Tim Berners-Lee
Date: 1998, last change: $Date: 2023/12/01 10:36:10 $
Status: personal view only. Editing status: first draft.

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This page assumes an imaginary namespace referred to as play: which is used only for the sake of example. The readers is assumed to be able to guess its specifictaion.Much of this page was originally a note to the Strawman Syntax.

Identifiers - what is identified?

When XML is used to represent a directed laballed graph which is used to represent information about things, then one must be able to make statements about parts of an XML document, parts of the DLG (such as RDF nodes) and of course the objects described.

In most cases it seems obvious to the human reader. The jam jar label text does not (normally) read "jam jar label text" or "jam jar label" or "jam jar" but "jam".

Take the case of a statement about a person in an imaginary syntax

<z:person id="foo">
   <play:mailbox resource="mailto:adoe@bar.com"/>
      <z:name value="Joe"/>
      <z:wrote href="#foo">
      <z:friend resource="#foo"/>

The XML element has one attribute and four child elements. The RDF node has three properties (stated here). The person Albert has two children. What so we refer to is we refer to "#foo"? Of course we refer to the element - but when we make RDF statements, we normally want to refer to the RDF node, or rather the object described by the node, in RDF terms the resource.

Of course, in a typical unix programming language we would simply add a syntax character to distinguish the forms of reference: #foo would be the node, and @#foo (or something) would be the object refered to. But in this case we are trying to do everthing with RDF, and what is left with XML, and so we would lose a few points by adding instead some totally new syntax. What we can do is to use different attribute names for the different forms of reference. The attribute names I used above are as follows:

Forms of reference to the object of a property
value litteral string
href taking the string as a URI with or without fragment identifier, the text (or XML fragment or whatever medium) to which it refers.
resource taking a string as a URI with fragment idenifier, the abstract RDF object (rdf:resource) corresponding to the identified XML document fragment.

Here I have used "href"to allow RDF to refer to the XML model. This is important, as for example it is bits of XML which one digitaly signs, not (in sigend XML) bits of RDF. Also, it is useful for RDF to be able to talk about XML elements. It brings up the question of what an RDF fragment identifier means.

RDF and XML fragment identifiers clash

This highlights (2000/02) a bug in the relationship between XML and RDF

Consider what is identified by


when ...foo.rdf contains among other things the following:

<rdf:description rdf:id="bar">
   <rdf:type resource="...#person">
   <y:common-name>Ora Lassila</y:common-name>

The meaning of the fragment identifier is taken from the specification assocaitedwith the MIME type.

Therfore, if this is takes as a document of type application/rdf, then the fragment identifier identifies the thing (person in this case, Ora) described RDF node. This is how refernces are used in RDF.

However, if its considered to be of type text/xml then the fragment identifier is defined bythe XML spec, and so references an element whose attrubute XML:ID. has value "bar". It happens that the rdf:id is not defined to be an xml:id but is defined to "act like one", whatever that means, by the RDF spec. So it isn't clear whether the reference to this would be to the XML subtree (consisting of the rdf:description element and its contents) or would be undefined or possibly a refernce to some other element which happened to have id="bar".

To have a different interpretation of a URI as a function of the notional type of the document belies the fact the point of using XML syntax for RDF was that RDF documents should be XML documents! Of course we embed RDF in regular XML documents. So this distinction is nonsense.

Of course, the RDF spec can simply use the XML definition indirectly and refer to the RDF ndoe described by the XML element. Howvere, this is not powerful enough for RDF. This is because RDF needs to be able to make statements about XML documents and XML elements. So for example, I might want to state that I wrote the above snipet. It would be very tempting to write that I am the author of foo.rdf#bar. But I am not the author of Ora Lassila. RDF uses and parseType to resolve this for inline data: parseType=Resource indicates that the reference is to the RDF object, and parseType=Literal indicates that it is to the XML. The thing could be resolved with an interpretion property which expresses relationship between an XML subtree and an RDF object which it describes. While it would be good to define that property, RDF syntax needs a shortcut. I would propose that "resource=" which is used to point to a resource be also used for a resource fragment id, and that a new syntax be introduced to refer to the actual RDF node. maybe "object=" which happens here to correspond to the (subject, predicate, object) sense -- as well as a "thing" sense. (The former is what is the reason for chosing it - the attribute should express the relationship, not the class of the thing refered to in general!).

Naming properties and elements

We have a similar problem in the XML-RDF relationship looking atthe identity oat the schema level.

In RDF M&S 1.0, a property name defined in a namespace is formed by directly concatenating the namepsace URI with local tag name of the XML element.

One natural way to use this is to end the namespace URI with "#" so that the local tag name becomes the fragment identifier. When the schema is written in XML, this implies that the tag name, being a simple alphanumeric, will identify something in the document by its XML ID. This is a constraint on the schema language: the XML ID of an element must be usable as a reference to the thing being defined.

When there is a 1:1 mapping netween RDF properties and XML element types, there is a choice of

  1. giving them the same URI and distinguishing which is refereed to by context (as in resource= and object= above), or
  2. giving the different URIs algorithimically related, like assuming that #foo-element means the element defining #foo, using a convention specified in eth schema languages, or
  3. giving them totally distinct URIs which can be connected by an assertion in the schema, or an in

Given that it is interesting to use RDF to make statements about XML element types, having different names it appealing. As writing down the relationship every time the algorithmic link is un appealing.

A generic problem with XML identifiers

(I notice in passing that XML has currently a mixture of identifier paces which is a little confusing.

The element and attribute namespace is very well handled in terms of abbreviations, and is grounded in URI space, using the XML namespaces spec.

The URI space is of course the same space, but when value is typed as a URI, then it cannot use the abbreviation system of the elelemnt namespace.)

IDREF considered harmful

The local identifier space is a subset of URI space. When an attribute is defined as a URI, the simple "#" prefix gives access to the local ID space - while still allowing great pwer of expression by reference to anything else on the Web. When the "idref" form is used, this is not possiible. The idref form is a weak form IMHO and not wise for new designs which are not to be deliberately constraining.

Others have noticed this problem and there have even been suggestions which confused the URI prefix and the namespace prefix. In fact the problem can be solved [ref eric whiteboard] with an escape of some sort. One prossibility is ambushing a void URI schme name by using a colon prefix (suggested by Eric Prud'hommeaux)


would be a perfectly valid URI (in an XML context) which referenced the rdf:description URI using the defined rdf: namespace. I feel this is messy, as it would have to be subject to different handling than any other URI: its expansion would be done in an XML-specific way.

The other link you need is the ability, when using an element name which only occurs once, and without changing the default namespace, it would clearly be logical to be able to just write


Because what follows uses the full power of what precedes with generality, we may need to see the first in use before the paper is over. But I can't see making the second change to XML.)

[We could derive resource as a shorthand for indicating that the object refeerred to is that refered to by the given element, with an explicit coersion

<z:person id=foo>
      <z:name value="Joe"/>
      <z:wrote href="#foo">
         <rdf:node href="#foo">

This could fromally model what is going on but it a mess: every rdf arc has to be doubled!. Rref is in fact more fundamental and basic to RDF, and href is an added level-breaker for breaking levels.]

In my opinion, when you look at this analysis, the fact that the abstract object in RDF is known as a resource and that that is different from the "Resource" which is the R in URI, this is very confusing.

RDF M&S solvesa similar problem to this with ID for the object and BAGID for the container if statements.

RDF uses the "resource" to indicate an object described by a node in the RDF graph. In the above example, "#foo" in the XML lamnguage idenifies the element <z:person ... Hwoever, in RDF #foo refers (I understand) to the person themselves. This means that the

@@@ - Note the relationship between an object and a URI is non-perfect except for an abstract resource.... give examples (home page, mailbox, common name).... Compare with SQL - no identifiers, only use properties.@@@

Expressing Identity of real things

Resources (as in Universal Resource Identifer) are precisely that identified by URIs. Web pages and email messages are thought of as resources. RDF unfortunately uses the term for anything which can be talked about - any concept no matterhow abstract. RDF was originally designed as a solution for metadat - information about information - where the subject of discourse was by defintion a Web resource. There was no problem with terminology. As we use RDF to describe things other than web pages, we in fact use properties to identify them, for example we use email addresses to idnetify people. We must not muddle the email address with the person.

Consider this example

   <play:name>Ora Yrjö Uolevi Lassila</play:name>
   <play:mailbox resource="mailto:ora.lassila@research.nokia.com"/>
   <play:homePage resource="http://www.w3.org/People/Lassila"/>

Now that represents five nodes in the RDF graph: the anonymous node for Ora himself (who has no web address) and the four arcs sepcifying that this thing is of type person, and has a commin name, email address and home page as given.

Some of the properties are unambiguous in some way: two people which have the same mailbox may be assumed to be the same person. (I won't get into the rat-hole of what identity properties should be assumed for what identifiers - that is not core to the discussion)

I imagine that many processors will use their knowledge (preprogrammed or from a schema) about uniqueness of such properties to make conclusions. For example, if we define play:mailbox to be such that no two people are allowed to share the same play:mailbox property. Then, the information

   <rdf:type resource="[...]book"/>
   <play:author parseType="Resource">

   <play:name>Ora Lassila</play:name>
   <play:mailbox resource="mailto:ora.lassila@research.nokia.com"/>
   <play:homePage resource="http://www.w3.org/People/Lassila/"/>

allows the system to conclude that the name of the author of the book is Ora Lassila.

This actually exposes what is really happening when we say as a short cut that "the author is ora.lassila@research.nokia.com". What we mean is that the author is somebody with that internet mailbox. To expose such a two-step process exposes the actual nature of the identity relationships, and also their limitations. This is, in my opinion, a much cleaner way to model the data. Sometimes we need a shortcut:

<rdf:description rdf:type="[...]book">

RDF treats people as "resources" and in using this terminolgy which normally means tautologically "things with URIs" makes people expect Ora to be synonymous with his web page or his mailbox. This is not of course good design. When a shortcut is made such as author-mailbox it is important to to realize what is happening. In this model, there is no RDF node for the author himself. The fact that there is a person involved who wrote the book and has the mailbox has to be expressed elsewhere. The RDF schema may indeed be a good place for that, once we have the vocabulary, as that will make the expansion of the short cut evident to any processing machinery.

The unambiguous nature of the play:mailbox meant that it could be used as a way of identifying something. As a raw URI, mailto:ora.lassila@research.nokia.com idenifies the abstract mailbox to and from which email can be sent. However, the play:mailbox property allows one to identify a person. Its unambiguousness allows us to step from the literal "written by a person whose email is ora@w3.org" to the more useful "written by the person whose mailbox is ora@w3.org".

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Tim BL