DTD

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DTD (Document Type Definition )is mostly used in XML for validating an xml document with the help of some set of rules. DTD defines these set of rules i.e. defines the legal building blocks of an XML document and defines document structure with a list of legal elements and attributes. DTD also defines that an xml can contain what elements and in which order, how the elements should be organized inside the other. It specifies that an element can contain what attributes.

A DTD can be defined inside an XML document, or an external reference can be declared.

• Internal DTD: This type of DTD can be included in the target xml file.
• External DTD: External dtd is nothing different from internal dtd except defining definition in an external file. External DTD can be use with more than one XML document.
  Rules are given below-
  1.The document type declaration must be written in between the XML declaration and the root element.
  2.Keyword DOCTYPE must be followed by the root element.
  3.keyword DOCTYPE must be in upper case.

DTD is very helpful in XML to make description of its own format. Through  DTD, independent groups of people can agree to use a standard DTD for interchanging data. If you have used DTD in your application then you can verify that the data you receive from the outside world is valid.You can also use a DTD to verify your own data.

Difference between DTD and XML Schema-    

• DTD can have only two types of data, the CDATA and the PCDATA. But in a schema you can use all the primitive data type that you use in the programming language and you have the flexibility of defining your own custom data types.
• Another difference between DTD and XML Schema, is namespace awareness, XML Schema has, while DTD is not.
• XML Schema has ability to implement strong typing.
• XML Schema has a wealth of derived and built-in data types to validate content.
• DTD has the ability to define DTDs inline, which XML Schema lacks
• DTD is good when working with small files, as it allows you to contain both the content and the schema within the same document.

DTDs are easier to code and validate than an XML schema. However, the advantages to using an DTD are shown in the following list:

• Its support is ubiquitous due to its inclusion in the XML 1.0 standard.
• DTD define a document type rather than the types used by a namespace, thus grouping all constraints for a document in a single collection.
• DTD allow the declaration of standard public entity sets for publishing characters.
• DTD are terse compared to element-based schema languages and consequently present more information in a single screen.

• DTD have no explicit support for newer features of XML, most importantly namespaces.
• DTD use a syntax based on regular expression syntax, inherited from SGML, to describe the schema.