10. XML Schema (XSD)

What you will learn

• What an XML Schema is, and why it replaced DTDs
• How to write a simple schema
• Simple and complex types, sequences, and attributes
• How to connect a document to its schema

What is an XML Schema?

XML Schema (often written XSD, for XML Schema Definition) is a language for describing the structure and content of XML documents. It does the job of a DTD, but better:

• A schema is itself written in XML.
• It has real data types: numbers, dates, true/false, and more.
• It supports namespaces.
• It can limit values, such as “a whole number from 1 to 100”.

A document and its schema

<?xml version="1.0" encoding="UTF-8"?>
<note>
  <to>Ali</to>
  <from>Sam</from>
  <message>See you at noon.</message>
</note>

<?xml version="1.0" encoding="UTF-8"?>
<xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema">

  <xs:element name="note">
    <xs:complexType>
      <xs:sequence>
        <xs:element name="to" type="xs:string"/>
        <xs:element name="from" type="xs:string"/>
        <xs:element name="message" type="xs:string"/>
      </xs:sequence>
    </xs:complexType>
  </xs:element>

</xs:schema>

The schema uses the namespace you met earlier: http://www.w3.org/2001/XMLSchema, with the prefix xs (some authors use xsd). The prefix is only a shortcut, as you learned in the Namespaces lesson.

Simple and complex elements

A simple element is declared with a type attribute:

<xs:element name="age" type="xs:integer"/>

Controlling the arrangement

Inside a complex type, three indicators decide how children are arranged.

How many times?

Use minOccurs and maxOccurs. The default for both is 1.

<?xml version="1.0" encoding="UTF-8"?>
<xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema">

  <xs:element name="library">
    <xs:complexType>
      <xs:sequence>
        <xs:element name="book" maxOccurs="unbounded">
          <xs:complexType>
            <xs:sequence>
              <xs:element name="title" type="xs:string"/>
              <xs:element name="subtitle" type="xs:string" minOccurs="0"/>
              <xs:element name="author" type="xs:string" maxOccurs="5"/>
              <xs:element name="price" type="xs:decimal"/>
            </xs:sequence>
            <xs:attribute name="id" type="xs:ID" use="required"/>
          </xs:complexType>
        </xs:element>
      </xs:sequence>
    </xs:complexType>
  </xs:element>

</xs:schema>

Attributes

Attributes are declared inside the complex type, after the child elements. Use use="required" or use="optional" (the default), and default for a fallback.

<xs:attribute name="language" type="xs:language" default="en"/>

An element with text and an attribute

A price with a currency attribute needs an extension of a simple type.

<xs:element name="price">
  <xs:complexType>
    <xs:simpleContent>
      <xs:extension base="xs:decimal">
        <xs:attribute name="currency" type="xs:string" use="required"/>
      </xs:extension>
    </xs:simpleContent>
  </xs:complexType>
</xs:element>

Linking a document to its schema

A document can point to its schema with attributes from the XML Schema instance namespace:

<?xml version="1.0" encoding="UTF-8"?>
<note xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
      xsi:noNamespaceSchemaLocation="note.xsd">
  <to>Ali</to>
  <from>Sam</from>
  <message>See you at noon.</message>
</note>

This is a hint, and many tools let you give the schema separately instead, which is safer when the XML comes from someone else. If your schema uses a targetNamespace, link it with xsi:schemaLocation instead. That advanced case is beyond this lesson.

Validating against a schema

As with a DTD, browsers do not validate. Use an editor extension, xmllint, or code.

xmllint --noout --schema note.xsd note.xml

<?php
libxml_use_internal_errors(true);

$doc = new DOMDocument();
$doc->load(__DIR__ . "/note.xml");

if ($doc->schemaValidate(__DIR__ . "/note.xsd")) {
    echo "The document is valid.";
} else {
    foreach (libxml_get_errors() as $error) {
        echo "Line " . $error->line . ": " . htmlspecialchars(trim($error->message), ENT_QUOTES, "UTF-8") . "<br>";
    }
    libxml_clear_errors();
}

If you remove the from element and run it again, the loop prints messages saying an element is missing or unexpected, with a line number. The exact words depend on your library version.

Why use schemas?

• They act as a contract between systems that exchange data.
• They catch bad data before your program uses it.
• Tools can use them to give auto-complete hints in editors, and to generate code.
• They document the format clearly.

Remember, a schema makes data predictable, not safe. Still treat outside XML as untrusted, and avoid loading external resources from it.

Common mistakes

• Forgetting the xs: prefix or its namespace declaration.
• Declaring attributes before the child elements, or outside the complex type.
• Using xs:sequence when order should not matter (use xs:all).
• Expecting a browser to validate the file.
• Mismatched element names between the schema and the document, since XML is case sensitive.

<?xml version="1.0" encoding="UTF-8"?>
<xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema">

  <xs:element name="student">
    <xs:complexType>
      <xs:sequence>
        <xs:element name="name" type="xs:string"/>
        <xs:element name="score" type="xs:integer"/>
      </xs:sequence>
      <xs:attribute name="id" type="xs:string" use="required"/>
    </xs:complexType>
  </xs:element>

</xs:schema>

Summary

• An XSD is an XML file that describes allowed structure, with simple and complex types.
• Use xs:sequence, xs:choice, and xs:all for arrangement, and minOccurs and maxOccurs for counts.
• Validate with a tool or code, because browsers do not.