The Unique Particle Attribution (UPA) rule is a mechanism to prevent ambiguity in
W3C XML Schema
XSD (XML Schema Definition), a recommendation of the World Wide Web Consortium ( W3C), specifies how to formally describe the elements in an Extensible Markup Language (XML) document. It can be used by programmers to verify each piece of item con ...
version 1.0.
Due to the UPA rule the
XML
Extensible Markup Language (XML) is a markup language and file format for storing, transmitting, and reconstructing arbitrary data. It defines a set of rules for encoding documents in a format that is both human-readable and machine-readable ...
schema fragment given below is prohibited:
Given the XML instance fragment:
42
it is ambiguous whether
should be associated with the element declaration (xsd:element name="x"), or the wildcard (xsd:any). This ambiguity violates the UPA rule and the corresponding XML schema therefore needs to be rejected by XML schema processors compliant to W3C XML Schema
XSD (XML Schema Definition), a recommendation of the World Wide Web Consortium ( W3C), specifies how to formally describe the elements in an Extensible Markup Language (XML) document. It can be used by programmers to verify each piece of item con ...
version 1.0.
This particular example no longer violates the Unique Particle Attribute constraint in XML Schema version 1.1, which disambiguates it by saying that when an element matches both an element particle and a wildcard, the element particle wins. However, the UPA constraint itself remains in version 1.1.
External links
Article "UPA in plain English"
Article "Understanding the Unique Particle Attribution Constraint"
XML Schema 1.0: Schema Component Constraint: Unique Particle Attribution
XML Schema 1.1 Part 1: Structures
XML Schema 1.1 Part 2: Datatypes
XML