DTD - Introduction

What is a DTD?

A DTD (Document Type Definition) is a markup language that defines the structure, elements, and rules for an XML document. It specifies the allowed elements, attributes, entity references, and their relationships within the document. DTDs provide a way to enforce consistency and validity of XML documents by defining a set of rules that the document must adhere to.

Why use a DTD?

There are several reasons to use a DTD when working with XML documents:

Structure definition: A DTD allows you to define the structure of an XML document, including the elements, attributes, and their relationships. It provides a blueprint for creating valid XML documents.

Validation: By using a DTD, you can ensure that XML documents adhere to a specific set of rules. It allows you to validate the structure and content of the document, helping to catch errors and inconsistencies.

Interoperability: DTDs provide a standardized way to define document structures, making it easier to exchange and process XML documents across different systems and platforms.

An Internal DTD Declaration:

An internal DTD declaration is placed directly within the XML document itself. It is enclosed within the <!DOCTYPE> tag at the beginning of the XML document. Here's an example of an internal DTD declaration:

<!DOCTYPE rootElement [
    <!-- DTD content goes here -->

The DTD content is placed within square brackets after the DOCTYPE declaration. Inside the DTD, you define the elements, attributes, and entity references for your XML document.

An External DTD Declaration:

An external DTD declaration refers to a separate DTD file that is linked to the XML document. It allows you to define the DTD rules in a separate file and reference it in multiple XML documents. Here's an example of an external DTD declaration:

<!DOCTYPE rootElement SYSTEM "dtdFile.dtd">

In this example, "rootElement" is the root element of the XML document, and "dtdFile.dtd" is the file that contains the DTD definition. The SYSTEM keyword indicates that the DTD file is located on the system.

Using an external DTD declaration enables reusability, as you can define the DTD once and reference it from multiple XML documents. It also helps keep the XML document clean and focused on content rather than DTD definitions.