An Introduction to Embedded RDF

Ian Davis, <ian.davis@talis.com>

Platform Programme Manager

Talis Information Ltd.

What is it?

A way of embedding a subset of RDF into XHTML documents
Uses idiomatic XHTML - no new elements
CSS-friendly

Some History

1996 - W3C
- A Proposed Convention for Embedding Metadata in HTML
- http://tinyurl.com/f7r5g
- <META NAME="DC.title" CONTENT="HTML 2.0 Specification">
- <LINK REL = SCHEMA.dc HREF = "http://purl.org/metadata/dublin_core">
2003 - Dublin Core
- Expressing Dublin Core in HTML
- http://www.dublincore.org/documents/dcq-html/

Principles

Visible metadata
Don't repeat yourself
Reuse not reinvention

Visible Metadata

Easier to trust
Easier to spot stale metadata

Don't repeat yourself

Easier to keep in synch
Express once for both humans and machines

Reuse not reinvention

Reuse existing formats
Can use existing tools to author and read
Reuse people's experience

How?

Profile
Schema declarations
Metadata in Head
Metadata in Body

Profile

Add URI in profile attribute of <head>
Marks document as containing embedded RDF

<html>
  <head profile="http://purl.org/NET/erdf/profile">
    ...
  </head>
  <body>
    ...
  </body>

</html>

Multiple Profiles

Profile attribute is space delimited list of URIs
```
<head profile="uri uri uri">
```
Applications mustn't assume single URI

Schema Declarations

Use meta tags in <head>
Associate short prefix with URI
```
<link rel="schema.prefix" href="uri" />
```
Prefix is case-insensitive
So schema.foaf and schema.FoaF are same

Schema Declarations Abbreviate Properties

Schema prefix helps encode property URIs
Encoded by concatenating...
- schema prefix
- hyphen (-) or dot (.)
- local name of property
foaf.name might represent
http://xmlns.com/foaf/0.1/name

Schema Declarations Compatibility

Use dot (.) notation in <head>
Backwards compatible with Dublin Core
Use dash (-) notation in <body>
Forwards compatible with CSS
But both are valid

Metadata in Head

Represents metadata about the current document
<meta> used to represent triples with literal values
<link> used to represent triples with resource values

Metadata in Head - Literal Values

Property encoded in name attribute
Value in content attribute

<meta name="dc.creator" content="Ian Davis" />

represents
```
<> dc:creator "Ian Davis" .
```

Metadata in Head - Resource Values

Property encoded in rel or rev attribute
URI of resource value in href attribute
```
<link rel="foaf.maker" href="#ian" />
```
represents
```
<> foaf:maker <#ian> . 
```

Metadata in Head - Multiple Rel

Multiple properties can be encoded in single rel value

represents

<> foaf:maker <#ian> .
<> dc:creator <#ian> .
<> foaf:topic <#ian> .

Metadata in Head - Rev

Swaps subject and object
Current document becomes object
Subject URI encoded in href
```
<link rev="foaf.made" href="#ian" />
```
represents
```
<#ian> foaf:made <> .
```

Metadata in Body

Triples can describe
- current document
- resources named with id attributes
Literal values embedded using elements with class attribute
Resource values embedded using anchor elements

Metadata in Body - Literal Values

Use class attribute to encode property
Value is either
- title attribute if present
- string value of element (as xslt function)

Metadata in Body - Simple Example

Using string value of element for value

<div>
  <address class="dc-creator">Ian Davis</address> 
  wrote this
</div>

represents
```
<> dc:creator "Ian Davis" .
```

Metadata in Body - Title Example

Using title attribute of element for value

<div>
  <span class="dc-creator" title="Ian Davis">I</span> 
  wrote this
</div>

represents same triple as before
```
<> dc:creator "Ian Davis" .
```

Metadata in Body - Resources

Similar to link
rel attribute encodes the property name
href attribute representes the URI of the value resource

<a rel="foaf.homepage" 
   href="http://example.org/home">my home page</a>

represents

<> foaf:homepage <http://example.org/home> .

Inferred Labels from Anchors

Anchors infer rdfs:label properties
Value is either
- title attribute if present
- string value of anchor (as xslt function)

<a rel="foaf.homepage" 
   href="http://example.org/home">my home page</a>

infers

<http://example.org/home> rdfs:label "my home page" .

Metadata About Other Things

id attribute used to define resources within current document
id used as subject URI based on current document
Triples within descendants inherit the subject

<p id="ian">
  <span class="foaf-name">Ian Davis</span> 
  wrote this
</p>

represents
```
<#ian> foaf:name "Ian Davis" .
```

Combinations

Elements with both class and id attributes
Anchors with class and rel attributes
Anchors with rel and rev attributes
Anchors with rel and id attributes
Anchors with rel, rev, class and id attributes
Anchors with rel or rev but no href attribute
See the wiki!

Worked Example

<html>
  <head profile="http://purl.org/NET/erdf/profile">
    <base href="http://example.org/about" />
    <title>Anna's Homepage</title>
    <link rel="schema.foaf" href="http://xmlns.com/foaf/0.1/" />
    <link rel="foaf-maker" rev="foaf-homepage foaf-made" href="#anna" />
  </head>
  <body>
    <h2>About me...</h2>
    <p id="anna">
      I'm <span class="foaf-name"><span class="foaf-firstName">Anna</span>
      <span class="foaf-surname">Wilder</span></span>. 
      You might know me from IRC as <span class="foaf-nick">wildling</span>!
    </p>
  </body>
</html>

Worked Example

<html>
  <head profile="http://purl.org/NET/erdf/profile">
    <base href="http://example.org/about" />
    <title>Anna's Homepage</title>
    <link rel="schema.foaf" href="http://xmlns.com/foaf/0.1/" />
    <link rel="foaf-maker" rev="foaf-homepage foaf-made" href="#anna" />
  </head>
  <body>
    <h2>About me...</h2>
    <p id="anna">
      I'm <span class="foaf-name"><span class="foaf-firstName">Anna</span>
      <span class="foaf-surname">Wilder</span></span>. 
      You might know me from IRC as <span class="foaf-nick">wildling</span>!
    </p>
  </body>
</html>

<http://example.org/about> foaf:maker <http://example.org/about#anna> .

Worked Example

<html>
  <head profile="http://purl.org/NET/erdf/profile">
    <base href="http://example.org/about" />
    <title>Anna's Homepage</title>
    <link rel="schema.foaf" href="http://xmlns.com/foaf/0.1/" />
    <link rel="foaf-maker" rev="foaf-homepage foaf-made" href="#anna" />
  </head>
  <body>
    <h2>About me...</h2>
    <p id="anna">
      I'm <span class="foaf-name"><span class="foaf-firstName">Anna</span>
      <span class="foaf-surname">Wilder</span></span>. 
      You might know me from IRC as <span class="foaf-nick">wildling</span>!
    </p>
  </body>
</html>

<http://example.org/about#anna> foaf:homepage <http://example.org/about> .

Worked Example

<html>
  <head profile="http://purl.org/NET/erdf/profile">
    <base href="http://example.org/about" />
    <title>Anna's Homepage</title>
    <link rel="schema.foaf" href="http://xmlns.com/foaf/0.1/" />
    <link rel="foaf-maker" rev="foaf-homepage foaf-made" href="#anna" />
  </head>
  <body>
    <h2>About me...</h2>
    <p id="anna">
      I'm <span class="foaf-name"><span class="foaf-firstName">Anna</span>
      <span class="foaf-surname">Wilder</span></span>. 
      You might know me from IRC as <span class="foaf-nick">wildling</span>!
    </p>
  </body>
</html>

<http://example.org/about#anna> foaf:made <http://example.org/about> .

Worked Example

<html>
  <head profile="http://purl.org/NET/erdf/profile">
    <base href="http://example.org/about" />
    <title>Anna's Homepage</title>
    <link rel="schema.foaf" href="http://xmlns.com/foaf/0.1/" />
    <link rel="foaf-maker" rev="foaf-homepage foaf-made" href="#anna" />
  </head>
  <body>
    <h2>About me...</h2>
    <p id="anna">
      I'm <span class="foaf-name"><span class="foaf-firstName">Anna</span>
      <span class="foaf-surname">Wilder</span></span>. 
      You might know me from IRC as <span class="foaf-nick">wildling</span>!
    </p>
  </body>
</html>

<http://example.org/about#anna> foaf:firstName "Anna" .

Worked Example

<html>
  <head profile="http://purl.org/NET/erdf/profile">
    <base href="http://example.org/about" />
    <title>Anna's Homepage</title>
    <link rel="schema.foaf" href="http://xmlns.com/foaf/0.1/" />
    <link rel="foaf-maker" rev="foaf-homepage foaf-made" href="#anna" />
  </head>
  <body>
    <h2>About me...</h2>
    <p id="anna">
      I'm <span class="foaf-name"><span class="foaf-firstName">Anna</span>
      <span class="foaf-surname">Wilder</span></span>. 
      You might know me from IRC as <span class="foaf-nick">wildling</span>!
    </p>
  </body>
</html>

<http://example.org/about#anna> foaf:surname "Wilder" .

Worked Example

<html>
  <head profile="http://purl.org/NET/erdf/profile">
    <base href="http://example.org/about" />
    <title>Anna's Homepage</title>
    <link rel="schema.foaf" href="http://xmlns.com/foaf/0.1/" />
    <link rel="foaf-maker" rev="foaf-homepage foaf-made" href="#anna" />
  </head>
  <body>
    <h2>About me...</h2>
    <p id="anna">
      I'm <span class="foaf-name"><span class="foaf-firstName">Anna</span>
      <span class="foaf-surname">Wilder</span></span>. 
      You might know me from IRC as <span class="foaf-nick">wildling</span>!
    </p>
  </body>
</html>

<http://example.org/about#anna> foaf:name "Anna Wilder" .

Other Examples

DOAP and the extracted RDF
my home page and the extracted RDF
Dan Connolly's home page and the extracted RDF
ARC parser page and the extracted RDF

Extraction

XSLT: http://purl.org/NET/erdf/extract-rdf.xsl
Online service: http://purl.org/NET/erdf/extract
GRDDL compatible

Limitations

Blank nodes - every node must be identified by a URI or literal text
Containers - rdf:Bag, rdf:Seq and rdf:Alt are all omitted
XML Literals, Datatyped literals - all literals are assumed to be plain
Subjects must be within document if object is resource
80% solution

What About Microformats?

Complementary
Can mix class names from microformats with embedded rdf

<li class="vevent  foaf-currentProject" id="sxsw2006">

What About RDFa?

Not compatible
Partial convergence, allow colon as prefix/name separator
RDFa requires new role and property attributes
RDFa more explicit but is new dialect of XHTML

Next Steps

Test suite!
Implementation experiences
Enable migration to RDFa if possible

The End!

That's enough for now...
Find out more at: http://research.talis.com/2005/erdf/wiki
Slides available at http://research.talis.com/2005/erdf/xtech2006.html