XML Basics — Study Notes

1. Introduction

XML, which stands for eXtensible Markup Language, is a markup language designed to store and transport data in a structured and readable format. Unlike HTML, which is used to display data in a browser, XML is used to describe and carry data so that different systems can exchange information with each other. The most important feature of XML is that it is extensible, meaning users can define their own tags according to the needs of their data, rather than being limited to a fixed set of predefined tags.

XML was developed by the World Wide Web Consortium (W3C) as a simpler and more flexible descendant of SGML. It quickly became a standard for data exchange between applications, databases, and web services. For example, when an online shopping website needs to send order information to a bank, XML is often the format used to structure that order data. Because XML separates the data from its presentation, the same XML document can be displayed in many different ways using XSLT or processed by programs in many languages.

In this chapter, we will study the structure of an XML document, the rules for writing well-formed XML, the concept of elements and attributes, and the differences between HTML and XML. We will also learn about DTD and other advanced concepts. By the end of this chapter, students will be able to create simple XML documents that follow all the rules of well-formedness and understand why XML is so important in modern data exchange.

2. Features of XML

XML has several distinguishing features that make it ideal for data storage and transfer.

• Extensible: Users can create their own tags and structure, so XML can represent data of any domain such as education, banking, or healthcare.
• Self-descriptive: The tags describe the meaning of the data. For example, <name>Riya</name> clearly states that "Riya" is a name.
• Strict rules: XML documents must be well-formed. This strictness makes XML machine-readable and reliable.
• Platform independent: XML is plain text, so it can be read by any operating system and any programming language.
• Supports Unicode: XML supports all languages of the world because it is based on Unicode.
• Separates data from presentation: The same XML data can be rendered differently by different stylesheets or applications.

These features make XML the backbone of many technologies including RSS feeds, web services using SOAP, configuration files, and data interchange between applications.

3. XML Syntax and Rules

XML documents must follow certain rules to be considered well-formed. A well-formed XML document obeys all the syntax rules of XML, and only such documents can be parsed by applications.

Basic rules of a well-formed XML document

1. XML documents must have a root element that contains all other elements. There can be only one root element.
2. Every start tag must have a matching end tag. For example, <name> must be closed by </name>.
3. Tags must be properly nested. <a><b></b></a> is correct, but <a><b></a></b> is wrong.
4. Attribute values must always be enclosed in quotation marks, either single or double.
5. XML is case-sensitive. <name> and <Name> are two different tags.
6. Tags cannot contain spaces or begin with a number. Comments in XML are written as <!-- comment -->.
7. Empty elements can be written in the shorthand form, such as <br/> or <img/>.

Example of a well-formed XML document

<?xml version="1.0" encoding="UTF-8"?>
<school>
    <student roll="1">
        <name>Riya Sharma</name>
        <class>X</class>
        <marks>92</marks>
    </student>
    <student roll="2">
        <name>Arjun Verma</name>
        <class>X</class>
        <marks>88</marks>
    </student>
</school>

In this document, school is the root element, student is a child element, and roll is an attribute of the student element. The first line is the XML declaration which specifies the version and encoding.

4. Elements and Attributes

In XML, an element is the basic building block of a document. An element consists of a start tag, its content, and an end tag. Elements can contain text, other elements (called child elements), or both. The element that contains other elements is called the parent element, and the contained elements are called child elements. Elements at the same level are called siblings.

Attributes provide additional information about an element. They are written inside the start tag as name-value pairs, for example <student roll="1">. An element can have any number of attributes, but no two attributes in the same tag can have the same name, and each attribute can appear only once in a tag. The choice between using an element or an attribute depends on the nature of the data; data that is part of the content is best kept in an element, while data that describes the content is best kept in an attribute.

5. Differences between HTML and XML

Although HTML and XML are both markup languages, they serve very different purposes.

• Purpose: HTML displays data, while XML stores and transports data.
• Tags: HTML has predefined tags such as <P> and <TABLE>, while XML has no predefined tags and allows users to create their own.
• Case sensitivity: HTML tags are not case-sensitive, but XML tags are case-sensitive.
• Closing tags: In HTML, some tags such as <BR> need not be closed, but in XML every tag must be closed.
• Strictness: HTML browsers tolerate errors and display the page, while XML parsers reject documents that are not well-formed.
• Whitespace: XML preserves whitespace, whereas HTML typically collapses multiple spaces into one.

These differences are frequently tested in examinations, so they should be memorised carefully.

6. XML Technologies

XML is supported by a family of related technologies.

• DTD (Document Type Definition): DTD defines the structure and legal elements and attributes of an XML document. It ensures that the document follows a valid structure.
• XSL (eXtensible Stylesheet Language): XSL is used to transform and present XML data. XSLT is the transformation part of XSL that converts an XML document into HTML or another format.
• XML Schema: A more powerful and modern alternative to DTD that also defines the structure, data types, and constraints of XML data.
• XPath: A language used to navigate through elements and attributes in an XML document.
• RSS (Really Simple Syndication): An XML-based format used to publish frequently updated content such as news headlines.

Web services such as SOAP and REST exchange data in XML format, making XML an essential technology for modern web development.

7. Advantages and Limitations of XML

Advantages

• It is both human-readable and machine-readable.
• It is platform and language independent.
• It supports international languages through Unicode.
• It is flexible and extensible for any type of data.
• It separates content from presentation, allowing flexible rendering.

Limitations

• XML files are larger than equivalent binary or JSON files because of repetitive tags.
• XML is more verbose and requires careful writing.
• Parsing XML can be slower and requires more processing power than simpler formats.
• XML does not by itself display anything in the browser; it needs XSLT or a program to present the data.

Quick Revision Tables

Mind Map

Important Diagrams (SVG)

Diagram 1: Structure of an XML Document Tree

Diagram 2: Well-Formed vs Ill-Formed XML

Common Mistakes

1. Creating more than one root element in a document. XML permits only a single root element.
2. Forgetting to close a tag, especially self-closing elements that must be written as <br/>.
3. Using mixed case inconsistently. Since XML is case-sensitive, <Name> and <name> are treated as different elements.
4. Writing attribute values without quotation marks, such as <student roll=1> instead of <student roll="1">.
5. Misplacing the XML declaration. It must be the very first line of the document.
6. Thinking XML is meant to display data like HTML. XML stores data and requires XSLT or a program to display it.
7. Starting a tag name with a number, which is not allowed in XML.

Exam Tips

1. Memorise the differences between HTML and XML in a table, as direct questions are common.
2. Be able to write a simple well-formed XML document with a root element, two child elements, and at least one attribute.
3. Remember that attribute values must be quoted and tags must be case-sensitive and properly nested.
4. Learn the full form of XML and know that it is a W3C recommendation.
5. Practise identifying well-formed versus ill-formed XML snippets, a frequent two-mark question.
6. Remember that DTD defines the valid structure of an XML document and XSLT transforms XML into another format.

Conclusion

XML is a powerful, extensible markup language designed for storing and transporting structured data. We learned that XML documents must be well-formed, meaning they must have exactly one root element, properly nested and matched tags, quoted attribute values, and case-sensitive naming. Elements and attributes provide the structure and description of the data, and technologies such as DTD, XSLT, and XPath extend the power of XML. Although HTML and XML both use tags, HTML displays data while XML carries it. This ability to exchange structured data makes XML the foundation of web services, RSS feeds, and countless business applications.