Using Qualified Names (QNames) as Identifiers in Content

1 Preface

This TAG Finding documents a portion of the web architecture where conflicting requirements and design goals intersect. It is a simple matter of fact that specifications which have chosen one set of design criteria interoperate less well with specifications that have chosen a different set.

Given that there are existing specifications which exhibit incompatible designs and strong arguments in favor of each design, the TAG elects not to assert architectural principles that would be in direct conflict with some significant set of specifications.

It's possible that these issues could be addressed in the scope of some larger, more global redesign of, for example, XML, but no short-term solution presents itself

2 QNames as Identifiers

This finding is concerned with the use of qualified names (QNames) as identifiers. That is, the contexts in which a colonized name can be understood to be a QName.

A related TAG issue, rdfmsQnameUriMapping-6, concerns the mechanism by which one can (or can not) construct a URI for a particular QName. We do not consider that issue in this finding.

3 QNames in XML

Qualified names were introduced by [XML Namespaces]. They were defined for element and attribute names (only) and provide a mechanism for concisely identifying a URI/local-name pair. For example, in the following document:

<?xml version='1.0'?>
<doc xmlns:x="http://example.com/ns/foo">
<x:p/>
</doc>

The QName "x:p" is a concise, unambiguous name for the URI/local-name pair {"http://example.com/ns/foo", "p"}.

When used solely in element and attribute names, all QNames are identified by the XML processor and can logically be replaced by the URI/local-name pair they identify.

4 QNames in Other Specifications

Other specifications, starting with [XSLT], have taken QNames and employed them in contexts other than element and attribute names. Specifically, QNames have been used in attribute values and element content.

For example, in the following document, "x:p" is understood to be a QName even though it appears in an attribute value, not an element or attribute name.

<?xml version='1.0'?>
<xsl:stylesheet xmlns:xsl="http://www.w3.org/1999/XSL/Transform"
                xmlns:x="http://example.com/ns/foo"
                version="1.0">

<xsl:output method="html"/>

<xsl:template match="x:p">
  <p>
    <xsl:apply-templates/>
  </p>
</xsl:template>

</xsl:stylesheet>
</doc>

In attribute values and element content, QNames are often used to identify a particular element type; they are, in principle, using QNames as they were intended. However, some specifications use QNames as shortcuts for unique identifiers derived from a URI/local-name pair that have no relationship to element or attribute types.

Using a QName as a shortcut for a URI/local-name pair is often convenient, but it carries a price. There is no single, accepted way to convert QNames into URI/local-name pairs or vice versa. Different specifications have chosen different algorithms. This means that even if a QName can be identified in content, it may be difficult or impossible to determine what URI/local-name it represents. At the very least, the mapping depends on the context in which it occurs. Therefore, at the very least, it is important for specifications to identify the mapping algorithm that they have chosen.

Principle

Specifications that use QNames to represent URI/local-name pairs MUST describe the algorithm that is used to map between them.

We observe also that there is an overlap in the lexical space of QNames and URIs.

Good Practice

Specifications that use QNames to represent URI/local-name pairs SHOULD NOT allow both forms in attribute values or element content where they would be indistinguishable.

5 Namespace Bindings

Some specifications rely on the in-scope namespace bindings in the XML document to associate prefixes with namespace names. Other specifications rely on application-specific mechanisms.

Using the in-scope namespace bindings has the advantage that it theoretically allows a generic processor to interpret QNames in content without having to be aware of any application-specific mechanisms. The alternative, where every specification defines its own mechansism, could clearly lead to a badly fragmented web.

However, there is at least one application where a compelling argument has been made for requiring an alternative mechanism for defining namespace bindings. That application is [XPointer Framework].

It is an architectural principle of URIs that they be context-independent. It follows that the QNames that appear in an XPointer must not refer to in-scope namespaces as this would make transcription impossible in the general case.

We must therfore accept that there are some applications which use in-scope namespaces and some which use their own mechanisms.

6 Architectural Observations

The TAG makes the following observations:

• Whatever the architectural ramifications of using QNames as identifiers in contexts other than XML element and attribute names, it is already established practice.

  It is simply not practical to suggest that this usage should be forbidden on architectural grounds.

• Using QNames in untyped (#PCDATA or xs:string) attribute values or element content places an additional burden on the processor that was not anticipated by [XML Namespaces].

  If QNames are only used in element and attribute names, the processor can fully resolve all of the prefixes as it parses. This gives it the freedom to discard the prefix-to-URI mappings when they go out of scope. A serializer, presented with an object model that conforms to [XML Namespaces] can manufacture new prefixes on the fly. (In practice, users expect most prefixes to be preserved through transformations, so things aren't quite this simple for most developers, but this is still theoretically the case.)

  As soon as QNames may appear in element or attribute values, the processor must retain all of the prefix-to-URI mappings (and any API must expose these mappings). This is necessary because some subsequent micro-parser, in the course of examining some content, may encounter a token that it recognizes as a QName and need to find its URI/local-name.

  In our previous XSL example, from the perspective of the XML processor, there are no qualified names that use the x: prefix. However, when the XSL processor examines the match attribute on xsl:template, it must be able to resolve the x: prefix.

• QNames in attribute values or element content by themselves, in other words, in contexts that could be typed as xs:QName ([XML Datatypes]) in a schema, could in principle be identified by the schema processor.

  For example, given:

  <elem ref="data:myInteger"/>

  If schema validation reveals that the following component applies to this instance of the elem element:

  <xs:complexType name="elemType">
    <xs:complexContent>
      <xs:restriction base="xs:anyType">
        <xs:attribute name="ref" type="xs:QName"/>
      </xs:restriction>
    </xs:complexContent>
  </xs:complexType>

  The schema processor can determine that data:myInteger is a QName and must therefore be a concise name for the URI/local-name pair consisting of the in-scope namespace URI for the prefix data and the local-name myInteger.

• Perhaps the most common use of QNames in untyped values at the moment is in locations where XPath expressions may occur. As XPath is reused in more and more specifications, it may eventually be reasonable to define an XPath data type to identify all of these values in a way that makes them accessible to higher-level parsers.

• Some specifications rely on mechanisms other than in-scope namespaces to associate prefixes with namespace names. In general, therefore, even when all of the in-scope namespace declarations are available, there may still be prefixes which can only be known in an application-dependent manner.

7 Architectural Statement

The TAG recognizes that there are pragmatic reasons why it is desireable to provide the same kind of URI/local-name shortcuts that QNames provide for element and attribute names in other contexts. In addition, the practice is already well established. Therefore, the TAG accepts that it is reasonable to use QNames in this way.

Specifications that have no compelling reason to use QNames where URIs would suffice, for example formats that are not expected to be edited by hand or formats where human readability is not a significant factor, can avoid an entire set of problems by simply using URIs.

The TAG encourages designers to consider the ramifications of their use of QNames carefully as it may have a dramatic impact on the extent to which their specification interoperates with other specifications.

8 References

XPointer Framework

Paul Grosso, Eve Maler, Jonathan Marsh, Norman Walsh, editors. XPointer Framework. World Wide Web Consortium, 2002. (See http://www.w3.org/TR/xptr-framework/.)

XML Datatypes

Paul V. Biron and Ashok Malhotra, editors. XML Schema Part 2: Datatypes. World Wide Web Consortium, 2000. (See http://www.w3.org/TR/xmlschema-2/.)

XML Namespaces

Tim Bray, Dave Hollander, Andrew Layman, editors. Namespaces in XML. World Wide Web Consortium, 1999. (See http://www.w3.org/TR/REC-xml-names/.)

XSLT

James Clark, editor. XML Transformations (XSLT) Version 1.0. World Wide Web Consortium, 1999. (See http://www.w3.org/TR/xslt.)