Conditional Type Assignment in XML Schema 1.1

XML Schema (XSD) 1.1 introduces Conditional Type Assignment (CTA), a feature that allows the validation type of an element to be determined dynamically at runtime based on its attribute values. By using the <xs:alternative> element alongside XPath 2.0 test expressions, XSD 1.1 eliminates the limitations of XSD 1.0, where an element’s type was statically bound to its name unless explicitly overridden in the XML instance using the xsi:type attribute.

The Mechanism: <xs:alternative>

In XSD 1.1, conditional type assignment is configured directly within an element declaration using one or more <xs:alternative> elements. Each <xs:alternative> specifies a conditional XPath expression via the test attribute and the corresponding target schema type via the type attribute.

When a validating parser encounters the element, it evaluates the XPath expressions sequentially. The element is validated against the type of the first test attribute that evaluates to true.

Syntax and Implementation

Below is an example demonstrating an <entry> element whose type changes based on the value of its @kind attribute:

<xs:element name="entry" type="xs:anyType">
    <!-- Alternative 1: If @kind is 'book' -->
    <xs:alternative test="@kind = 'book'" type="BookType"/>
    
    <!-- Alternative 2: If @kind is 'journal' -->
    <xs:alternative test="@kind = 'journal'" type="JournalType"/>
    
    <!-- Default Alternative: If no conditions match -->
    <xs:alternative type="GenericEntryType"/>
</xs:element>

Corresponding XML Instances

1. Validates as BookType:

<entry kind="book">
    <isbn>978-0134685991</isbn>
    <author>Joshua Bloch</author>
</entry>

2. Validates as JournalType:

<entry kind="journal">
    <issn>0010-4620</issn>
    <volume>64</volume>
</entry>

3. Validates as GenericEntryType (fallback):

<entry kind="misc">
    <description>General note</description>
</entry>

Key Rules and Processing Constraints

• XPath Subset Restrictions: The XPath expression in the test attribute is restricted to a subset of XPath 2.0. It can inspect the element’s own attributes and inherited in-scope namespaces, but it cannot inspect child elements or sibling nodes.
• Evaluation Order: Alternatives are evaluated in the exact order they appear in the schema definition. The first matching alternative is selected, and subsequent alternatives are ignored.
• Default Type Fallback: An <xs:alternative> without a test attribute acts as a default/catch-all branch. If included, it must appear as the last <xs:alternative> in the sequence. If no alternatives match and no default alternative exists, the base type declared on the <xs:element> is used.
• Inherited Types: If an element definition uses type substitution and extensions, derived types can inherit and extend alternative rules from their base types.

Advantages Over XSD 1.0

In XSD 1.0, conditional data models required consumers to add schema-specific attributes (xsi:type) directly into the business XML documents. XML Schema 1.1 CTA enables domain-specific attributes (such as type, mode, or category) to drive the type validation natively, maintaining separation between domain data and validation logic.