What Is XML Schema Component Designator (SCD)?

An XML Schema Component Designator (SCD) is a W3C-standardized identifier used to uniquely reference specific components within an XML Schema, such as element declarations, complex types, or attribute groups. Unlike XPath, which locates nodes within an XML document’s tree structure, an SCD targets the abstract components defined in the schema information set. This guide explains what SCD is, its syntax structure, and how it navigates schemas to identify individual components accurately.

Understanding XML Schema Component Designator (SCD)

XML Schema Component Designator (SCD) defines a URI-compatible scheme for addressing components in an XML Schema definition. In standard XML development, referencing a specific component—such as an anonymous type nested inside an element—can be difficult using standard URIs or fragmented identifiers. SCD resolves this by defining an unambiguous, canonical path to any structural component, regardless of how or where it was declared.

How SCD Identifies Schema Components

SCD operates similarly to a path language, using structural steps to traverse a schema’s component graph rather than the raw XML markup.

1. Structural Axis and Step Syntax

An SCD expression consists of a series of steps separated by slashes (/), moving from an outer scope to an inner component. Each step identifies a component using an axis, a namespace, and a name test:

/axis::namespace:name

• Axis: Specifies the kind of schema component being selected (e.g., type::, elem::, attr::, model::).
• Name: The local name or qualified name (QName) of the component.

2. Common Axis Types

SCD defines specific axes to target different schema components directly:

• type::: Targets simple or complex types (e.g., /type::CustomerType).
• elem::: Targets global or local element declarations (e.g., /type::CustomerType/elem::Address).
• attr::: Targets attribute declarations (e.g., /elem::Invoice/attr::id).
• group:: / attrGroup::: Targets model groups or attribute groups.
• facet::: Targets specific restricting facets on simple types.

3. Addressing Anonymous and Inline Components

A significant advantage of SCD is its ability to identify anonymous components. When a schema contains an element with an inline, unnamed complex type containing a child element, SCD can address it step-by-step:

/elem::Order/type::/elem::Item

In this example, type:: without a name targets the anonymous type directly nested inside the Order element.

4. Canonical and Relative Paths

• Canonical SCDs: Fully qualified paths starting with a leading slash (/) that define an absolute route from the root of the schema.
• Relative SCDs: Paths that resolve relative to a context component, allowing modular schema references across multiple documents.

Key Differences Between SCD and XPath

While the syntax of SCD resembles XPath, they serve different layers of XML processing:

• Target Domain: XPath evaluates instantiated XML nodes (elements, attributes, text nodes) in an XML instance document. SCD evaluates abstract schema components (types, assertions, model groups) in a compiled schema model.
• Validation Independence: SCD can resolve components across multi-file schemas united by <xs:include> or <xs:import> without relying on the physical file locations or the lexical structure of the schema files.