tanszek:oktatas:techcomm:dtd

DTD (Document Type Definition)
DTD Through Examples
Example: A music CD contains **tracks**:
Example: An album has at least one title, followed by at least one **track title** and **duration**:
Example: A library may contain **books** (zero or more)
Defining Attributes in DTD
Complex DTD Example for a Hypothetical Mail Processing System

DTD (Document Type Definition)

In simple cases, DTD is used to describe the syntax of a data structure stored in XML. This DTD can be saved in a file with a .dtd extension, but it can also be part of the XML document itself.

Basic Properties of DTD

- Allows for simple syntactic validation.

- A descriptive language similar to extended Backus-Naur form.

- You can define structures, required sequences, type constraints, and cardinality.

If an XML document has a document type declaration, it must be indicated in the file with a special declaration starting with `<!DOCTYPE>`. The type declaration can be internal or external (referenced via URI or a file).

Internal declaration

<!DOCTYPE uzenet [
        ....
]>

External URI

<!DOCTYPE html PUBLIC "-//W3//DTD XHTML 1.0 Strict//EN" "http://www.w3.org/TR/......dtd">

External file

<!DOCTYPE uzenet SYSTEM "filename.dtd">

The DTD defines the structure and rules that an XML document must follow to be considered valid.

DTD Through Examples

Here is a DTD file named message_syntax.dtd and the corresponding XML on the right:

<!ELEMENT message ( text )>
<!ELEMENT text ( #PCDATA )>

<?xml version = "1.0" encoding = "UTF-8"?>
<!DOCTYPE message SYSTEM "message_syntax.dtd">

<message>
   <text>Hello XML</text>
</message>

In the XML structure, a comma indicates the required order of elements:

<!DOCTYPE studygroup [
        <!ELEMENT group (teacher, student)>
        <!ELEMENT teacher ( #PCDATA ) >
        <!ELEMENT student ( #PCDATA ) >   
]>

<studygroup>
    <teacher>Kiss Janos</teacher>
    <student>Gipsz Jakab</student>
</studygroup>

The pipe symbol `|` expresses an either-or relationship. For example, the storage device can be either a pendrive or an SSD, but not both:

<!DOCTYPE datastore [
    <!ELEMENT datastorage  (pendrive | SSD)>
    <!ELEMENT pendrive ( #PCDATA ) >
    <!ELEMENT SSD ( #PCDATA ) >   
]>

<datastorage>
    <pendrive>64MB</pendrive>
</datastorage>

There are three ways to express frequency:

Symbol	Meaning
`+`	The element appears at least once.
`*`	The element can appear any number of times, including zero.
`?`	The element appears zero or one time.

Example: A music CD contains tracks:

<!DOCTYPE datastorage [
    <!ELEMENT CD (recording + )>
    <!ELEMENT recording ( #PCDATA ) >   
]>

<CD>
    <felvetel>Song 1</felvetel>
    <felvetel>Song 2</felvetel>
</CD>

Example: An album has at least one title, followed by at least one track title and duration:

<!DOCTYPE datastorage [
    <!ELEMENT album (title+, (tracktitle, duration)+)>
    <!ELEMENT title ( #PCDATA ) >
    <!ELEMENT tracktitle ( #PCDATA ) >
    <!ELEMENT duration ( #PCDATA ) >   
]>

Syntactically correct XML:

<album>
    <title>Title 1</title>
    <title>Subtitle</title>
    <tracktitle>Track Title 1</tracktitle>
    <duration>3.42</duration>
    <tracktitle>Track Title 2</tracktitle>
    <duration>2.32</duration>
</album>

Example: A library may contain books (zero or more)

<!DOCTYPE datastorage  [
    <!ELEMENT library (book*) >
    <!ELEMENT book (author, title) >
    <!ELEMENT author ( #PCDATA ) >
    <!ELEMENT title ( #PCDATA ) >   
]>

Syntactically correct XML:

<library>
    <book >
        <author>Orwell, George</author>
        <title >1984</title >
    </book>
    <book>
        <author>Brown, Dan</author>
        <title >The Da Vinci Code</title >
    </book >
</library>

Defining Attributes in DTD

If a class has an attribute such as “number of students,” it can be specified as follows:

<!ELEMENT class (student *) >
<!ATTLIST class number CDATA #REQUIRED>

Attributes can be: - #IMPLIED: not mandatory - #REQUIRED: mandatory - #FIXED: fixed value

Specifying a Default Attribute Value

Example:

<!ATTLIST paymentType type CDATA "bankTransfer">
</sxh

The XML could look like this:

<sxh>
<paymentType/> or <paymentType type="bankTransfer">

Enumerated Values

Syntax:

<!ATTLIST element-name attribute-name (eval | eval | ..) default-value>

DTD Example:

<!ATTLIST payment type (check | cash) "cash">

XML Example:

<payment type="check"/> or <payment type="cash"/>

Complex DTD Example for a Hypothetical Mail Processing System

Task: Provide an XML example that satisfies the following DTD:

<!ELEMENT mails (email*, postcard*)>
<!ELEMENT email (address, sender, message?, attachment?)>
<!ELEMENT postcard (address, sender?, message?)>
<!ELEMENT address (name, postalcode, city, country)>
<!ATTLIST address nick CDATA #IMPLIED>
<!ATTLIST sender nick CDATA #IMPLIED>
<!ATTLIST attachment type CDATA #REQUIRED>
<!ATTLIST postcard scanimage CDATA #IMPLIED>
<!ATTLIST sender name CDATA #REQUIRED>

A Possible XML for the given DTD

<mails>
    <email>
        <address nick="Alice">alice@usa.com</address>
        <sender nick="Bob">bob@jp.com</sender>
        <message>Hash code</message>
        <attachment type="text/doc"></attachment>
    </email>
    <postcard scanimage="kep.jpg">
       <address>
         <name>John Doe</name>
         <postalcode>1234</postalcode>
         <city>Miskolc</city>
         <country>Hungary</country>
       </address>
       <sender name="Bob Cat" />
       <message>Happy Name Day</message>
    </postcard>
</mails>

This provides an example of how attributes, including mandatory, optional, and default values, can be defined in a DTD and represented in an XML document.

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