Web Services

Web Services Session 4

Introduction to Document Type Definitions (DTDs)

XML allows you to define your own elements and to define the structure of the documents that you create. A Document Type Definition is a Schema used to define the elements and how they are nested to produce a valid document.
DTDs were invented before XML Schemas, and are still useful.
DTDs define the structure of an XML document -- what elements and attributes are permitted.
XML documents are not required to have DTD.
Usually recommended for document conformity.
Use Extended Backus-Naur Form (EBNF) grammar.
DTDs are linked to an XML document in its prolog. Syntax is basically:
```
<?xml version="1.0" encoding="utf-8"?>
<!DOCTYPE dtd-name .... >
  
```

Well Formed and Valid XML Documents

A well-formed XML document is syntactically correct even if the structure of the document does not obey the DTD specification.
A Valid XML document obeys the DTD (or schema) and must also be well-formed.
The Validating Parser determines whether the XML document conforms to its DTD, hence is valid.
By definition, a valid XML document is also well-formed, but...
An XML document can be well-formed, but not valid.
Non-validating Parsers do not check the document against the DTD, but can determine if the document is (or is not) well-formed. Validation can be turned on or off in most parsers.
Validation is normally checked during application development to pick up errors in generated XML, but may not be necessary in the "final product", eg:
Financial transactions in B2B applications -- validation is probably necessary
Generating an HTML document for displaying non critical information, validation may be unnecessary overhead.

Internal DTDs

Declarations can be "inside" the document, specified in the prolog. It is "visible" only within the document in which it resides. In general:

<?xml version="1.0" encoding="utf-8"?>
<!DOCTYPE dtd-name [ ... element declarations ... ]>
<dtd-name>
    .
    .
    .
</dtd-name>

where dtd-name is the name of the root element. For example:

<?xml version="1.0" encoding="UTF-8"?>
<!-- using an internal document type definition
       for an invitation list to a party -->
<!DOCTYPE invitationList [
     <!ELEMENT invitationList (person*)>
     <!ELEMENT person ( #PCDATA )> 
]>
<invitationList>
   <person>mal sutherland</person>
   <person>brian retallick</person>
   <person>mary martin</person>
   <person>fran soddell</person>
   <person>phil scott</person>
   <person>peter goddard</person>
</invitationList>

External DTDs

Definition exists in a different file (with .dtd extension) located by a URI. The preceding document might look like:

<?xml version="1.0" encoding="UTF-8"?>
<!-- using an external document type definition
       for an invitation list to a party -->
<!DOCTYPE invitationList SYSTEM "party.dtd">
<invitationList>
   <person>mal sutherland</person>
   <person>brian retallick</person>
   <person>mary martin</person>
   <person>fran soddell</person>
   <person>phil scott</person>
   <person>peter goddard</person>
</invitationList>

Contents of party.dtd are:

<!ELEMENT invitationList (person*)>
<!ELEMENT person ( #PCDATA )>

Note how parenthesis are used to indicate an element's content.

Basic DTD Structure

#PCDATA
"Parsable Character Data" -- text that will be parsed by the XML parser as content. In other words, data -- not markup.
SYSTEM

This identifier is used with a URI to specify the location of the DTD.
The URI can be specified as relative or absolute, in exactly the same way as (eg) an HTML href.

PUBLIC

This identifier can appear instead of SYSTEM in the above example
Indicates an industry-standard DTD.
DTD owners (individuals or organisations) can be registered with ISO as an ISO9070 standard through a registration authority.

Basic DTD Structure cont...

A PUBLIC identifier takes the form:

registration type // owner // DTD description // language

"Registration types:"

ISO The DTD is an ISO standard.
+ The DTD is an approved non-ISO standard
- The DTD is a non-approved, non-ISO standard

Owner is the name of the company or individual who owns the DTD.
"DTD Description" is a short description of the DTD.
"Language" is a two-character ISO language code (ISO 639).
Example:
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 //EN" "http://www.w3c.org/TR/html4/strict.dtd">

DTD Symbols

The following sysmbols have special meaning in a DTD. If you are familiar with Regular Expression formalism you will find these quite familiar.

Symbol	Meaning
`?`	zero or one occurrence of this entity
`*`	zero or more occurrences of this entity
`+`	one or more occurrences of this entity
`,`	sequence order
`\|`	or (choice, alternation)

Example DTD #1

meals.dtd

<!ELEMENT meals ( breakfast, lunch, tea )>
      <!-- meals must contain exactly one of each of the elements
              breakfast, lunch and tea in sequence -->

<!ELEMENT breakfast (food+)>
      <!-- breakfast must contain one or more food items -->

<!ELEMENT lunch (food?)>
      <!-- lunch can be zero or one food item -->

<!ELEMENT tea (food*)>
      <!-- tea can be zero or more food items -->

<!ELEMENT food (fruit | vegetable | fish | poultry | meat | dairy |cereal)>
      <!-- food is one of fruit, veg, fish, poultry, meat, dairy or cereal -->

<!ELEMENT fruit (#PCDATA)>
<!ELEMENT vegetable (#PCDATA)>
<!ELEMENT fish (#PCDATA)>
<!ELEMENT poultry (#PCDATA)>
<!ELEMENT meat (#PCDATA)>
<!ELEMENT dairy (#PCDATA)>
<!ELEMENT cereal (#PCDATA)>

An Example XML Document

Using the DTD of the previous slide we could have:

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE meals SYSTEM "meals.dtd">
<meals>
  <breakfast>
    <food>
      <fruit>grapefruit</fruit>
    </food>
    <food>
      <cereal>rice bubbles</cereal>
    </food>
    <food>
      <dairy>yogurt</dairy>
    </food>
  </breakfast>
  <lunch>
    <food>
      <meat> meat pie </meat>
    </food>
  </lunch>
  <tea>
    <food>
      <poultry>chicken</poultry>
    </food>
    <food>
      <vegetable>chips</vegetable>
    </food>
    <food>
      <vegetable>peas</vegetable>
    </food>
    <food>
      <dairy>ice cream</dairy>
    </food>
  </tea>
</meals>

DTD For weather.xml

<!-- weather.dtd -->
<!ELEMENT report (weather-station)*>
<!ELEMENT weather-station (location, rainfall, max-temp, min-temp)>
<!ELEMENT location (latitude, longitude, place-name)>
<!ELEMENT latitude (#PCDATA)>
<!ELEMENT longitude (#PCDATA)>
<!ELEMENT place-name (#PCDATA)>
<!ELEMENT rainfall (#PCDATA)>
<!ELEMENT max-temp (#PCDATA)>
<!ELEMENT min-temp (#PCDATA)>

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

<weather-station>
  <location>
     <latitude>-36.82</latitude>
     <longitude>144.30</longitude>
     <place-name>Mandurang</place-name>
  </location>
     
  <rainfall>5.0</rainfall>
  <max-temp>18.1</max-temp>
  <min-temp>6.2</min-temp>
</weather-station>

<weather-station>
  <location>
     <latitude>-36.77</latitude>
     <longitude>144.28</longitude>
     <place-name>Bendigo</place-name>
  </location>
     
  <rainfall>8.0</rainfall>
  <max-temp>19.1</max-temp>
  <min-temp>7.0</min-temp>
</weather-station> 

</report>

Attribute Declarations

<!ATTLIST element attributeName [attributeType] [value] [-- descriptive comment ]>

Elements can have attributes.
Attributes take the form name="value".
An element can have many attributes but each attribute can appear only once, and the order of attributes is not important.
Use attributes for elements that have a single child or for empty elements or where the data value is restricted to a list of possible values.

Syntax for Attribute Declarations

<!ELEMENT x (#PCDATA)>
<!ATTLIST x
    a CDATA #REQUIRED
    b CDATA #IMPLIED
    c CDATA #FIXED "value"
    d CDATA "value" >

<!ELEMENT y (#PCDATA)>
<!ATTLIST y a #REQUIRED>

<!ELEMENT z (someElement)>
<!ATTLIST z a "value"

<!ELEMENT myTag (#PCDATA)>
<!ATTLIST myTag year "2004">
    <!--year is an optional attribute with default value of "2005"-->

REQUIRED ... the attribute is required to be present
IMPLIED ... the attribute is optional and the default value assumed by the application is used
FIXED ... the attribute is required and is set to a fixed value (that must be specified in the DTD)

A specific value, eg:

<!ATTLIST myTag year CDATA "2005">

<myTag year ="2005">......</myTag>

<myTag> ...... </myTag>
<!-- the missing attribute "year" is assumed to have the value "2005" by the parser. -->

XML Schemas: DTDs