Document type definition

Document Type Definition

A Document Type Definition (DTD) is a set of rules that defines the structure and content of an XML document. It serves as a blueprint for creating valid XML documents by specifying the allowed elements, attributes, and their relationships.

Overview

DTDs were introduced as a means to define the structure of XML documents before the advent of more advanced schema languages like XML Schema and Relax NG. Despite being less expressive than these newer alternatives, DTDs are still widely used due to their simplicity and compatibility with older systems.

Syntax

DTDs are written in a specific syntax that follows a set of rules. The basic structure of a DTD consists of declarations for elements, attributes, entities, and notations. These declarations define the rules and constraints that an XML document must adhere to.

Elements

Elements in a DTD are defined using the `<!ELEMENT>` declaration. This declaration specifies the name of the element, its content model, and any additional constraints. The content model can be defined as either empty, containing only text, or containing other elements.

Attributes

Attributes in a DTD are defined using the `<!ATTLIST>` declaration. This declaration specifies the element to which the attribute belongs, the name of the attribute, its data type, and any additional constraints. Attributes can be required, optional, or have default values.

Entities

Entities in a DTD are used to define reusable pieces of content. There are two types of entities: internal and external. Internal entities are defined within the DTD itself, while external entities are defined in separate files and referenced in the DTD.

Notations

Notations in a DTD are used to define the format of non-XML data within an XML document. They are typically used for multimedia content such as images, audio, or video. Notations are declared using the `<!NOTATION>` declaration.

Usage

To use a DTD in an XML document, the document must reference the DTD using the `<!DOCTYPE>` declaration. This declaration specifies the location of the DTD file or provides the DTD directly within the document.

DTDs can also be used to validate XML documents against their defined rules. Various tools and libraries exist that can parse an XML document and check its conformance to the specified DTD.

Advantages and Disadvantages

DTDs have several advantages and disadvantages compared to other schema languages:

Advantages

- DTDs are simple and easy to understand, making them accessible to beginners. - DTDs have wide support and compatibility with older systems. - DTDs can be used to validate XML documents efficiently.

Disadvantages

- DTDs lack some of the advanced features provided by newer schema languages. - DTDs have limited support for data types and constraints. - DTDs can be verbose and difficult to maintain for complex document structures.

Conclusion

Despite their limitations, Document Type Definitions (DTDs) continue to be used in various applications for defining the structure and content of XML documents. While newer schema languages offer more advanced features, DTDs remain a viable option for simpler projects or when compatibility with older systems is required.

See Also

• XML
• XML Schema
• Relax NG