Document Type Definitions DTDs

1
Document Type Definitions (DTDs)
2
Whats DTD for?

• DTD explains precisely which elements and
  entities may appear where in the document and
  what the elements' contents and attributes are
• DTD defines the valid structure of a XML document
  rules
• Valid element names
• ltBOOKgtltTITLEgtltAUTHORgtltPRICEgtltISBNgt
• Valid attribute names and values
• ltAUTHOR id234 sexFgt
• Relationship between elements
• ltBOOKgtltTITLEgtlt/TITLEgtlt/BOOKgt
• Different XML applications can use different DTDs
  to specify what they do and do not allow

3
Its Complex, but More Powerful

• With DTD you can
• Define your own meaningful tags
• You can Validate whether your document is
  structure correct
• An XML document is valid if it has an DTD and the
  document conforms with the DTD
• The document type declaration must appear before
  the first element in an XML

4
Getting Started with DTD

• 3-1.dtd, 3-1.txt
• DTD can be stored in a separate file from the
  document it describes
• DTD can be included inside the document it
  describes
• DTDs allow forward, backward, and circular
  references to other declarations

5
Document Type Declaration

• A valid document includes a reference to the DTD
  to which it should be compared
• lt!DOCTYPE person SYSTEM "http//../person.dtd"gt
• The root element is person
• The DTD of this document can be found in
  "http//../person.dtd"
• The document type declaration is included in the
  prolog of the XML document after the XML
  declaration but before the root element
• You can use a relative URL (or just the filename)
  instead of the absolute form, if the document
  resides in the same server (or directory) as the
  DTD

6
Internal DTD Subsets

• DTD can be included inside the document it
  describes
• 3-4.xml
• Some document type declarations contain some
  declarations directly but link in others
  (3-5.xml)
• The part of the DTD between the brackets is
  called the internal DTD subset
• All the parts that come from outside this
  document are called the external DTD subsets
• As a general rule, the two subsets must be
  compatible. Neither can override the element
  declarations the other makes
• When you use an external DTD subset, you should
  give the standalone attribute of the XML
  declaration the value no

7
Internal and External DTD

• Internal DTD Put DTD and XML in one file
• lt!DOCTYPE RootElement lt!ELEMENT author
  (PCDATA)gt..gt

8
Element Declaration

• DTD is a mechanism to describe every object
  (element, attribute,) that can appear in an XML
  document
• Element Declaration
• lt!ELEMENT element-name content-specification gt
• Content specification specifies what children the
  element may or must have in what order
• lt!ELEMENT address-book (entry)gt
• Parentheses are used to group elements in content
  specification
• lt!ELEMENT name (lname, (fname title))gt

9
Special Keywords in Content Model

• Special Keywords in Content Model
• PCDATA parsed character data (text)
• lt!ELEMENT phone_number (PCDATA)gt
• CDATA sections appear anywhere PCDATA appears
• EMPTY empty element
• ANY can contain any other elements declared in
  the DTD (including mixed content, child elements)
• Mixed Content
• Element contents that have PCDATA
• Element Content
• Element contents that contain only elements

10
The Secret of , , ?

• , , ? Occurrence indicators
• No occurrence indicator appear once and only
  once
• appear one or several times
• appear zero or more times
• ? appear once or not at all
• Example
• lt!ELEMENT entry (name,address,tel,fax,email)gt
• lt!ELEMENT address (street,region?,postal-code,loca
  lity,country)gt

11
The Secret of ,

• , Connectors
• , Elements must appear in the same order
• Only one element must appear
• Examples
• lt!ELEMENT name (PCDATA fname lname)gt
• lt!ELEMENT entry (name,address,tel,fax,email)gt
  
• Mixed Content Components must always separated
  by a , and the model must repeat
• lt!ELEMENT name (PCDATA, fname, lname)gt
• lt!ELEMENT name (PCDATA fname lname)gt
• PCDATA must be the first child in the list

12
More Examples

• lt!ELEMENT cover (title, (author subtitle))gt
• lt!ELEMENT circle (center, (radius diameter))gt
• lt!ELEMENT name (last_name (first_name,
  ((middle_name, last_name) (last_name?)))gt
• lt!ELEMENT paragraph (PADATA name profession
  footnote emphasize date) gt
• lt!ELEMENT image EMPTY)
• ltimage source"bus.jpg" width"152" /gt

13
Attribute Declaration

• Attribute Declaration
• lt!ATTLIST element-name attribute-name
  attribute-type default-valuegt
• lt!ATTLIST tel preferred (truefalse) falsegt
• lt!ATTLIST email href CDATA REQUIRED
  preferred (truefalse) falsegt
• Can appear anywhere in the DTD
• Best to list attributes immediately after the
  element declaration

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Attribute Type

• CDATA String
• ID Identifier unique in the document
• IDREF Value of an ID
• IDREFS List of IDREF separated by space
• ENTITY Name of an external entity
• ENTITIES List of ENTITY
• NMTOKEN Word without spaces
• NMTOKENS List of NMTOKEN
• Enumerated-type list Closed list of NMTOKEN
  separated by
• Notation name of a notation declared in the DTD

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Attribute Types (Cont.)

• CDATA
• Any text string acceptable in a well-formed XML
  attribute value
• The most general attribute type
• Can be used for prices, URIs, email addresses,
  citations
• NMTOKEN (named token)
• Consist of the same characters as an XML name,
  but any allowed characters can be the first
  character in a name token
• 12, .cshrc
• NMTOKENS
• one ore more XML name tokens separated by
  whitespace
• ltperformances dates"08-21-2001 08-23-2001
  08-27-2001"gt Kat and the Kings lt/performancesgt

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Attribute Types (Cont.)

• Enumeration
• A list of all possible values for the attribute,
  separated by
• Each possible value must be an XML name token
• lt!ATTLIST date year (2000 2001 2002 2003)
  REQUIREDgt
• ID assign unique identifiers to elements
• An XML name that is unique within the XML
  document (no other ID type attribute in the
  document can have the same value)
• Each element must have no more than one ID type
  attribute
• lt!ATTLIST employee ssn ID REQUIREDgt
• ltemploy ssn"_078-05_123" /gt

17
Attribute Types (Cont.)

• IDREF
• Refer to the ID type attribute of some element in
  the document
• Used to establish relationships between elements
  when simple containment won't suffice
• 3-6.xml
• Do not constrain the person attribute of the
  team_member element to match only employee IDs or
  constrain the project_id attribute of the
  assignment element to match only project IDs
• IDREFS
• Contain a whitespace-separated list of XML names,
  each of which must be the ID of an element in the
  document

18
Attribute Types (Cont.)

• ENTITY
• Contain the name of an unparsed entity declared
  elsewhere in DTD
• lt!ATTLIST move source ENTITY REQUIREDgt
• ltmove source"X-Men-trailer" /gt
• ENTITIES
• Contain the name of one ore more unparsed
  entities declared elsewhere in the DTD, separated
  by whitespace
• lt!ATTLIST slide_show slides ENTITIES REQUIREDgt
• ltslide_show slides"slide1 slide2 slide3 slide4 /gt

19
Attribute Types (Cont.)

• NOTATION
• Contain the name of a notation declared in the
  document's DTD
• It could be used to associate types with
  particular elements, as well as limiting the
  types associated with the element
• lt!NOTATION gif SYSTEM "image/gif"gt
• lt!NOTATION tiff SYSTEM "image/tiff"gt
• lt!NOTATION jpeg SYSTEM "image/jpeg"gt
• lt!NOTATION png SYSTEM "image/png"gt
• lt!ATTLIST image type NOTATION (gif tiff jpeg
  png) REQUIREDgt

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Default Value

• REQUIRED
• The attribute is required. Each instance of the
  element must provide a value for the attribute
• No default value is provided
• lt!ATTLIST person name CDATA REQUIREDgt
• IMPLIED
• The attribute is optional. Each instance of the
  element may or may not provide a value for the
  attribute
• No default value is provided
• lt!ATTLIST person born CDATA IMPLIEDgt

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Default Value (Cont.)

• FIXED (follow by a value)
• The attribute value is constant and immutable.
  The attribute has the specified value regardless
  of whether the attribute is explicitly noted on
  an individual instance of the element
• If it is included, it must have the specified
  value
• lt!ATTLIST biography xmlnsxlink CDATA FIXED
  "http//www.w3.org/1999/xlinkgt
• Literal Value
• The attribute will take this value if no value is
  given in the document
• lt!ATTLIST web_page protocol NMTOKEN "http"gt

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General Entity Declaration

• Replace texts in XML documents defined by the DTD
• lt!ENTITY entity_name "replacement_text"gt
• lt!ENTITY super "superman in the jungle"gt
• super ? superman in the jungle

23
Limitation of DTD

• Limitations of DTD
• Content is limited to textual
• Difficult to put in repetition constraints
• DTD does not use XML syntax
• DTD does not say the following
• What the root element of the document is
• How many of instances of each kind of element
  appear in the document
• What the character data inside the elements looks
  like
• The semantic meaning of an element
• A DTD never says anything about the length,
  structure, meaning, allowed values, or other
  aspects of the text content of an element

24
Advanced DTD

• General Entity Declaration
• External Parsed General Entities
• External Unparsed Entities and Notations
• Parameter Entities
• Conditional Inclusion

25
Validation

• A validating parser compares a document to its
  DTD and lists any places where the document
  differs from the constraints specified in the DTD
• The parser can decide what it wants to do about
  any violations
• A validity error is not necessarily a fatal error
• Validation is an optional step in processing XML
• Everything not permitted in the DTD is forbidden

26
Validating a Document

• Online validating parsers
• http//www.stg.brown.edu/service/xmlvalid
• http//www.cogsci.ed.ac.uk/7Erichard/xml-check.ht
  ml
• Validating parser software
• Most XML parser class libraries include a simple
  program to validate documents
• http//www.cogsci.ed.ac.uk/7Erichard/xml-check.ht
  ml
• http//www.topologi.com