1 XML DTD & XML Schema Monica Farrow G30

2 A Complete XML Document Lisa Simpson Required Optional Link to document defining the XML elements

3 Defining the structure of an XML file We can check if an XML file is well-formed by looking at it, maybe By loading it into a browser If well-formed, it will be displayed However, how can we check that the well-formed file contains the correct elements in the correct quantities? We need to write a specification for the XML file

4 Defining the structure of an XML file There are 2 main alternatives Document Type Definitions Original and simple XML Schema More versatile and complex We will look at both Concentrating on XML Schema

5 Example: An Address Book Homer Simpson Up to 4 tel nos Optionally one Exactly one name An attribute One or more persons

6 DTD - Specifying the Structure In a DTD, we can specify the permitted content for each element, using regular expressions Describes the pattern For a person element, the regular expression is name, title?, tel*, +

7 What’s in a person Element? This means name = there must be a name element title? = there is an optional title element (i.e., 0 or 1 title elements) name, title? = the name element is followed by an optional title element tel* = there are 0 or more tel elements + = there are 1 or more elements

8 DTD For the Address Book <!DOCTYPE addressbook [ <!ELEMENT person (name, title?, tel*, +)> ]> PCDATA means parsed character data Regular expressions

9 Attributes in a DTD XML elements can have attributes. General Syntax for DTD: <!ATTLIST element-name attribute-name1 type1 default-value1 …. attribute-namen typen default-valuen> Example: CDATA means Character data Default value could be REQUIRED or IMPLIED (meaning optional)

10 Connecting a Document with its DTD A DTD can be internal (part of the document file) … ]>... Or external (the DTD and the document are in different files) A DTD from the local file system: A DTD from a remote file system:

11 Valid Documents A document with a DTD is valid if it conforms to the DTD, i.e., the document conforms to the regular-expression grammar, types of attributes are correct, and constraints on references are satisfied

12 DTDs Problems DTDs are rather weak specifications by DB & programming-language standards Some limitations: Only one base type – PCDATA Also no constraints, e.g range of values, frequency of occurrence Not easily parsed (since they are not XML) Not easy to express that element a has exactly the children c, d, e in any order

13 XML Schema DTDs are now being superceded by XML schemas. They provide the following features XML Syntax So can be parsed, validated with standard XML tools Data types other than #PCDATA There are built in types such as integer, float, boolean, string and many others Greater control over permitted constructs Can specify maximum and minimum occurrences Can use regular expressions to set patterns to be matched Support for modularity and inheritance

14 XML Schema continued XML Schema are more precise and therefore more complicated than DTDs They were designed to replace DTDs but DTDs are very well established, and simpler

15 Schema types There are some basic built-in types such as xs:string, xs:decimal, xs:integer, xs:ID Each element is composed of either simple types or complex types. A complex type is often a sequence of elements The content of the type can be declared as shown in the following example. A type can also be declared, named and referred to. Notice the use of minOccurs and maxOccurs. Their default is 1.

16 Simple Schema Example <xs:schema xmlns:xs= " details of the person element -pto standard stuff Top-level element Namespace

17 Namespaces You’ll see namespaces when using XML schemas and stylesheets. There is a namespace associated with the tags used in each that lets them be used unambiguously. e.g. a schema element, a chemical element A namespace is identified by a short prefix e.g. xs A unique URL

18 Namespace declaration So at the start of a document we must specify what namespaces we are using. In the schema example, we are using the XML schema namespace with the xs prefix We declare this namespace in an attribute in the top-level element <xs:schema xmlns:xs= " We then use the xs prefix in all the XML Schema elements e.g. complexType, sequence, element etc

19 Schema Example Continued Details of the person element <xs:element name="person" maxOccurs="unbounded"> <xs:element name = " " type="xs:string" minOccurs="0" maxOccurs="1"/> A person is a complex type which is a sequence of elements and an attribute Empty element

20 Exercise 1 Create a schema for the holiday house example. Each home has an id, a name and a location Additionally, each home has between one and three sets of contact details. Contact details consist of a name and a phone number, and optionally an address and website.

21 Restrictions on elements You can also restrict the values of the data in a range an enumerated list a pattern Means 0 or more lowercase alphabetic chars

22 Declaring your own types Named types can be used for elements or attributes. Here’s an example which specifies restrictions on the attribute A named type is declared And used as the attribute type

23 More complex Schemas The previous example shows a simple schema. It is also possible to make the schema easier to maintain by declaring all the simple elements first and then referring to them in the body of the document By naming the declaration of simple and complex types, which could then be used later in the document, and more than once if necessary See ple.asp if you are interested ple.asp

24 Referring to a schema Save your schema in a file with the extension xsd. Linking schema definition with a document is done using a special attribute of the root node of the document: <people xmlns:xsi=" xsi:noNamespaceSchemaLocation=“people.xsd">

25 Validating Validators I don’t seem to be able to revalidate with the same filenames No problems, nicer layout Others also on the web

26 XML: Summary XML lets you choose application specific element names and define special purpose document types. Need document type definition or schema to define allowed markup. What can we do with our valid document? – next 2 lectures

27 Exercises 2 Alter the schema given in the lecture notes so that there must be between 1 and 4 tel numbers which must be in the range 1000 – 9999 Create a simple type for tel