Institute of Information Science - University of Miskolc

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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 ) >   
]>