A Document Type Definition (DTD) is a set of markup declarations that define a document type for an SGML-family markup language (SGML, XML, HTML). A Document Type Definition is a set of rules that defines how elements are used in an XML document. It defines the legal elements and attributes, their order and nesting, and the types of data that can be used in the document.
Types of DTD
The two types of DTD are Internal DTDs and External DTDs.
Internal DTD
An internal DTD, also known as an inline DTD, is defined inside an XML document. It is placed within the document itself, and is restricted to the document in which it is defined. An internal DTD can be used to define the structure of the document, and can be used to validate the content of the document.
Advantages
• Easy to read: The internal DTD is easy to read and understand, as it is embedded within the document.
• Easy to maintain: The internal DTD is easy to maintain, as it is defined within the document itself.
• Quick to implement: The internal DTD can be quickly implemented, as it is embedded within the document.
Disadvantages
• Limited scope: The internal DTD is limited to the document in which it is defined, and cannot be used to validate other documents.
• Redundancy: The internal DTD can be redundant, as it is defined in every document.
• Hard to debug: The internal DTD can be difficult to debug, as it is embedded within the document.
External DTD
An external DTD, also known as an external subset, is a separate file that contains the document type definition for a document. It is stored in a separate file, and can be shared across multiple documents. An external DTD can be used to define the structure of the document, and can be used to validate the content of the document.
Advantages
• Easy to maintain: The external DTD is easy to maintain, as it is stored in a separate file.
• Quick to implement: The external DTD is quick to implement, as it is stored in a separate file.
Disadvantages
• Harder to read: The external DTD is harder to read and understand, as it is stored in a separate file.
• Limited scope: The external DTD is limited to the documents that it is defined for, and cannot be used to validate other documents.
• Harder to debug: The external DTD can be difficult to debug, as it is stored in a separate file.
Conclusion
A Document Type Definition is a set of rules that defines how elements are used in an XML document. There are two types of DTDs: Internal DTDs and External DTDs. Internal DTDs are defined within the document itself, and are easy to read, easy to maintain, and quick to implement. External DTDs are stored in a separate file, and can be shared across multiple documents. They are harder to read, limited to the documents that they are defined for, and harder to debug.