Documents Type Defination (DTD) Full details with suitable example

Introduction

  • DTD (Document Type Definition) is a set of rules that define the structure, elements, and attributes of an XML or HTML document.
  • It ensures that the document follows a specific format and maintains data consistency.
  • In simple words, DTD defines what tags and attributes are allowed in a document and how they should be arranged.
  • It acts as a blueprint for how the document should be written so that it remains valid, consistent, and machine-readable.
  • It tells the XML parser what elements and attributes can appear, in what order, and what kind of data they contain.

Why DTD is Used

  1. To define a consistent structure for XML documents.

  2. To make XML documents self-descriptive and machine-verifiable.

  3. To validate XML data before sharing or storing it.

  4. To separate data content from data structure rules.

Purpose of DTD

  • To define the legal structure of an XML or HTML document.
  • To verify whether the document is well-formed and valid.
  • To describe the relationships between different data elements.
  • To ensure that data interchange between systems is consistent.

Types of DTD

There are two main types of DTD:

  1. Internal DTD

  2. External DTD

1. Internal DTD

  • The DTD is written inside the same XML document.
  • Declared within the <!DOCTYPE> tag, inside the document.

Example (Internal DTD)

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE student [
  <!ELEMENT student (name, age, course)>
  <!ELEMENT name (#PCDATA)>
  <!ELEMENT age (#PCDATA)>
  <!ELEMENT course (#PCDATA)>
]>
<student>
  <name>Lav Pratap</name>
  <age>23</age>
  <course>MCA</course>
</student>

Explanation:

  • <!DOCTYPE student [...]> defines the root element.
  • <!ELEMENT student (name, age, course)> means the student element contains three child elements: name, age, and course.
  • <!ELEMENT name (#PCDATA)> means #PCDATA represents parsed character data (normal text).

2. External DTD

  • The DTD rules are stored in a separate file (with .dtd extension).
  • The XML document then links to that file.

Example (External DTD)

File 1: student.xml

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE student SYSTEM "student.dtd">
<student>
  <name>Lav Pratap</name>
  <age>23</age>
  <course>MCA</course>
</student>

File 2: student.dtd

<!ELEMENT student (name, age, course)>
<!ELEMENT name (#PCDATA)>
<!ELEMENT age (#PCDATA)>
<!ELEMENT course (#PCDATA)>

Explanation:

  • SYSTEM "student.dtd" tells the XML parser to load structure rules from an external file named student.dtd.
  • This allows the same DTD to be reused for multiple XML files.

DTD Syntax Elements

Component Description Example
<!ELEMENT> Defines an element <!ELEMENT student (name, age, course)>
<!ATTLIST> Defines an attribute of an element <!ATTLIST student id ID #REQUIRED>
#PCDATA Text data that will be parsed by the XML parser <!ELEMENT name (#PCDATA)>
EMPTY Element that cannot contain anything <!ELEMENT image EMPTY>
ANY Element can contain any data <!ELEMENT info ANY>

Attribute Declaration in DTD

Attributes of an element are defined using the <!ATTLIST> declaration.

Example

<!ELEMENT student (name, age, course)>
<!ELEMENT name (#PCDATA)>
<!ELEMENT age (#PCDATA)>
<!ELEMENT course (#PCDATA)>
<!ATTLIST student id ID #REQUIRED>

Explanation:

  • id is an attribute of the <student> element.
  • ID specifies that it must be unique.
  • #REQUIRED means the attribute must be present.

Advantages of DTD

  1. Defines structure clearly and maintains uniformity.
  2. Validates documents to ensure correctness.
  3. External DTDs can be reused across multiple files.
  4. Useful for exchanging data between different systems.

Disadvantages of DTD

  1. Limited data types (supports only text-based validation).
  2. No namespace support.
  3. Complex for large documents.
  4. XML Schema (XSD) provides more advanced features.

Example of Complete XML with DTD

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE library [
  <!ELEMENT library (book+)>
  <!ELEMENT book (title, author, price)>
  <!ELEMENT title (#PCDATA)>
  <!ELEMENT author (#PCDATA)>
  <!ELEMENT price (#PCDATA)>
  <!ATTLIST book id ID #REQUIRED>
]>
<library>
  <book id="b1">
    <title>Introduction to XML</title>
    <author>Lav Pratap</author>
    <price>250</price>
  </book>
  <book id="b2">
    <title>HTML Basics</title>
    <author>John Smith</author>
    <price>200</price>
  </book>
</library>

Summary

Feature Description
Full Form Document Type Definition
Used In XML / HTML
Purpose Defines document structure and element rules
Types Internal DTD and External DTD
File Extension .dtd
Alternative XML Schema (XSD)


In short:

DTD defines the structure and rules of an XML or HTML document, ensuring that it is valid and well-formed.
Document Type Definition (DTD) defines the rules, elements, and attributes of an XML/HTML document.
It ensures that documents are well-structured, valid, and consistent according to a predefined model.