If you’ve ever used or been exposed to XML, you’ve probably heard of DTDs and XSDs (Document Type Definitions and XML Schema Definitions, respectively). These two items can confuse new users, so we want to take some time and explain the differences between them and how they are used in an XML document.
There are two main ways to define the structure of an XML document: DTD and XSD. DTD, or Document Type Definition, is the older of the two methods. It is a language for defining the rules of an XML document. A DTD can be used to define the allowed elements, attributes, and relationships between them.
XSD, or XML Schema Definition, is a more modern way to define the structure of an XML document. It uses XML syntax to define the rules of an XML document. An XSD can be used to define the allowed elements, attributes, and data types.
Both DTD and XSD have their advantages and disadvantages. DTD is easier to learn and use but less flexible than XSD. XSD is more flexible but can be more difficult to learn and use.
What is DTD (Document Type Definition)?
A document type definition is a set of markup declarations that define a document type for an SGML-family markup language (SGML, XML, HTML). A DTD can be declared inline within an XML document or as an external reference. It defines the elements, attributes, and content models which compose a given class of documents.
It also may include non-document elements such as character entities. On the other hand, the XML Schema Definition file does not deal with document content but rather with data types; it describes what data items are allowed and how they should be used in a particular application.
What is XSD (XML Schema Definition)?
XML Schema is a language for describing XML documents. It lets you specify rules for how an XML document should look. For example, you can use XML Schema to define what elements and attributes can appear in an XML document, in what order they should appear, and what data types they should have.
If you are using more than one schema (XML file) within your project, then it is vital that all of the schemas are at least semantically compatible with each other. When multiple schemas are used together, we call them composite or complex schemas. In this case, it is also essential that every schema from which your composite schema derives has already been validated against itself and against its parent schemas (if any).
Differences Between DTD and XSD
DTDs are used to define the structure of an XML document, while XSDS are used to define the data that can be contained within an XML document. In other words, a DTD defines the rules of an XML document, while an XSD defines what data can go into an XML document.
1. General Background: DTD vs. XSD
Regarding XML, there are two ways to define the rules for a valid document: DTD and XSD. DTD is short for Document Type Definition, while XSD is the XML Schema Definition. They both serve the same purpose, but there are some key differences between them. For example, with DTDs, you can’t specify what order different tags should appear in the document.
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A common misconception is that DTDs are deprecated or obsolete in favor of XSDs, which couldn’t be further from the truth. It all depends on your needs – if you want to specify how an element should look before being saved as an XML file, you’ll need an XSD; if not, then a DTD will do just fine.
2. Definition: DTD vs. XSD
DTD defines the structure and syntax of an XML document, while XSD defines the semantics of an XML document. In other words, DTD tells you what you can put in your XML document, while XSD tells you what that stuff means.
For example, a name tag might include a given-name field to identify the person’s first name and a family-name field to identify their last name. With this information in hand, someone could search for all people with the last name Jones.
3. XML Specifications: DTD vs. XSD
XML is a metalanguage to design markup languages, i.e.; it provides a framework to define new markup languages. The two most common ways of doing this are via Document Type Definitions (DTDs) and XML Schemas (XSDs). Both have advantages and disadvantages, which can be summarized as follows:
- DTDs are easier to learn and use, but they are less flexible than XSDs. They also allow for more errors in documents that conform to them.
- XSDs are challenging to learn and use, but they are much more flexible than DTDs because they allow you to specify what should go where.
- They also help you avoid errors by checking the validity of documents against them.
4. In Action: DTD vs. XSD
DTD defines the structure and legal elements of an XML document, while XSD defines the rules for how an XML document can be structured. DTD is more flexible than XSD, but XSD is more widely used. A web browser will use a DTD to parse and display an XML document.
A web browser will use an XSD to parse and display a page built in HTML5 or another markup language that uses the same types of tags as HTML5 (such as SVG).
Other formats, such as RELAX NG, are similar to DTDs and XSDs in some ways.
5. Overlay Schemas and Modularization: DTD vs. XSD
An XML schema is a description of a type of XML document, typically expressed in terms of constraints on the structure and content of documents of that type, above and beyond the primary syntactical constraints imposed by XML itself. These constraints are generally expressed using grammatical rules governing the order of elements, attribute values, nesting, inclusion, and so on.
Key Differences Between DTD and XSD
- DTD stands for Document Type Definition. A document type definition is a set of rules that define how XML documents should be formatted. In short, it defines what tags make up a valid XML document.
- XSD stands for XML Schema Definition. An XML schema definition is a description of the structure of an XML document. It describes the elements that make up the document and its attributes.
- DTD and XSD standards describe how XML documents should be structured. However, they do not specify how those documents should look. Instead, they simply tell software programs how to read them.
- DTD was developed before XML was created. It was designed to work with SGML, a predecessor to XML. Because of its limitations, XML was created to replace DTD.
- XSD was created after XML was created. It is a standard that specifies how XML documents should be written.
In short, a DTD defines the structure of an XML document, while an XSD defines the rules for how an XML document can be structured. An XML document can be validated against a DTD or an XSD (or both), but only an XSD can be used to generate an XML document. An XSD may contain references to other schemas. These are called Schema Imports, allowing one schema to reuse the definitions in another.