A canonical schema component designator is an absolute schema component designator that is a URI that has been mapped from a LEIRI to an IRI in accordance with the rules given in , mapped from an IRI to a URI in accordance with the rules given in RFC 3987, and normalized according to the rules of syntax-based normalization given there in RFC 3986; and where the relative schema component designator consists of an xmlns XPointer pointer part (if required) followed by a canonicalized xscd XPointer pointer part.

A canonicalized xscd XPointer pointer part is an xscd XPointer pointer part whose pointer data is a canonical schema component path.

Many use cases for schema component designators call for them to be compared for equality. A simple string comparison may fail to correctly recognize equivalent designators with this scheme because namespace prefixes may vary. However, comparison is still straightforward.

Two schema component designators are equal if they are absolute and their URIs are equal or if they are relative to the same schema, and their schema component paths are equal.

An assembled schema consists of a graph of schema components. Schema property records and helper components are included in the graph, but a schema component path never refers to them. Traversal in the graph passes through property records. A schema component property that has other schema components as its value or as part of its value creates links in the graph which may be traversed through one of the defined axes to construct a schema component path.

A traversal from one component to another takes place across some particular component property. This property is the arc of traversal.

A default arc is a privileged arc of traversal. The default arcs are:

includes a summary of which arcs act as defaults and when.

A schema component path selects a sequence of schema components in the context of an assembled schema; it is a series of steps separated by '/' or '//'. Complete schema component paths always start with either '/' or '//'.

A step consists of a schema component axis and a schema component name test and an optional positional predicate. It selects a sequence of schema components that are linked to a source component through an arc that matches the axis, pass the name test, and are selected by the positional predicate, if any. Schema component axes are described in .

In general the syntax of a step of the schema component path has the following form:

The predicate is optional and there are abbreviated forms for some steps.

A schema component name test is a condition on name of the components selected by the schema component path step. A schema component name test either gives a localname, possibly with a namespace prefix, or consists of a just the wildcard character ('*'), or just the character zero ('0').

A schema component name test of this form is true if the QName formed from the namespace name and local name specified in the the schema component name test is equal to the value of the component-name() accessor of the schema component. For more information on namespace binding for QName interpretation see .

A schema component name test of this form is true for anonymous type definitions. An anonymous type definition's component-name() accessor returns "0". This name cannot have any conflict with any possible named component, as 0 is not a valid name start character.

A schema component name test of this form is true for all linked components matching the given axis.

A schema component positional predicate is a condition on the relative position of the component in the sequence of matching components from the schema component path step. A schema component position predicate is true if the position of the target component in the sequence of components selected by the axis and name test is equal to the positive integer given in the predicate.

A path consisting of just '/' selects the schema description component.

A path of the form /S1 selects components linked to the schema description component in accordance with the step S1.

A path of the form S1/S2 is interpreted by gathering the sequence of source components and then selecting any schema component that is linked to one of the source components in accordance with the step S2. The sequence of source components consists of the sequence of components selected by S1 followed by the components selected by the elided-component axis on those components.

A path of the form S1//S2 is also interpreted by gathering the sequence of source components and then selecting any schema component that is linked to one of the source components in accordance with the step S2. In this case the sequence of source components consists of the sequence of components selected by S1 followed by the application of the component axis on members of that sequence.

A path of the form S1 selects all schema components linked to the initial source component in accordance with the step S1.

Implementations will have to exercise care to properly handle circularities in the component graph in determining the set of components matched by a path.

A schema component path axis contains specific components linked to the current context component through certain kinds of arcs. Unless otherwise indicated, an axis applied to a component that has no arc of the required kind designates no components.

A set of accessors is defined on schema components. For consistency, each accessor is defined for every kind of schema component, although several accessors return a constant empty value on some kinds of schema components.

Where accessors return sequences, the order of components accessed on one arc is as defined by the schema component model. Components accessed through default arcs precede components accessed through non-default arcs. Components are otherwise ordered as if arcs were accessed in the following order:

The canonical schema component path of a component is a distinguished valid component path that uniquely identifies that particular component, that has as few steps as possible, and that can be deterministically constructed starting from the schema description component, and that contains no extension axes or accessors.

The remainder of this section consists of a constructive definition of canonical schema component paths: a path produced by this construction will be the canonical one.

The current schema component is a schema component for which there is a valid canonical path through the schema component graph.

The target schema component is a schema component linked to the current schema component via one of the schema component properties defined previously.

The component relationship is the name of the schema component property on the current schema component which references the target schema component.

The ancestor set of the target schema component is the set of schema components on the valid canonical path to the current schema component together with the current schema component.

The base type set of a schema component is the set consisting of the {base type definition} of that schema component and the {base type definition} of every member of the set.

Constructively, this set can be computed by adding the {base type definition} to the set, and iterating on that type component, until you see a component whose {target namespace} property is "http://www.w3.org/2001/XMLSchema" and whose {name} property is "anyType".

The extended base type set of a schema component is the set consisting of the {base type definition} of that schema component if its {derivation} method is "extension" and the {base type definition} of every member of the set whose {derivation} method is "extension".

The base facet set of a schema component is the set consisting of all members of the {facets} property of members of the base type set of that schema component.

Two facets are the same if they are the same kind of component (e.g. both are length facets) and their {value} properties have identical values.

The base attribute use set of a schema component is the set consisting of all members of the {attribute uses} property of members of the base type set of that schema component.

Two attribute uses are the same if the {name} and {target namespace} of their {attribute declaration} properties are equal.

(Note that this is not equality which requires checking the {type definition} as well. However, the rules for component consistency will ensure that checking the {name} and {target namespace} suffices for our purposes here.)

The particle set of a schema component is the set consisting of the {content type} of that schema component with the members of the {particles} property of the {term} of any member of the set.

The base particle set of a schema component is the union of all particle sets of all the members of the extended base type set of that schema component.

Two particles are the same if the implementation reports them as the same. Informally, implementations will report particles that are "inherited" from base types as "the same" as the corresponding particle in the base type. Future revisions of the XML Schema recommendation will specify more precisely the conditions under which particles should be regarded as "the same".

Traversal where the current schema component is the schema description component to any target schema component is always permitted in the construction of a canonical path. If the current schema component is not the schema description schema component, then traversal from the current schema component to the target schema component is forbidden in the construction of a canonical path if any of the following conditions is met:

Informally, the first two conditions ensure that the canonical path to an identity constraint or a named type is the one that flows directly from the schema description component, the third condition ensures that the canonical path to element and attribute declarations is likewise the global one (if there is one), the fourth condition accounts for element and attribute declarations stemming from named model groups and attribute groups, the fifth and sixth ensure that the canonical path doesn't follow "backwards" paths (while accounting for redefined types), and the final three conditions ensure the canonical path to "inherited" components goes through their component of origin.

Because the 1.0 component model provides no reliable ordering of annotations, selection of a specific annotation using a numerical predicate is forbidden in canonical paths against a 1.0 schema. For 1.1 schemas, an individual annotation component is selected using a numerical predicate.

In general, it is not possible to obtain the canonical path for a schema component without access to the schema component graph. In particular, canonicalizing some non-canonical path, or determining whether a schema component path is canonical cannot be performed without such access.

For example, consider the path /type::x:foo/schemaElement::x:bar.

For this schema fragment, this represents a canonical path to the local element bar:

But for the following schema fragment, the same path represents a non-canonical path. The canonical path would instead be /group::x:barGroup/schemaElement::x:bar.

Two schema component paths are equal if they have the same number of steps, and each step in one path is equal to the corresponding step in the other. Steps are equal if they have the same axis, name test (namespace and local name, or wildcard), and predicate. .

Note: This definition of equality neither relies on the notion of 'the same component' nor defines component equivalence. A schema component may have multiple schema component paths. Although these schema component paths address 'the same component', they are not equal. It is true, however, that two canonical paths to the same component will be equal.

Schema component paths may be used by other specifications. For such usage, this specification does not define any criteria for conformance and relies on other specifications to specify criteria for conformance of implementations.

Such specifications must define:

This specification normatively depends on the XPointer Framework .

Conforming XPointer processors claiming to support the xscd() scheme must identify schema components as defined in this specification and conform to the XPointer xmlns() scheme specification.

Schema component paths and designators navigate the XML Schema component model and are wholly dependent on that model to define the set of allowable axes and referenced components. Therefore, as the schema component model changes, this specification may add additional axes, mapping rules, and accessors as required to support those changes. The EBNF defines syntax for extension accessors after the last step in a path and for extension axes.

This section walks through an example XML Schema Document from the XML Schema Part 0: Primer and enumerates the abbreviated and canonical schema component designators for schema components.

All schema constructs in this section are considered to be in the following schema document and its URI is schema-URI:

Since this schema has no target namespace, no namespace binding is required and there is no xmlns() pointer part for any of the schema component designators for this schema.

The canonical schema component designator for this schema description component is schema-URI#xscd(/).

The canonical schema component designator for this annotation schema component is schema-URI#xscd(/annotation::*).

The following is a global element declaration:

The abbreviated schema component designator for this element declaration component is schema-URI#xscd(/purchaseOrder) and the canonical is schema-URI#xscd(/schemaElement::purchaseOrder).

The following is another global element declaration,

The abbreviated schema component designator for this element declaration component is schema-URI#xscd(/comment) and the canonical is schema-URI#xscd(/schemaElement::comment).

This following is a global complex type definition component:

The abbreviated schema component designator for this complex type definition and its element and attribute declaration components are:

The canonical schema component designator for this complex type definition and its element and attribute declaration components are:

The following is another global complex type definition component:

The abbreviated schema component designator for this complex type definition and its element and attribute declaration components are:

The canonical schema component designator for this complex type definition and its element and attribute declaration components are:

The following is a global complex type definition with anonymous complex type and simple type definition components:

The abbreviated schema component designator for this complex type definition and its element and attribute declaration, simple type definition, and facet components are:

The canonical schema component designator for this complex type definition and its element and attribute declaration, simple type definition, and facet components are (paths broken across lines for readability):

The following is a global simple type definition component:

The canonical schema component designator for this simple type definition and its facet component are:

The schema for the examples in the previous section had no target namespace. If we change the schema so that the target namespace is "http://example.com/schema/po", then the schema component designators need to take namespace bindings into account.

For example, the abbreviated schema component designator for the complex type definition USAddress and its element and attribute declaration components are:

Note that since the attributeFormDefault for the schema is unqualified (the default value), no namespace prefix is used with the country attribute.

The canonical schema component designator for this complex type definition and its element and attribute declaration components are (paths broken across lines for readability):

Schema component paths may also be used in other contexts that provide namespace binding contexts. For example, an XML representation may apply normal namespace binding rules to apply to embedded schema component paths:

Another possible means of binding namespaces is to use the namespace URI as the schema designator with the convention that it is the default namespace. This could be useful combined with the use of just the schema component paths in the fragment identifier to obtain terse identifiers for global schema types in single-namespace schemas:

The following examples assume that the namespace prefixes r and ipo are appropriately bound.

Here is a path that designates an attribute declaration whose local name is period in the namespace denoted by r in an anonymous complex type:

Using the elided-component axis to select the attribute declaration directly from the element declaration, this path reduces to:

Here is a path that designates an element declaration whose local name is name in the namespace denoted by ipo in a globally defined type:

Using the elided-component axis to select the element declaration directly from the complex type definition, this path reduces to:

This path designates an element declaration whose local name is regions in the namespace denoted by r in an anonymous complex type:

Using the elided-component axis to select the element declaration r:regions directly from the element declaration r:purchaseReport, this path reduces to:

This path designates global or local element declarations whose local name is quantity in the namespace denoted by r:

This path designates global or local element declarations whose local name is quantity in the namespace denoted by r, where these declarations are in the subgraph represented by the global complex type component whose local name is Items in the namespace denoted by r:

This path designates global or local attribute declarations whose local name is partNum in the namespace denoted by r where these declarations are in the subgraph represented by the global complex type component whose local name is Items in the namespace denoted by r:

This extended example works against the following schema fragment for a schema with no target namespace:

Which of the following paths can be shortened to //foo using the component and elided-component axes?

The "no" answers are all because bar is not selected by either the component axis or the elided-component axis and must therefore appear in the path.