W3C XML Schemas have become the core of many business applications because of their powerful data typing and definition capabilities. But a data model isn't always static. Schemas often need ways to allow for extensibility over time to accommodate new information and element types. Several approaches can extend schemas to include new elements as needed: The six strategies described in this article provide techniques to extend single-namespace schemas. Using multiple namespaces to extend the data being processed requires an article of its own.
Note: This article focuses solely on W3C XML Schema version 1.0. The W3C XML Schema Working Group is nearing completion of version 1.1, but it is not yet ratified and might change. The examples here are all based on the current specification.
A good example of data that changes over time is code lists. A code list is a list of unique code values that have specific meanings, such as product descriptors, frequently used terms, and lists of countries or cities. These values are often stored in a database row that you can add to over time and use to populate choices in an application window.
The simple code list of colors in Listing 1 illustrates how
to extend a schema as new data choices emerge. It defines a simple code list with
the element type
color, which contains four possible
elements, the first three of which are given known color names. The last element
in the group is sometimes called a generic element and is designed to allow
any value to be inserted in the
name attribute, thereby
allowing you to add new colors to the list as needed over time. If you want a new
color choice many months after the application has been completed and deployed,
you can specify a new color—perhaps purple—and use the
other element with the attribute
name="purple". When validated, the
<other> element is allowed, and you keep working
with no changes to the schema required.
Listing 1. Sample schema defining extensible color code list elements
<xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema"> <xs:element name="color"> <xs:complexType> <xs:choice maxOccurs="unbounded"> <xs:element name="red" type="xs:string"/> <xs:element name="blue" type="xs:string"/> <xs:element name="green" type="xs:string"/> <xs:element name="other"> <xs:complexType> <xs:attribute name="name" type="xs:string"/> </xs:complexType> </xs:element> </xs:choice> </xs:complexType> </xs:element> </xs:schema>
A sample of valid data associated with this schema that uses the generic element extension is in Listing 2.
Listing 2. Valid data instance associated with the color code list schema
<color xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="color.xsd"> <red/> <green/> <other name="purple">cc00cc</other><!-- Extension data --> <other name="orange">ee9944</other><!-- Extension data --> </color>
As you can see, the schema does not define elements with the names purple or orange, but these names were included in the data instance and parsed as valid because of the extension technique used. This technique works where a static list exists but new items are added on an ongoing basis. The creation of the data can be slightly more complicated, but maintaining the schema and related applications is greatly simplified. Of course, this data could manage all color information in an attribute instead of an element.
Processing this data requires special handling of the generic
element when it occurs in the data instance. An XPath statement in an XQuery or
XSLT stylesheet might test for one of the predefined elements and also display
the known color. Either language has the ability to select one of the known element
names to process accordingly, or it can select the
element and read the attribute value for
the element content for the color value (expressed here as a CSS style value for
the respective colors).
Modular schema assembly
You might modularize schemas for a lot of reasons, but this section focuses on using
modularity to extend them. In short, creating several schema modules and
including them into your base schema is a form of extending the base schema. The
example in Listing 3 uses the schema construct
<xs:include> to bring in the schema module.
Listing 3. Bringing in a schema module using xs:include
<xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema"> <!-- Reference to External Module containing USaddress definition --> <xs:include schemaLocation="USaddress.xsd"/> <!-- Element containing USaddress element from included module --> <xs:complexType name="contact"> <xs:sequence> <xs:element name="Name" type="xs:string"/> <xs:element ref="USaddress"/> </xs:sequence> </xs:complexType> </xs:schema>
Listing 4. The included schema module
<xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema"> <xs:element name="USaddress"> <xs:complexType> <xs:sequence> <xs:element name="street1" type="xs:string" minOccurs="0"/> <xs:element name="street2" type="xs:string" minOccurs="0"/> <xs:element name="city" type="xs:string"/> <xs:element name="state" type="xs:string"/> <xs:element name="zip" type="xs:string" minOccurs="0"/> </xs:sequence> </xs:complexType> </xs:element> </xs:schema>
The combined resulting schema allows the data instance in Listing 5 to validate successfully.
Listing 5. Data instance validated using the included schema
<contact> <name>Dale Waldt</name> <USaddress> <city>New York</city> <state>NY</state> </USaddress> </contact>
Note that the resolved schema will contain all declarations from both the original and the included schemas. Because the example in Listing 5 uses only one namespace, element and attribute names must be unique in the combined resolved form. Also, the occurrence rules must be consistent in the resolved form.
Although adding modules is a form of extending a schema, the potential for building a set of dynamically assembled modules to create flexible schemas for different environments and applications is a powerful concept for optimizing development and maintenance effort. You can create a library of predefined, consistent declarations for developers to selectively use throughout the enterprise. Even so, take special care to prevent naming collisions and other errors from occurring, especially in a single namespace.
Abstract elements and substitution groups
The W3C XML Schema allows for a class of element types that generally appear in the same
locations to be treated as a group of equivalent elements in type definitions. For
example, you might have several types of named objects (that is, people, places,
things) that appear in text as inline elements, including person, city, lodging,
restaurant, and museum. You can define a content model, like the one in
Listing 6, for the textual
element that is defined as mixed content with child elements named
<inline>, because paragraphs are likely to have
bold, italic, and other inline elements.
Listing 6. Defining elements that are members of a substitution group
<!-- Paragraph Element Type Definition with Abstract Element --> <xs:element name="p"> <xs:complexType> <xs:choice maxOccurs="unbounded" mixed="true"> <xs:element name="b" type="xs:string"/> <xs:element name="i" type="xs:string"/> <xs:element ref="inline"/> </xs:choice> </xs:complexType> </xs:element> <xs:element name="inline" type="xs:string"/> <!-- Substitution Element Types --> <xs:element name="person" type="xs:string" substitutionGroup="inline"/> <xs:element name="hotel" type="xs:string" substitutionGroup="inline"/> <xs:element name="city" type="xs:string" substitutionGroup="inline"/> <xs:element name="url" type="xs:string" substitutionGroup="inline"/> <xs:element name="email" type="xs:string" substitutionGroup="inline"/> <xs:element name="phone" type="xs:string" substitutionGroup="inline"/>
Note that in Listing 6 the
element is referenced in the attribute
found in the element declarations that follow the one for
<inline>. This means that all elements that are
members of the substitution group that has the name of an another element can
be placed wherever that element is allowed (or an abstract element can serve this
purpose). In this example, the substitution group element types
and so on are allowed anywhere the
element is—in this case, inline in the text of the paragraph. The data
instance in Listing 7 is valid with this schema and its
substitution group extensions. (You can also use this technique with substitutions
that reference an abstract element.)
Listing 7. A valid data instance using substitution group elements
<p>This is a <b>paragraph</b> with several inline elements. This sentence mentions <city>Chicago</city>, <person>Mayor Daly</person> and <hotel>The Drake Hotel</hotel> which are named entities that have specific markup applied to them. </p>
Also note that all affected elements must be declared globally, not locally in
the context of another type definition. The substitution group elements can
appear in any order, because they are in a repeatable choice group with
Over time, new uses might require the addition of inline elements. What makes this example extensible is that the schema developer only has to add a new element declaration indicating that it is a member of the substitution group. Of course, the block of element declarations that are substitution group members can be managed in a schema module that is stored separately and included in a main schema. Doing so might simplify the process of adding new element declarations to the substitution group and might even be managed and produced from an application interface, much in the same way code lists are managed and extended.
Extension to an existing type
The W3C XML Schema lets you extend existing type definitions to add additional sub-elements, adding additional elements to the data model's structure. You can apply extensions to the types of element or attributes. Given the example type definition in Listing 8, you can define the contents of elements that contain person name information.
Listing 8. A simple type definition
<xs:complexType name="nameType"> <xs:sequence> <xs:element name="fname" type="xs:string"/> <xs:element name="lname" type="xs:string"/> </xs:sequence> </xs:complexType>
This definition will parse the instance below as valid (assuming that the element
<name> is defined using the
You can supply additional sub-elements to the complex type called
nameType using the example in Listing 9.
In this example, you can see that a new
extendedNameType shows that the extension
is to be applied to the base type
above). Once extended, the base type will inherit the properties of the new
extended type in addition to its own definition. In this case, you intend to add
<gen> to the
nameType element defined above, which already has
as sub-elements the
Listing 9. Adding elements to nameType using xs:extension
<xs:complexType name="extendedNameType"> <xs:extension base="nameType"> <xs:sequence> <xs:element name="gen" type="xs:string"/> </xs:sequence> </xs:extension> </xs:complexType> <xs:element name="para" ref="extendedNameType"/>
<para> element defined in Listing 9
extendedNameType, which has been defined
to include all the sub-elements from its base type
as well as the extension element
<gen>. This would
allow the following instance to validate with no errors:
To better understand what is really happening during validation, the example in
Listing 10 represents what results when the extension is
resolved by the validator (this is a view of the schemas as it is processed,
sometimes called the Post Schema Validation Infoset, or PSVI). As you can
see, the parsing didn't simply insert the new declaration for the
<gen> element right after the last element
lname in the original
<xs:sequence> element. Instead, it added a
<xs:sequence> element immediately following
the original one, resulting in a sequence of sequences, and encapsulated it in an
<xs:sequence> element to preserve the
order. In fact, extensions can only be applied to the compositor
<xs:sequence>, not its counterparts
(Actually, you can extend an
compositor, but it will end up inserting an
Listing 10. Resolved extended type
<xs:complexType name="ExtendedNameType"> <xs:sequence> <xs:sequence> <xs:element name="fname" type="xs:string"/> <xs:element name="lname" type="xs:string"/> </xs:sequence> <xs:sequence> <xs:element name="gen" type="xs:string"/> </xs:sequence> </xs:sequence> </xs:complexType>
Again, the code in Listing 10 allows the following instance to validate:
You can also add attributes when you extend a type, as in Listing 11,
ParaType complex type definition is
extended to add an attribute for
Listing 11. Adding attributes to a named type using xs:extension
<xs:complexType name="ParaType"> <xs:simpleContent> <xs:extension base="xs:string"> <xs:attribute name="label" type="xs:string" use="required"/> </xs:extension> </xs:simpleContent> </xs:complexType> <xs:element name="para" type="ParaType"/>
The extension example in Listing 11 allows the following text instance to validate successfully:
<para label="abc">Paragraph string text.</para>
Redefining existing types
Types defined in one schema can be reused and redefined in another schema module.
This behavior can be handy if you inherit a schema but want to modify the
definition somewhat to work better in your environment. Suppose you're given an
industry-standard schema that defines a
yearType simple type, as in Listing 12.
Listing 12. Schema module that defines a simple type named yearType
<xsd:schema xmlns:xsd="http://www.w3.org/2001/XMLSchema"> <xsd:simpleType name="yearType"> <xsd:restriction base="xsd:string"/> </xsd:simpleType> <xsd:element name="year" type="yearType"/> </xsd:schema>
yearType in Listing 12 is
defined strictly as an
<xs:string>, which might
not be rigorous enough for your local environment. Perhaps in the broader world,
you might find years that have two or four digits and might even contain an apostrophe,
as in '09. But in your internal environment, you might want to force the year
always to be four numerical digits to be as unambiguous as possible.
Consider a separate schema module that calls the original schema through the
schemaLocation= attribute and redefines it with the
code in Listing 13.
Listing 13. Schema module that redefines the simple type named yearType
<xsd:schema xmlns:xsd="http://www.w3.org/2001/XMLSchema"> <xsd:redefine schemaLocation="prod1.xsd"> <xsd:simpleType name="yearType"> <xsd:restriction base="yearType"> <xsd:length value="4"/> </xsd:restriction> </xsd:simpleType> </xsd:redefine> <xsd:element name="fullYear" type="yearType"/> </xsd:schema>
yearType has been redefined, it is still referred
to using the same name, but when applied to the
element, it ensures that the
fullYear will always contain
Note that redefines require that the old and new definitions have the same original type
as their base data type. In the example in Listing 13, it is defined
The W3C XML Schema allows you to declare some elements using wildcards, or elements
that can contain just about any other element or attribute—declared or
otherwise. The wildcard
ANY type is a placeholder whose
content might or might not be validated against a schema. Validation is controlled by
processContents attribute to one of the
- Skip: Do not validate contents.
- Lax: Validate only if you can find a corresponding declaration.
- Strict: Validate against the current schema.
You define wildcards using the
<xs:anyAttribute> element for elements or attributes,
respectively. The example in Listing 14 shows an element named
<HTMLExample> that has a wildcard where subordinate
elements or attributes would be declared. In other words, the
<HTMLExample> element can contain any other
elements as long as they have well-formed markup. You can add the XHTML Schema
and allow the elements to parse against it, and then set the
lax to check that it is valid HTML markup. But
this example doesn't bother checking the wildcard elements, so leave it set to
Listing 14. Element declaration using xs:any and xs:anyAttribute wildcards
<xs:element name="HTMLExample"> <xs:complexType mixed="true"> <xs:sequence> <xs:any minOccurs="0" maxOccurs="unbounded" processContents="skip"/> </xs:sequence> <xs:anyAttribute processContents="skip"/> </xs:complexType> </xs:element>
Specifically, the example in Listing 14 shows the element declaration to contain a complex type that contains a sequence. The sequence contains the
<any> element, which is the wildcard placeholder indicating that any element markup can appear at this location. It also
<xs:anyAttribute> declaration, which
allows the addition of any well-formed attributes to the element markup. Because the
processing of the contents of this element should be skipped, no schema validation
is performed on the contents between the start and end tags. Therefore, the entire
element can contain any well-formed markup using any element or attribute names.
In this way, you can add content from other document models inside this element,
thus extending the types of elements allowed overall in the document—albeit
in a specific location. The data instance in Listing 15 is
valid given this example.
Listing 15. Valid HTMLExample data instance
<HTMLExample href="http://www.w3.org"> <tr> <th align="left">Table Head</th> </tr> </HTMLExample>
In the example in Listing 15, the element type
<HTMLExample> is validated normally
against the schema, but the contents of that element, the table row, and the table
head HTML elements are skipped.
Take care when you use wildcards if you expect to require validation or allow
lax validation if a schema can be found. Resources, such as alternative schemas to
validate against, must be made available to the processor. Namespaces must be
managed correctly. Also, using wildcards in conjunction with optional or repeatable
elements can cause ambiguities and non-deterministic conditions. The simplest use
of wildcards with
processContents="skip" will allow you
to avoid most of this complexity.
As you can see from the examples in this article, the designers of the W3C XML Schema language had extensibility in mind when they created the standard. Take care to observe the rules for each extension type in order for them to work. These powerful techniques, although only working in a single namespace, can allow tremendous flexibility—especially when you work with schemas used in distributed and diverse environments.
Looking forward, keep an eye on the emerging XML Schema version 1.1 standard being produced in the W3C XML Schema Working Group. It has some interesting changes to the wildcards and other constructs that might affect the examples shown here.
- Definitive XML Scheema (Prentice Hall, 2001): Check out Priscilla Walmsley's definitive book on the W3C's XML Schema.
- W3Schools XML Schema Tutorial:Take the W3C tutorial for the XML Schema.
- W3C XML Schema Working Group: Learn more about the doings of the W3C XML Schema Working Group.
- Design XML schemas for enterprise data (Bilal Siddiqui, developerWorks, October 2006): Take Bilal Siddiqui's tutorial on designing XML schemas.
- Compound XML document profiles for rich content, Part 1: Exploring extensibility alternatives using XML Schema (Steve Speicher and Kevin E. Kelly, developerWorks, September 2005): Learn how to build compound XML Schema profiles from core specification schemas.
- Real-world XML Schema (Paul Golick and Richard Mader, developerWorks, January 2002): Discover a set of 17 broadly applicable practices for using XML.
- XML Schema 1.1: An introduction to XML Schema 1.1 (Neil Delima, Sandy Gao, Michael Glavassevich, and Khaled Noaman, developerWorks, December 2008): Explore the improvements and new capabilities being introduced with this version of the standard in this series of articles.
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