Document Type Definition: Definition from Answers.com

Document Type Definition (DTD) is a set of markup declarations that define a document type for SGML-family markup languages (SGML, XML, HTML). A DTD is a kind of XML schema.

DTDs use a terse formal syntax that declares precisely which elements and references may appear where in the document of the particular type, and what the elements’ contents and attributes are. DTDs also declare entities which may be used in the instance document.

XML uses a subset of SGML DTD.

As of 2009^[update] newer XML Namespace-aware schema languages (such as W3C XML Schema and ISO RELAX NG) have largely superseded DTDs. A namespace-aware version of DTDs is being developed as Part 9[1] of ISO DSDL. DTDs persist in applications which need special publishing characters such as the XML and HTML Character Entity References, which were derived from the larger sets defined as part of the ISO SGML standard effort.

1 Associating DTDs with documents
- 1.1 Example
2 Markup Declarations
3 XML DTDs and schema validation
4 XML DTD Example
5 See also
6 References
7 External links

Associating DTDs with documents

A Document Type Declaration associates a DTD with an XML document. Document Type Declarations appear in the syntactic fragment doctypedecl near the start of an XML document.^[1] The declaration establishes that the document is an instance of the type defined by the referenced DTD.

DTDs make two sorts of declaration:

an internal subset
an external subset

The declarations in the internal subset form part of the Document Type Declaration in the document itself. The declarations in the external subset are located in a separate text file. The external subset may be referenced via a public identifier and/or a system identifier. Programs for reading documents may not be required to read the external subset.

Example

The following example of a Document Type Declaration contains both public and system identifiers:

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN"
"http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">

All HTML 4.01 documents are expected^{[by whom?]} to conform to one of three SGML DTDs. The public identifiers of these DTDs are constant and are as follows:

The system identifiers of these DTDs, if present in the Document Type Declaration, will be URI references. System identifiers can vary, but are expected^{[by whom?]} to point to a specific set of declarations in a resolvable location. SGML allows for public identifiers to be mapped to system identifiers in catalogs that are optionally made available to the URI resolvers used by document parsing software.

Markup Declarations

DTDs describe the structure of a class of documents via element and attribute-list declarations. Element declarations name the allowable set of elements within the document, and specify whether and how declared elements and runs of character data may be contained within each element. Attribute-list declarations name the allowable set of attributes for each declared element, including the type of each attribute value, if not an explicit set of valid value(s).

DTD markup declarations declare which element types, attribute lists, entities and notations are allowed in the structure of the corresponding class of XML documents.^[2]

Element Type Declarations

An Element Type Declaration defines an element and its possible content. A valid XML document contains only elements that are defined in the DTD.

Various key-words and characters specify an element’s content:

EMPTY for no content
ANY for any content
, for orders
| for alternatives ("either...or")
( ) for groups
* for any number (zero or more)
+ for at least once (one or more)
? mark for optional (zero or one)
If there is no *, + or ?, the element must occur exactly one time

Examples:

<!ELEMENT html (head, body)>
<!ELEMENT p (#PCDATA | p | ul | dl | table | h1|h2|h3)*>

Attribute List Declarations

An Attribute List specifies the name, data type and default value of each attribute associated with a given element type,^[3] for example:

<!ATTLIST img 
   id     ID       #IMPLIED
   src    CDATA    #REQUIRED
>

There are the following attribute types:

CDATA (Character set of data)
ID
IDREF and IDREFS
NMTOKEN and NMTOKENS
ENTITY and ENTITIES
NOTATION and NOTATIONS
Listings and NOTATION-listings

A default value can define whether an attribute must occur (#REQUIRED) or not (#IMPLIED), whether it has a fixed value (#FIXED), and which value should be used as a default value ("…") in case the given attribute is left out in an XML tag.

Entity Declarations

Entities are variables used to define abbreviations; a typical use is user-readable names for special characters.^[4] Thus, entities help to avoid repetition and make editing easier. In general, there are basically two different types:

Internal (Parsed) Entities define entity references in order to replace certain strings by a replacement text. The content of the entity is given in the declaration.
External (Parsed) Entities refer to external storage objects.

Notation Declarations

Notations read the file format of unparsed external documents in order to include non-XML data in a XML document. For example a GIF image:

<!NOTATION GIF system "image/gif">

XML DTDs and schema validation

The XML DTD syntax is one of several XML schema languages.

A common misconception holds that non-validating XML parsers do not have to read DTDs, when in fact, the DTD must still be scanned for correct syntax as well as for declarations of entities and default attributes. A non-validating parser may, however, elect not to read external entities, including the external subset of the DTD. If the XML document depends on declarations found only in external entities, it should assert standalone="no" in its XML declaration. Identification of the validating DTD may be performed by the use of XML Catalogs.

<!DOCTYPE html PUBLIC
  "-//W3C//DTD XHTML 1.0 Transitional//EN"
  "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE people_list SYSTEM "example.dtd">
<people_list>
  <person>
    <name>Njandu Varuthude</name>
    <birthdate>04/02/1977</birthdate>
    <gender>Male</gender>
  </person>
</people_list>

XML DTD Example

An example of a very simple XML DTD to describe a list of persons might consist of:

<!ELEMENT people_list (person*)>
<!ELEMENT person (name, birthdate?, gender?, socialsecuritynumber?)>
<!ELEMENT name (#PCDATA)>
<!ELEMENT birthdate (#PCDATA)>
<!ELEMENT gender (#PCDATA)>
<!ELEMENT socialsecuritynumber (#PCDATA)>

Taking this line by line:

people_list is a valid element name, and an instance of such an element contains any number of person elements. The * denotes there can be 0 or more person elements within the people_list element.
person is a valid element name, and an instance of such an element contains one element named name, followed by one named birthdate (optional), then gender (also optional) and socialsecuritynumber (also optional). The ? indicates that an element is optional. The reference to the name element name has no ?, so a person element must contain a name element.
name is a valid element name, and an instance of such an element contains "parsed character data" (#PCDATA).
birthdate is a valid element name, and an instance of such an element contains parsed character data.
gender is a valid element name, and an instance of such an element contains parsed character data.
socialsecuritynumber is a valid element name, and an instance of such an element contains parsed character data.

An example of an XML file which makes use of and conforms to this DTD follows. It assumes that we can identify the DTD with the relative URI reference "example.dtd"; the "people_list" after "!DOCTYPE" tells us that the root tags, or the first element defined in the DTD, is called "people_list":

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE people_list SYSTEM "example.dtd">
<people_list>
  <person>
    <name>Fred Bloggs</name>
    <birthdate>27/11/2008</birthdate>
    <gender>Male</gender>
  </person>
</people_list>

One can render this in an XML-enabled browser (such as IE5 or Mozilla) by pasting and saving the DTD component above to a text file named example.dtd and the XML file to a differently-named text file, and opening the XML file with the browser. The files should both be saved in the same directory. However, many browsers do not check that an XML document conforms to the rules in the DTD; they are only required to check that the DTD is syntactically correct. For security reasons, they may also choose not to read the external DTD.

Alternatives to DTDs are available:

XML Schema, also referred to as XML Schema Definition (XSD), has achieved Recommendation status within the W3C, and is popular for "data oriented" (that is, transactional non-publishing) XML use because of its stronger typing and easier round-tripping to Java declarations. Most of the publishing world has found that the added complexity of XSD would not bring them any particular benefits^{[citation needed]}, so DTDs are still far more popular there. An XML Schema Definition is itself an XML document while a DTD is not.
RELAX NG, which is also a part of DSDL, is an ISO international standard. It is more expressive than XSD, while providing a simpler syntax, but commercial software support has been slow in coming.

References

^ "doctypedecl". Extensible Markup Language (XML) 1.1. W3C. http://www.w3.org/TR/2004/REC-xml11-20040204/#NT-doctypedecl.
^ Sams teach yourself XML in 10 minutes Von Andrew H. Watt
^ http://www.stylusstudio.com/w3c/xml11/attdecls.htm#attdecls
^ http://www.w3schools.com/dtd/dtd_entities.asp

External links

This article's external links may not follow Wikipedia's content policies or guidelines. Please improve this article by removing excessive or inappropriate external links. (August 2009)

Definition of the XML document type declaration from Extensible Markup Language (XML) 1.0 (Fourth Edition) on W3.org
XML DTD Quick Reference
The XML FAQ has some DTD-specific entries
DTD Tutorial from W3schools
Zvon DTD Tutorial - in 7 languages
Interactive DTD tutorial from XMLzoo
Different doctypes for HTML
XMLPatterns.com - Design Patterns for developing DTDs
dtd2xs Converts a DTD to an XML Schema
PlainXML Converts a DTD to POJO objects
DTD Statistics

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	UDDI (technology)

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