1. MSc in Communication Sciences 2010-2011 Program in Technologies for Human Communication Davide Eynard Facoltà di scienze della comunicazione Università della Svizzera italiana 04 – XML Schemas Software Technology 2

2. I Facoltà di scienze della comunicazione Università della Svizzera italiana XML: recap and evaluation During last lesson we saw the basics of XML... Tree structure Elements and attributes Content vs presentation... And the basics of XML evaluation Well-formedness (just syntax) Validity wrt a schema 2

3. I Facoltà di scienze della comunicazione Università della Svizzera italiana Why do we need a schema? XML can be used to describe different data and is totally unaware of what you are speaking about You can check if the syntax is right...... but you cannot constrain its usage in any way! Example: John Doe 123-45-6789 987-65-4321 SSN should be unique, but a simple check on syntax would not find errors in this code 3

4. I Facoltà di scienze della comunicazione Università della Svizzera italiana What does a schema do? A schema allows you to define all the elements and attributes that can be used inside an XML document Moreover, you can add constraints specifying: Which are the children of a particular element In which order they appear How many children an element can have If the element is empty or contains text Datatypes for elements and attributes Default values for elements and attributes Given this information, an XML document conforming to a given schema can be validated The document is valid if it is well-formed and it follows the structure given inside the schema Validation can be done automatically by any tool which “understands” the schema language. 4

5. I Facoltà di scienze della comunicazione Università della Svizzera italiana DTD vs XML Schema There are many ways of defining the structure of an XML document i.e. DTD, XML Schema, RELAX NG, Schematron,... DTD and XML Schema are the most used ones, but XML Schema is having more success (W3C recommendation, 2001) because: it is written with XML syntax it supports datatypes it supports namespaces it supports inheritance and data type extension 5

6. I Facoltà di scienze della comunicazione Università della Svizzera italiana Using a DTD Writing a DTD is just as easy as writing another text file, but how can we use a DTD? How can we say a file should follow a schema? How can we use this information to validate the file? To match a document with a DTD, we should add the following to the xml prolog: To test it, we can use online validators or validating editors Example:... 6

7. I Facoltà di scienze della comunicazione Università della Svizzera italiana DTD Elements – 1 An element can be declared in the following way: or Category = EMPTY Elements containing only a sequence of characters Elements containing any mixture of text and other elements Elements containing one or more children elements Follows the specified order! 7

8. I Facoltà di scienze della comunicazione Università della Svizzera italiana DTD Elements – 2 Example: +393357654321 +393471234567 Hi there! 8

9. I Facoltà di scienze della comunicazione Università della Svizzera italiana DTD Elements – 3 Disjunction: We can use disjunction to specify subelements in generic order:... What if we have 10 subelements? Cardinality: ?zero times or once *zero or more times +one or more times 9

10. I Facoltà di scienze della comunicazione Università della Svizzera italiana DTD Attributes – 1 Attributes are defined in a DTD with an attribute list: For each attribute you have to define: The name of the element it is related to Its name Its type Its value type 10

11. I Facoltà di scienze della comunicazione Università della Svizzera italiana DTD Attributes – 2 Example: +393471234567 Hi there! <!ATTLIST text-message fromCDATA #REQUIRED> 11

12. I Facoltà di scienze della comunicazione Università della Svizzera italiana DTD Attributes – 3 Attribute types: CDATA, a string ID, a name that is unique across the XML document IDREF, a reference to another element with the ID attribute IDREFS, a sequence of IDREF (v1|...|vn), an enumeration of all possible values i.e. weekday (monday|tuesday|...|sunday) Limitations No dates No numbers No booleans 12

13. I Facoltà di scienze della comunicazione Università della Svizzera italiana DTD Attributes – 4 Attribute value types: #REQUIRED (attribute must appear in every occurrence of the element type in the XML document) #IMPLIED (the appearance of the attribute is optional) #FIXED “value” (every element must have this attribute with this value) “value” (specifies the default value for the attribute) 13

14. I Facoltà di scienze della comunicazione Università della Svizzera italiana From DTD to XML Schema Main differences between XML DTD and XML Schema: XML Schema's syntax is based on XML itself (you can use the same tools for XML documents and schemas!) It allows the reuse of existing schemas (inheritance) and their refinement (extension) It supports more specific datatypes It supports namespaces Note: XML Schema is also called XML Schema Definition (XSD) 14

15. I Facoltà di scienze della comunicazione Università della Svizzera italiana Namespaces Elements in XML files can be defined by the developers What if two developers use the same name for different kinds of elements? Example: Apples Bananas African Coffee Table 80 120 15

16. I Facoltà di scienze della comunicazione Università della Svizzera italiana Namespaces definition We need a way to specify that element names come from two different contexts we put a prefix before element names we specify what namespace that prefix represents Apples Bananas African Coffee Table 80 120 16

17. I Facoltà di scienze della comunicazione Università della Svizzera italiana Root and default namespaces You can also define all the namespaces you are going to use in the root element of your XML document: <root xmlns:h = "http://www.w3.org/TR/html4/" xmlns:f = "http://my.name.space/furniture/">... If the xmlns attribute is not followed by a prefix, then the specified namespace is considered as the default one 17

18. I Facoltà di scienze della comunicazione Università della Svizzera italiana Documents using XML Schema How is the prolog of XML documents using an XML Schema? <messages xmlns = "http://my.name.space" xmlns:xsi = "http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation = "http://my.name.space./messages.xsd">... 18

19. I Facoltà di scienze della comunicazione Università della Svizzera italiana XML Schema opening tag An XML schema is an XML document whose root element is called schema and is defined like follows: <xs:schema xmlns:xs = "http://www.w3.org/2001/XMLSchema" targetNamespace = "http://my.name.space" xmlns = "http://my.name.space" elementFormDefault = "qualified"> Source, target, and default ns Notes: the xs:schema element is the root of every XML schema qualified = “Associated with a namespace, either by the use of a declared prefix or via a default namespace declaration”. More details herehere 19

20. I Facoltà di scienze della comunicazione Università della Svizzera italiana The four constructs of XML Schema XML Schema is built on four constructs: A simple type definition defines a family of text strings (Unicode) A complex type definition defines a collection of requirements for attributes, sub-elements, and char data An element declaration associates an element name with either a simple type or a complex type An attribute declaration associates an attribute name with a simple type (attributes always contain unstructured text) 20

21. I Facoltà di scienze della comunicazione Università della Svizzera italiana XML Schema Elements and Types To declare an element (equivalent to in a DTD) you have to use the “element” tag: The most important (optional) attribute is type, as it defines the element's content type: 21

22. I Facoltà di scienze della comunicazione Università della Svizzera italiana Cardinality and default values To change cardinality, you can use the (optional) attributes minoccurs and maxoccurs: Note: minoccurs=”x”, where x is an integer >=0 maxoccurs=”x”, where x is an integer >0 or “unbounded” The default is “1” in both cases Also, default or fixed values can be specified: 22

23. I Facoltà di scienze della comunicazione Università della Svizzera italiana XML Schema Attributes and Types To declare an attribute use the “attribute” tag (very similar to the “element” one): Similarly to element, attributes can have types, default, and fixed values: Attributes are optional by default. You can use the use attribute to make them required: 23 Note: Attributes can be defined only within a complex element type (see later)

24. I Facoltà di scienze della comunicazione Università della Svizzera italiana XML Schema built-in data types 24

25. I Facoltà di scienze della comunicazione Università della Svizzera italiana Simple derived types Derived datatypes (such as integer), are built from the original ones using Restrictions Lists Unions 25

26. I Facoltà di scienze della comunicazione Università della Svizzera italiana Complex types Complex types are used to define elements which contain attributes, text, other elements, or any combination of these They are built using the following operators Element references, such as Concatenation, using the sequence element Union, using a choice element The all element (like sequence but unordered) The any construct The group element (to allow references to item groups) MinOccurs and maxOccurs attributes to define cardinalities The mixed (boolean) attribute to allow mixed content 26

27. I Facoltà di scienze della comunicazione Università della Svizzera italiana An example 27 <xsd:element name=“firstname” type=“xsd:string” minOccurs=“0” maxOccurs=“unbounded”/> <xsd:attribute name=“title” type=“xsd:string” use=“optional” /> Davide Eynard

28. I Facoltà di scienze della comunicazione Università della Svizzera italiana Schema extension Modularization is allowed by the following three constructs:... Inheritance and extensions Restrictions 28

29. I Facoltà di scienze della comunicazione Università della Svizzera italiana Limitations of XML Schema XML Schema is much more powerful and expressive than DTDs, however it still has some limitations: Too difficult for non-experts (problem: non-experts need to read the schema to write valid XML documents!) Element and attribute declarations are context insensitive Although XML Schema is built with XML, it still does not have a complete XML Schema Technical limitations When describing mixed content, the character data cannot be constrained in any way A schema cannot enforce a particular root element Element defaults cannot contain markup, but only character data... and many others 29

30. I Facoltà di scienze della comunicazione Università della Svizzera italiana References Some Web references: G. Antoniou and F. van Harmelen “A Semantic Web Primer”, The MIT Press 2004. Chapter 2 slides: http://www.ics.forth.gr/isl/swprimerhttp://www.ics.forth.gr/isl/swprimer M.C. Daconta, L.J. Obrst, and K.T. Smith. “The Semantic Web”, Wiley, 2003. Chapter 3 online: http://www.wiley.com/legacy/compbooks/daconta/sw/sample.html http://www.wiley.com/legacy/compbooks/daconta/sw/sample.html W3 School website, in particular http://www.w3schools.com/dtd and http://www.w3schools.com/Schemahttp://www.w3schools.com/dtd http://www.w3schools.com/Schema Anders Møller and Michael I. Schwartzbach. “An Introduction to XML and Web Technologies”, Addison-Wesley, 2006. Chapter 4 (Schema Languages) online: http://www.brics.dk/ixwt http://www.brics.dk/ixwt Examples from Elizabeth Castro, “XML for the World Wide Web: Visual Quickstart Guide”, Peachpit Press, 2000. http://www.cookwood.com/xmlhttp://www.cookwood.com/xml Tools: XML Validation Services: http://www.stg.brown.edu/service/xmlvalid, http://validator.w3.orghttp://www.stg.brown.edu/service/xmlvalid http://validator.w3.org XML Copy Editor, a free (as in freedom), multiplatform editor which supports validation: http://xml-copy-editor.sourceforge.net http://xml-copy-editor.sourceforge.net Validator, a free (as in freedom), multiplatform, drag and drop XML validator which works on Windows, Linux, and Mac OS X: http://homepage.mac.com/rcrews/software/validator/ http://homepage.mac.com/rcrews/software/validator/ 30