This document discusses the following topics about namespace usage in XML Schema and TSD4:
One major advantage of XML Schema compared to DTDs is that XML Schema fully supports XML Namespaces. In order to do so, XML Schema introduces the concept of the target namespace. Each schema document can declare at most one target namespace, and all definitions made in this schema document belong to this namespace. Thus, a schema may contain namespace-less definitions, or definitions belonging to the specified target namespace.
Does this really mean that we cannot define multi-namespace schemas? No;
the emphasis is here on the schema document. We can always compose
multi-namespace schemas by importing (see below) other schema documents into a
schema, see the description of the xs:import element.
The xs:import statement is used
in XML Schema to compose multi-namespace schemas. A typical example is given
below:
<?xml version="1.0" encoding="UTF-8"?>
<xs:schema targetNamespace="http://www.softwareag.com/tamino/doc/examples/models/jazz/encyclopedia"
xmlns="http://www.softwareag.com/tamino/doc/examples/models/jazz/encyclopedia"
xmlns:xs="http://www.w3.org/2001/XMLSchema"
xmlns:i="http://www.softwareag.com/tamino/doc/examples/models/instruments">
<xs:import namespace="http://www.softwareag.com/tamino/doc/examples/models/instruments"
schemaLocation="instruments.xsd"/>
...
</xs:schema>
xs:import
statements are always given at the beginning of a schema definition. An
xs:import
statement usually specifies a namespace to import, and may optionally specify a
URL using the schemaLocation
attribute. Note that this attribute only gives a hint to
the XML processor, telling it where to find the schema
document associated with the imported namespace. XML processors are not
required to use this attribute, but may use their own logic to find the
associated schema. Tamino supports only relative
URLs in the schemaLocation attribute in
xs:import. The referenced schemas must already be defined in the
Tamino database when defining the referencing
schema.
The structure of the schemaLocation attribute may be any
of the following alternatives:
../collectionName/schemaName
./schemaName
schemaName
The last two alternatives are equivalent. Note: The
xsi:schemaLocation and xsi:noNamespaceSchemaLocation
attributes in XML instances of a schema (or doctype) are
ignored by Tamino. Instead, a different logic based
on collections and doctypes is applied to locate the schema used for validating
the XML instance.
The following schema shows the simultaneous use of the
namespace and schemaLocation attributes of the
<xs:import>
element:
<?xml version="1.0" encoding="UTF-8"?>
<xs:schema
targetNamespace="http://www.softwareag.com/tamino/doc/examples/models/jazz/encyclopedia"
xmlns:e="http://www.softwareag.com/tamino/doc/examples/models/jazz/encyclopedia"
xmlns:xs="http://www.w3.org/2001/XMLSchema"
xmlns:tsd="http://namespaces.softwareag.com/tamino/TaminoSchemaDefinition"
xmlns:i="http://www.softwareag.com/tamino/doc/examples/models/instruments"
elementFormDefault="qualified"
attributeFormDefault="unqualified">
<xs:import
namespace="http://www.softwareag.com/tamino/doc/examples/models/instruments"
schemaLocation="instruments.xsd"/>
<xs:annotation>
<xs:appinfo>
<tsd:schemaInfo name="jazzMusician">
<tsd:collection name="music"/>
<tsd:doctype name="e:jazzMusician"/>
</tsd:schemaInfo>
</xs:appinfo>
</xs:annotation>
<xs:element name="jazzMusician">
<xs:complexType>
<xs:sequence>
<xs:element name="name" type="e:tName"/>
...
<xs:element name="plays" minOccurs="0" maxOccurs="unbounded">
<xs:complexType>
<xs:choice>
<xs:element ref="i:saxophone"/>
</xs:choice>
</xs:complexType>
</xs:element>
</xs:sequence>
...
</xs:complexType>
</xs:element>
...
</xs:schema>
This example schema defines a global element named
jazzMusician, and local elements name and
plays. The local elements are qualified because of the
elementFormDefault="qualified" attribute. All the elements defined
in the schema belong to the namespace given by the
targetNamespace attribute.
The xs:import element imports a
schema with definitions for the
http://www.softwareag.com/tamino/doc/examples/models/instruments
namespace. The content model of element plays refers to an element
named saxophone in the imported namespace.
The imported schema might look as follows:
<?xml version="1.0" encoding="UTF-8"?>
<xs:schema
targetNamespace = "http://www.softwareag.com/tamino/doc/examples/models/instruments"
xmlns:i = "http://www.softwareag.com/tamino/doc/examples/models/instruments"
xmlns:xs = "http://www.w3.org/2001/XMLSchema"
xmlns:tsd = "http://namespaces.softwareag.com/tamino/TaminoSchemaDefinition">
<xs:annotation>
<xs:appinfo>
<tsd:schemaInfo name="instruments.xsd">
<tsd:collection name="music"/>
</tsd:schemaInfo>
</xs:appinfo>
</xs:annotation>
<xs:element name="saxophone">
...
</xs:element>
...
</xs:schema>
Usage of the xs:import element is subject to various
constraints with respect to:
the targetNamespace of the involved schema
documents;
usage of the namespace attribute;
usage without the schemaLocation attribute.
The following constraints must be taken into account when using the
xs:import element:
If there is no namespace attribute present, the
enclosing xs:schema element must have
a target namespace declaration.
If the xs:import element contains a
namespace attribute, it may not match the value of the
targetNamespace attribute of its enclosing
xs:schema
element.
Elements and attributes defined as child nodes of the
xs:schema root
element of a schema document are called global elements and attributes. If the
xs:schema element
has a targetNamespace attribute, all the global
elements and attributes defined in that schema document are qualified, i.e.
they belong to that namespace.
Once declared in this way, a global element or attribute can be used by
referencing it in declarations (using the ref
attribute of xs:element or
xs:attribute).
References to global elements or attributes are not restricted to a single
schema; rather, they may refer to objects in a different namespace by using the
prefix mapped to the targetNamespace of the
imported schema in which the global object is defined.
By contrast, declarations of elements or attributes that are not
immediate descendants of the xs:schema root node and do
not just reference a global element or attribute definition via
ref="..." are called local elements or local attributes. They can
be defined e.g. inside a local or global (i.e. named) type. By default, these
local nodes do not belong to the namespace given by the
targetNamespace unless the corresponding element
or attribute definition has a form="qualified" attribute. The
default of form="unqualified" can be superseded by the
elementFormDefault="qualified" or
attributeFormDefault="qualified" attributes of the
xs:schema
element. These attributes specify whether the corresponding elements or
attributes in the XML instance documents are qualified or not, if the
form attribute is not present.
Namespace aspects of global and local elements and attributes in XML Schema are addressed in detail in the section Advanced Concepts I: Namespaces, Schemas & Qualification of the W3C XML Schema Part 0: Primer.
Another mechanism to allow for foreign namespaces is the usage of
wildcards. A wildcard (i.e. an element or attribute whose content is not
defined further) can be declared with the XML Schema elements
xs:any or xs:anyAttribute. This allows for the
inclusion of elements and attributes from foreign namespaces. For example,
sections of XHTML, SVG, RDF or other content could be included into a document.
A typical application is the description property in the style asset of our
"Jazz" model, where we could use XHTML to mark up the content.
It is possible to constrain the namespace of the content of such a wildcard. This is done with the namespace attribute of the wildcard. This attribute can contain either:
a whitespace-separated list of the following items:
explicit namespace URIs;
the string "##targetNamespace", which denotes the
target namespace of the current schema file;
the string "##local", which denotes the absence of a
namespace;
or one of the following string values:
the string "##any", which allows for any namespace.
This is also the default value of the namespace attribute. This value is often
used together with processContents="skip";
the string "##other", which allows for any namespace
other than the target namespace.
References to types must be qualified with the namespace prefix bound to the namespace in which the referenced type is defined.
Note:
This applies to both XML Schema predefined types and user-defined
global types.
For example, the importing schema above contains a reference to a type
e:tName, which might be defined as follows:
<xs:complexType name="tName">
<xs:sequence>
<xs:element name="firstName" type="xs:string"/>
<xs:element name="middleName" type="xs:string" minOccurs="0"/>
<xs:element name="lastName" type="xs:string"/>
</xs:sequence>
</xs:complexType>
In the same way, references to a model group or attribute group
definition in the same or a different schema document must use a prefix bound
to a namespace that is identical to the respective schema's
targetNamespace.