Gleaning Resource Descriptions from Dialects of Languages (GRDDL)

Dan Connolly
W3C TAG meeting, 9 Feb 2004
$Revision: 1.7 $ of $Date: 2004/02/24 18:47:50 $ by $Author: connolly $

Talk Overview

History, motivation, principles
- Early tensions, compromises for metadata in HTML
- Some architectural principles from PICS
- Syntax issues of mixing RDF and XHTML
Proposed Solution: GRDDL
- intro/how-to, deployment so far
- enhancements, future work

Metadata in HTML 1.x (1993)

Hypertext Markup Language (HTML): A Representation of Textual Information and MetaInformation for Retrieval and Interchange
June 1993
Requirement: metadata for title, author, date, etc.
Reality: etc. is a never-ending story
Compromise: <META name="who-knows" content="whatever">

PICS: Metadata for Decentralized Content Selection (1996)

The Web spread into home with children
Governments started considering censorship
W3C developed PICS to demonstrate that the market could take care of this

PICS influence on Semantic Web Architecture

basic statement structure: subject, predicate, object
Labels can be about the document they occur in, or about some other document.
- "anyone can say anything about anything"
- akin to HTML form action, which separates form front end from back end
- some RDDL designs have not met this requirement.
  @@find details in www-tag

PICS in HTML: Syntax

HTML uses <>; PICS uses ()

<!DOCTYPE html ...>
<html lang="en">
<head>
  <meta http-equiv="PICS-Labels"
  content='(PICS-1.1 "http://www.rsac.org/ratingsv01.html" l
     by "martinm@raleigh.ibm.com" on "1996.06.24T10:11-0500"
     exp "1998.06.06T08:00-0500"
     r (n 0 s 0 v 0 l 0))'>
  <meta http-equiv="PICS-Label"
  content='(PICS-1.1 "http://www.classify.org/safesurf/" l
    by "martinm@raleigh.ibm.com"
    on "1996.06.24T15:38-0500"
    exp "1998.06.06T08:00-0500"
    r (SS~~000 1))'>
  <title>...

RDF REC anticipates XHTML, XML Schemas (1998)

RDF REC, appendix B: stick <rdf:RDF> in <html><head>
XHTML REC: use DTDs
RDF IG: ouch!
- faq-html-compliance: The suggested way of including RDF meta data in HTML is not compliant with HTML 4.01 or XHTML
- rdfms-validating-embedded-rdf: RDF embedded in XHTML and other XML documents is hard to validate

Observation: lots of structured data in XHTML dialects (March 2000)

bibliographies
schedules
issues lists
news items
blog rolls

March 2000 design sketch: XSLT for screen-scraping RDF out of real-world data

RDF Users: We need features more than we need standards

Creative Commons, Trackback:
```

```
- ew... HotComments: "an antipattern that gets employed when somebody needs a way to extend a non-extensible format"
- huh? where's the X in XHTML?
- comments belong to the author, no?
Semantic Web CG, Hypertext CG start public-rdf-in-xhtml-tf

Meanwhile, in the TAG...

mixedNamespaceMeaning-13: What is the meaning of a document composed of content in mixed namespaces?
- RDFinXHTML-35: Syntax and semantics for embedding RDF in XHTML
namespaceDocument-8: What should a "namespace document" look like?

GRDDL: an 80/20 solution

Pick a way of expressing data in XHTML, and get an XSLT transformation to RDF; for example:
- Encoding Dublin Core Metadata in HTML and dc-extract.xsl
- GeoURL markup and grokGeoURL.xsl
- RDF Site Summaries in XHTML, using home2rss.xsl

Make links from your XHTML data to the transformation, using the transformation link type:

<html xmlns="http://www.w3.org/1999/xhtml">
  <head profile="http://www.w3.org/2003/g/data-view">
    <title>Some Document</title>
    <link rel="transformation"
       href="http://www.w3.org/2000/06/dc-extract/dc-extract.xsl" />
    <meta name="DC.Subject"
       content="ADAM; Simple Search; Index+; prototype" />
    ...
  </head>
  ...
</html>

Reference the GRDDL profile to make it clear what that transformation link type means:

<html xmlns="http://www.w3.org/1999/xhtml">
  <head profile="http://www.w3.org/2003/g/data-view">
    <title>Some Document</title>
    ...

GRDDL XSLT service demo, example

online GRDDL demo service by Dom, Nov 2003.
- check its output with the W3C RDF Validation service
  @@pretty picture?
- feed output into FOAF explorer
An example home page for Joe Lambda shows DC, GeoURL, RSS, FOAF, and Creative Commons Licenses.

GRDDL drawback: turing completeness

XSLT is a big hammer
consumer runs XSLT from who knows where. trust issues
- well-known transformations should cover 80%
  - short-cut local implementation
  - known-good cached copies
- DSig might help for the rest

see: Principle of Least Power. hmm... belongs in webarch somewhere?

GRDDL Syntax: users's choice

DTD-happy, if you like
or just stick rdf:RDF in the head
@@todo: "standard library" of transformations like this
works with XML in general, in fact
Future work: RDAL: automatically generated XSLT from Relax-NG for syntax-checking

GRDDL Semantics: explicit, grounded in the Web

exploits flexibility points:
- XHTML profiles
- XML namespaces

We didn't finish our discussion of URI-based flexibility points before webarch last call. Let's resume, please!