Understanding Well-Formed XML Documents:

A well-formed XML document adheres to the syntax rules defined by the XML specification.

To create a proper growing structure of your XML file there are certain very important points to follow and they are all framed under the heading of XML document well-formedness.

Key Rules for Well-Formed XML:

• Single Root Element: The document must have exactly one root element.
• Proper Tagging: All elements must have a start tag and a corresponding end tag.
• Case Sensitivity: XML tags have a particular way in which case they should be written; <Title> and <title> are two different tags.
• Proper Nesting: Tags must be nested correctly without overlapping.
• Attribute Quotation: Attribute values must be enclosed in quotes, either single (') or double (").

Example of a Well-Formed XML Document:

                    
<?xml version="1.0" encoding="UTF-8"?>
<bookstore>
  <book>
    <title>XML Fundamentals</title>
    <author>John Smith</author>
    <price>29.99</price>
  </book>
</bookstore>
                    
                  

It contains one element, which is named <bookstore>, all of the tags are properly constructed and closed, and all attributes are surrounded by quotes.

Ensuring XML Documents Are Well-Formed

To ensure an XML document is well-formed, follow these steps:

Check for a Single Root Element:

                    
<!-- Incorrect: Multiple root elements -->
<bookstore></bookstore>
<library></library>

<!-- Correct: Single root element -->
<catalog>
  <bookstore></bookstore>
  <library></library>
</catalog>
                    
                  

Verify Proper Tagging

                    
<!-- Incorrect: Missing end tag -->
<title>XML Fundamentals

<!-- Correct: Properly closed tag -->
<title>XML Fundamentals</title>
                    
                  

Ensure Proper Nesting

                    
<!-- Incorrect: Overlapping tags -->
<book>
  <title>XML Basics
  <author>John Doe</title>
</author>

<!-- Correct: Proper nesting -->
<book>
  <title>XML Basics</title>
  <author>John Doe</author>
</book>

                    
                  

Attribute Quotation

                    
<!-- Incorrect: Unquoted attribute value -->
<book id=123>

<!-- Correct: Quoted attribute value -->
<book id="123">
                    
                  

Valid XML and Schema Validation:

Valid XML goes beyond being well-formed.It has to have a structure and includes specific fields and data types, only allowed by the schema. Schemas ensure that an XML document is built using a recommended format for designing its structure.

Types of Schemas:

• DTD (Document Type Definition): An older schema language used to define the structure and allowable content of an XML document.
• XML Schema (XSD): A more powerful and flexible schema language that supports data types, namespaces, and complex structures.

Example of an XML Schema (XSD):

                    
<!-- books.xsd -->
<xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema">
  <xs:element name="bookstore">
    <xs:complexType>
      <xs:sequence>
        <xs:element name="book" maxOccurs="unbounded">
          <xs:complexType>
            <xs:sequence>
              <xs:element name="title" type="xs:string"/>
              <xs:element name="author" type="xs:string"/>
              <xs:element name="price" type="xs:decimal"/>
            </xs:sequence>
          <xs:attribute name="id" type="xs:string" use="required"/>
        </xs:complexType>
      </xs:element>
    </xs:sequence>
  </xs:complexType>
</xs:element>
</xs:schema>

                    
                  

Example of a Valid XML Document:

                    
<?xml version="1.0" encoding="UTF-8"?>
<bookstore xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="books.xsd">
  <book id="001">
    <title>XML Fundamentals</title>
    <author>John Smith</author>
    <price>29.99</price>
  </book>
</bookstore>

                    
                  

Steps for Schema Validation:

• Create a Schema: Define the allowed structure and data types in a DTD or XSD file.
• Reference the Schema: Xi: schemaLocation or DOCTYPE should be used to refer XML to the schema.
• Validate the Document: To validate the document, use an XML parser that supports schema validation to verify if the document complies with the schema.