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xmlrdf

Differences between rdf:resource, rdf:about and rdf:ID

What are the conceptual differences between rdf:resource, rdf:about, and rdf:ID. I did some investigation but the difference between them are not clear for me yet. For example, whether rdf:ID is used when declaring a resource for the first time, rdf:resource is used for referencing an already existing resource, etc.

I would be glad if you provide some little examples.

Solution

  • To be clear this is only about a particular way of writing rdf: namely RDF/XML. Other syntaxes don't feature these differences.

    With that disclaimer out of the way:

    What we're trying to do is write statements of the form:

    subject predicate object

    and in particular:

    subjectURI predicate objectURI

    So how do we introduce subject and object URIs in RDF/XML?

    1. rdf:about sets the subject URI of a statement, which may be absolute (http://example.com/) or resolved relative to the BASE of the document (e.g. /foo/bar, #frag). (Like href in html)
    2. rdf:resource sets the object URI of a statement, once again either absolute or relative.
    3. rdf:ID sets the subject URI, but it can only be within this document. An ID can also only be used once. Very like <a name="baz"> or id="baz" in html.

    rdf:ID is discouraged since

    1. you can replace it with an rdf:about or rdf:resource with a fragment #baz and
    2. it can cause xml issues if you use the same id more than once.

    That is, it's redundant and a potential source of errors.

    In retrospect there typically only needs to be one attribute to specify a URI, since whether something is a subject or object is apparent from the RDF/XML syntax:

    <ex:Foo ...> - subject
  <ex:prop ... /> - property then object
</ex:Foo>

<ex:Foo ...> - subject
  <ex:prop> - property
    <ex:Bar ... /> - subject (and implictly an object chaining from previous) 
...

    (rule of thumb: odd lines rdf:about, even lines, rdf:resource)

    and using both rdf:about and rdf:resource on an element is almost always an error (you're either in a subject position or object position).

    tl;dr

    Avoid rdf:ID. Use rdf:about and rdf:resource much like an href, the former for subject, the latter for objects.

    Additional

    Forgot to mention that rdf:ID can be used on a property element, but it does something you may find unexpected: it reifies the triple. Avoid rdf:ID.