  • a document in English or Irish
  • representing today's weather report for Dublin
• 8 representations for one (time-varying) resource.
• Selecting a representation to return based on the Accept header in a request is called Content Negotiation

12. Information resources

• WebArch uses the phrase 'information resource' for the subset of resources which can be pretty much completely captured in a message.
• So (today's) Dublin weather report is an information resource, but Dublin itself is not.
• Another way to look at this is that for information resources, the retrievable representations are (isomorphic to) the information resource itself -- consider
  • Beethoven's Symphony No. 6: http://downloads.bbc.co.uk/rmhttp/radio3/symphony6_beethoven_radio3.mp3
  • The Charge of the Light Brigade: http://eserver.org/poetry/light-brigade.html
  • The XML 1.0 Recommendation: http://www.w3.org/TR/REC-xml/
  • The W3C at 10 logo: http://www.w3.org/2004/09/logos/W3C10_logo_short_fc.png

13. The Network Effect

• The history of XML is illuminating
  • XML was designed to solve the online-documentation problem
  • It succeeded for a completely different reason
    • The commercial value of data interchange
• The use of http: URIs as the foundation of the Semantic Web is intended to enable the same effect
  • But for machines this time, instead of people

14. Architecture of the World Wide Web

• A grandiose concept
  • In practice an ongoing post-hoc analysis project with respect to a vast, messy, poorly delimited real-world phenomenon
• A document
  • The first publication from the Technical Architecture Group of the W3C (World Wide Web Consortium)
  • Architecture of the World Wide Web, Volume One
  • The goal of the document is to "preserve [the properties underlying the success] of the information space as the technologies evolve"
  • It contains a number of "Principles, Constraints and Good Practice Notes"

15. Grandmother observations about the Web

• "Global naming leads to global network effects"
  • A prerequisite for citation
  • A challenge for many existing schemes
• "To benefit from and increase the value of the World Wide Web, agents should provide [http:] URIs as identifiers for resources"
  • I.e. not just any global name, but a particular kind of global name

16. More from Grandma Webarch

• It's good for the ownership of a name (URI) to be manifest in the form of that URI
  • We need to know who is responsible for the name and its use
• "A URI owner should not associate arbitrarily different URIs with the same resource."
• "Agents do not incur obligations by retrieving a representation."
  • HTTP 'GET' is side-effect free (c.f. cookies)
• "A URI owner should provide representations of the resource it identifies"

17. More on terminology and technology

• A retrieved representation is always accompanied by one crucial kind of metadata:
  • Its media type, e.g.
    • text/html
    • image/png
    • audio/mp3
    • See the IANA registry for a full list
  • That is, information sufficient to enable an agent to interpret the representation.
    • Some agents, e.g. web browsers, present interpreted representations to human beings
    • Others, e.g. search engines, process interpreted representations automatically for their own purposes
  • Retrieval may involve content negotiation
    • When multiple representations are available
      • JPG vs. PNG vs. SVG vs. GIF . . .
      • French vs. German vs. Chinese vs. . . .
      • DTD vs. XML Schema vs. . . .
• So information resources are not necessarily concrete
  • It's perfectly coherent to get real value from a URI which indentifies an abstraction
  • For example, http://www.guardian.co.uk/uklatest

18. The infamous 'httpRange-14' question

• Consider the following RDF triple:

  http://www.ltg.ed.ac.uk/~ht/index.html
   http://purl.org/dc/elements/1.1/creator
  http://www.inf.ed.ac.uk/people/staff/Henry_Thompson.html

• For this to 'make sense', we have to understand the first URI, http://www.ltg.ed.ac.uk/~ht/index.html, as identifying an information resource, that is, my home page, but the second URI, http://www.inf.ed.ac.uk/people/staff/Henry_Thompson, as identifying me.
• But what about this:

  http://www.inf.ed.ac.uk/people/staff/Henry_Thompson.html
   http://purl.org/dc/elements/1.1/creator
  http://www.inf.ed.ac.uk/about/webmaster.html

• How can an automated system 'know' whether URIs it encounters are being used to identify the information resource for which a representation can be retrieved, or a non-information resource described or documented by that resource?

19. The TAG's resolution of httpRange-14

• The details are in an email message
• Start from the meaning of HTTP response codes:

  404

  You lose!

  200

  You win: here comes a represention of the resource identified by the URI you asked for

  303

  I don't have that, but here's URI for something you might find helpful

• It would be wrong to respond to an HTTP GET for e.g. http://www.ihmc.us/users/phayes/PatHayes.html with a 200, because what you are getting back is not Pat Hayes, or anything close.
• The server should reply with a 303, and a URI such as http://www.ihmc.us/users/phayes/PatHayes_biography.html, which identifies an information resource in English, or http://www.advogato.org/person/phayes/foaf.rdf#me, which identifies an information resource using RDF.
• This is not a completely satisfactory solution, but it is a step forward.

20. Conclusions

• The Web works because you can
  • View Source
  • Follow your nose
  • Write URIs on the side of a bus
  • Use generic tools
  • Redirect, cache and proxy
• Integrating Web Services and the Semantic Web into (the original) Web while preserving those values is not straightforward.