Semantic Web. XML and XML Schema. Morteza Amini. Sharif University of Technology Fall 94-95

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1 ه عا ی Semantic Web XML and XML Schema Morteza Amini Sharif University of Technology Fall 94-95

2 Outline Markup Languages XML Building Blocks XML Applications Namespaces XML Schema 2

3 Outline Markup Languages XML Building Blocks XML Applications Namespaces XML Schema 3

4 SGML (ISO 8879:1986) Standard Generalized Markup Language The international standard for defining descriptions of structure and content in text documents. Interchangeable: device-independent, system-independent. Tags are not predefined. Using DTD to validate the structure of the document. Large, powerful, and very complex. Heavily used in industrial and commercial usages for over a decade. 4

5 HTML (RFC 1866) HyperText Markup Language A small SGML application used on web (a DTD and a set of processing conventions). Only uses a predefined set of tags. 5

6 What is XML? extensible Markup Language A simplified version of SGML. Maintains the most useful parts of SGML. Designed so that SGML can be delivered over the Web. More flexible and adaptable than HTML. XHTML: a reformulation of HTML 4 in XML 1.0 6

7 HTML vs. XML (1) HTML is used to mark up text so it can be displayed to users. HTML describes both structure (e.g. , <h2>, ) and appearance (e.g. , , ) HTML uses a fixed, unchangeable set of tags. XML is used to mark up data so it can be processed by computers. XML describes only content, or meaning In XML, you make up your own tags. 7

8 HTML vs. XML (2) HTML is for humans HTML describes web pages. You don t want to see error messages about the web pages you visit. Browsers ignore and/or correct as many HTML errors as they can, so HTML is often sloppy. XML is for computers XML describes data. The rules are strict and errors are not allowed. In this way, XML is like a programming language. Current versions of most browsers can display XML However, browser support of XML is spotty at best. 8

9 XML-Related Technologies DTD (Document Type Definition) and XML Schemas are used to define legal XML tags and their attributes for particular purposes XSLT (extensible Stylesheet Language Transformations) and XPath are used to translate from one form of XML to another SAX (Simple API for XML) is an event-based sequential access parser API for XML documents. 9

10 Outline Markup Languages XML Building Blocks XML Applications Namespaces XML Schema 10

11 XML Building Blocks - Declaration The XML declaration looks like this: <?xml version="1.0" encoding="utf-8" standalone="yes"?> The XML declaration is not required by browsers, but is required by most XML processors (so include it!) If present, the XML declaration must be first--not even whitespace should precede it. Note that the brackets are <? and?> version="1.0" is required (this is the only version so far) encoding can be "UTF-8" (ASCII) or "UTF-16" (Unicode), or something else, or it can be omitted standalone tells whether there is a separate DTD 11

12 XML Building Blocks - Processing Instructions PI s (Processing Instructions) may occur anywhere in the XML document (but usually first). A PI is a command to the program processing the XML document to handle it in a certain way. XML documents are typically processed by more than one program. Programs that do not recognize a given PI should just ignore it. General format of a PI: <?target instructions?> Example: <?xml-stylesheet type="text/css" href="mysheet.css"?> 12

13 XML Building Blocks - Elements Delimited by angle brackets. Identify the nature of the content they surround. General format: <element> </element> Empty element: <empty-element /> XML Elements have Relationships Elements are related as parents and children Elements have Content Elements can have different content types: Element, mixed, simple, empty 13

14 XML Building Blocks - Attributes Name-value pairs that occur inside start-tags after element name, like: <element attribute= value /> Provide additional information about elements that often is not a part of data. Attributes and elements are somewhat interchangeable Should I use an element or an attribute? Example using just elements: <name> <first>ali</first> <last>rahmati</last> </name> Example using attributes: <name first= Ali" last= Rahmati"></name> metadata (data about data) should be stored as attributes, and that data itself should be stored as elements 14

15 XML Building Blocks - Entities Five special characters must be written as entities, because they have special meaning in XML docs: & for & (almost always necessary) < for < (almost always necessary) > for > (not usually necessary) " for (necessary inside double quotes) ' for ' (necessary inside single quotes) These entities can be used even in places where they are not absolutely required. These are the only predefined entities in XML. 15

16 XML Building Blocks - Comments  Comments can be put anywhere in an XML document. Comments are useful for: Explaining the structure of an XML document Commenting out parts of the XML during development and testing The character sequence -- cannot occur in the comment. Comments are not necessarily displayed by browsers, but can be seen by anyone who looks at the source code. 16

17 CDATA By default, all text inside an XML document is parsed. You can force text to be treated as unparsed character data by enclosing it in <![CDATA[... ]]> Any characters, even & and <, can occur inside a CDATA Whitespace inside a CDATA is (usually) preserved. The only real restriction is that the character sequence ]]> cannot occur inside a CDATA. CDATA is useful when your text has a lot of illegal characters (for example, if your XML document contains some HTML text). 17

18 XML Syntax All XML documents must have a root tag. All XML elements must have a closing tag. XML tags are case sensitive. All XML elements must be properly nested. Attribute values must always be quoted. With XML, a new line is always stored as LF. 18

19 Well-Formed XML Every element must have both a start tag and an end tag, e.g. <name>... </name> But empty elements can be abbreviated: <break />. XML tags are case sensitive. XML tags may not begin with the letters xml, in any combination of cases. Elements must be properly nested, e.g. not bold and italic Every XML document must have one and only one root element. The values of attributes must be enclosed in single or double quotes, e.g. <time unit="days"> Parsed character data cannot contain < or & 19

20 Displaying XML XML documents do not carry information about how to display the data. We can add display information to XML with CSS (Cascading Style Sheets) XSL (extensible Stylesheet Language) --- preferred 20

21 Outline Markup Languages XML Building Blocks XML Applications Namespaces XML Schema 21

22 XML Applications (1) Separate data XML can Separate Data from HTML Store data in separate XML files Using HTML for layout and display Data Islands can be embedded inside HTML pages using XML Benefits: Changes in the underlying data will not require any changes to your HTML 22

23 XML Applications (2) Exchange data XML is used to Exchange Data Text format Software-independent, hardware-independent Exchange data between incompatible systems, given that they agree on the same tag definition. Can be read by many different types of applications Benefits: Reduce the complexity of interpreting data Easier to expand and upgrade a system 23

24 XML Applications (3) Store Data XML can be used to Store Data Plain text file Store data in files or databases Application can be written to store and retrieve information from the store Other clients and applications can access your XML files as data sources Benefits: Accessible to more applications 24

25 XML Applications (4) Create new language XML can be used to Create new Languages, e.g. : WML (Wireless Markup Language) used to markup Internet applications for handheld devices like mobile phones (WAP). MusicXML used to publishing musical scores. 25

26 Outline Markup Languages XML Building Blocks XML Applications Namespaces XML Schema 26

27 Names in XML Names (as used for tags and attributes) must begin with a letter or underscore, and can consist of: Letters, both Roman (English) and foreign Digits, both Roman and foreign. (dot) - (hyphen) _ (underscore) : (colon) should be used only for namespaces Combining characters and extenders (not used in English) 27

28 Namespaces Namespaces are a simple mechanism for creating globally unique names for the elements and attributes of your markup language. Benefits: De-conflicts the meaning of identical names in different markup languages. Allows different markup languages to be mixed together without ambiguity. Namespaces are implemented by requiring every XML name to consist of two parts: a prefix and a local part: <xsd:integer> 28

29 Namespaces and URIs A namespace is defined as a unique string To guarantee uniqueness, typically a URI (Uniform Resource Indicator) is used, because the author owns the domain. It doesn't have to be a real URI; it just has to be a unique string. Example: There are two ways to use namespaces: Declare a default namespace. Associate a prefix with a namespace, then use the prefix in the XML to refer to the namespace. 29

30 Namespace Syntax In any start tag you can use the reserved attribute name xmlns: <book xmlns=" This namespace will be used as the default for all elements up to the corresponding end tag. You can override it with a specific prefix. You can use almost the same form to declare a prefix: <book xmlns:dave=" Use this prefix on every tag and attribute you want to use from this namespace, including end tags--it is not a default prefix <dave:chapter dave:number="1">to Begin</dave:chapter> You can use the prefix in the start tag in which it is defined: <dave:book xmlns:dave= 30

31 Review of XML Rules Start with <?xml version="1"?> XML is case sensitive. You must have exactly one root element that encloses all the rest of the XML. Every element must have a closing tag. Elements must be properly nested. Attribute values must be enclosed in double or single quotation marks. There are only five pre-declared entities. 31

32 XML as a Tree An XML document represents a hierarchy; a hierarchy is a tree novel foreword chapter number="1" paragraph paragraph paragraph This is the great American novel. It was a dark and stormy night. Suddenly, a shot rang out! 32

33 Extended Document Standards You can define your own XML tag sets, but here are some already available: XHTML: HTML redefined in XML SMIL: Synchronized Multimedia Integration Language MathML: Mathematical Markup Language SVG: Scalable Vector Graphics DrawML: Drawing MetaLanguage ICE: Information and Content Exchange ebxml: Electronic Business with XML cxml: Commerce XML CBL: Common Business Library 33

34 Outline Markup Languages XML Building Blocks XML Applications Namespaces XML Schema 34

35 XML Validation "Well Formed" XML document correct XML syntax "Valid" XML document well formed Conforms to the rules of a DTD or an XSD XML DTD (Document Type Definition) defines the legal building blocks of an XML document Can be inline in XML or as an external reference XML Schema Definition (XSD) an XML based alternative to DTD, more powerful Support namespace and data types 35

36 An Example XML with DTD <?xml version="1.0"?> <!DOCTYPE note [ <!ELEMENT note (to,from,heading,body)> <!ELEMENT to (#PCDATA)> <!ELEMENT from (#PCDATA)> <!ELEMENT heading (#PCDATA)> <!ELEMENT body (#PCDATA)> ]> <note> <to>tove</to> <from>jani</from> <heading>reminder</heading> <body>don't forget me this weekend</body> </note> 36

37 XML Schemas Schema is a general term DTDs are a form of XML schemas When we say XML Schemas, we usually mean the W3C XML Schema Language This is also known as XML Schema Definition language, or XSD. 37

38 XSD vs. DTD DTDs provide a very weak specification language You can t put any restrictions on text content You have very little control over mixed content (text plus elements) You have little control over ordering of elements DTDs are written in a strange (non-xml) format You need separate parsers for DTDs and XML The XML Schema Definition language solves these problems XSD gives you much more control over structure and content XSD is written in XML 38

39 Referring to a Schema To refer to a DTD in an XML document, the reference goes before the root element: <?xml version="1.0"?> <!DOCTYPE rootelement SYSTEM "url"> <rootelement>... </rootelement> To refer to an XML Schema in an XML document, the reference goes in the root element: <?xml version="1.0"?> <rootelement xmlns:xsi=" (The XML Schema Instance reference is required) xsi:nonamespaceschemalocation="url.xsd"> (This is where your XML Schema definition can be found)... </rootelement> 39

40 The XSD Document Since the XSD is written in XML, it can get confusing which we are talking about. The file extension is.xsd The root element is <schema> The XSD starts like this: <?xml version="1.0"?> <xs:schema xmlns:xs=" 40

41 <schema> The <schema> element may have attributes: xmlns:xs=" This is necessary to specify where all our XSD tags are defined. elementformdefault="qualified" This means that all XML elements must be qualified (use a namespace). It is highly desirable to qualify all elements, or problems will arise when another schema is added. 41

42 Simple and Complex Elements A simple element is one that contains text and nothing else. A simple element cannot have attributes. A simple element cannot contain other elements. A simple element cannot be empty. However, the text can be of many different types, and may have various restrictions applied to it. If an element isn t simple, it s complex. A complex element may have attributes. A complex element may be empty, or it may contain text, other elements, or both text and other elements. 42

43 Defining a Simple Element A simple element is defined as <xs:element name="name" type="type" /> where: name is the name of the element. the most common values for type are xs:boolean xs:integer xs:date xs:string xs:decimal xs:time Other attributes a simple element may have: default="default value" if no value is specified, value is assigned fixed="value" if the element appears its value must be equal to value, otherwise value is assigned Note that default and fixed are mutually exclusive. 43

44 Defining an Attribute Attributes themselves are always declared as simple types. An attribute is defined as <xs:attribute name="name" type="type" /> where: name and type are the same as for xs:element Other attributes a simple element may have: default="default value" if no other value is specified fixed="value" no other value may be specified use="optional" the attribute is not required (default) use="required" the attribute must be present 44

45 Restrictions, or facets The general form for putting a restriction on a text value is: <xs:element name="name"> (or xs:attribute) <xs:simpletype> <xs:restriction base="type">... the restrictions... </xs:restriction> </xs:simpletype> </xs:element> Example: <xs:element name="age"> <xs:simpletype> <xs:restriction base="xs:integer"> <xs:mininclusive value="0"> <xs:maxinclusive value="140"> </xs:restriction> </xs:simpletype> </xs:element> 45

46 Restrictions on Numbers mininclusive -- number must be the given value minexclusive -- number must be > the given value maxinclusive -- number must be the given value maxexclusive -- number must be < the given value totaldigits -- number must have exactly value digits fractiondigits -- number must have no more than value digits after the decimal point 46

47 Restrictions on Strings length -- the string must contain exactly value characters minlength -- the string must contain at least value characters maxlength -- the string must contain no more than value characters pattern -- the value is a regular expression that the string must match whitespace -- not really a restriction --tells what XML Processor to do with whitespaces (lines feeds, tabs, spaces, and carriage return) value="preserve" Keep all whitespace value="replace" Change all whitespace characters to spaces value="collapse" Remove leading and trailing whitespace, and replace all sequences of whitespace with a single space 47

48 Enumeration An enumeration restricts the value to be one of a fixed set of values. Example: <xs:element name="season"> <xs:simpletype> <xs:restriction base="xs:string"> <xs:enumeration value="spring"/> <xs:enumeration value="summer"/> <xs:enumeration value="autumn"/> <xs:enumeration value="fall"/> <xs:enumeration value="winter"/> </xs:restriction> </xs:simpletype> </xs:element> 48

49 Complex Elements A complex element is defined as <xs:element name="name"> <xs:complextype>... information about the complex type... </xs:complextype> </xs:element> Example: <xs:element name="person"> <xs:complextype> <xs:sequence> <xs:element name="firstname" type="xs:string" /> <xs:element name="lastname" type="xs:string" /> </xs:sequence> </xs:complextype> </xs:element> <xs:sequence> says that elements must occur in this order. Remember that attributes are always simple types. 49

50 Declaration and Use We can define types like as follows. <xs:complextype name="person"> <xs:sequence> <xs:element name="firstname" type="xs:string" /> <xs:element name="lastname" type="xs:string" /> </xs:sequence> </xs:complextype> To use a type we have declared, use it as the value of type="..." Examples: <xs:element name="student" type="person"/> <xs:element name="professor" type="person"/> 50

51 xs:sequence We ve already seen an example of a complex type whose elements must occur in a specific order: <xs:element name="person"> <xs:complextype> <xs:sequence> <xs:element name="firstname" type="xs:string" /> <xs:element name="lastname" type="xs:string" /> </xs:sequence> </xs:complextype> </xs:element> 51

52 xs:all xs:all allows elements to appear in any order <xs:element name="person"> <xs:complextype> <xs:all> <xs:element name="firstname" type="xs:string" /> <xs:element name="lastname" type="xs:string" /> </xs:all> </xs:complextype> </xs:element> Despite the name, the members of an xs:all group can occur once or not at all. You can use minoccurs="0" to specify that an element is optional (default value is 1) In this context, maxoccurs is always 1 52

53 Mixed Elements Mixed elements may contain both text and elements We add mixed="true" to the xs:complextype element The text itself is not mentioned in the element, and may go anywhere (it is basically ignored) <xs:complextype name="paragraph" mixed="true"> <xs:sequence> <xs:element name="somename type="xs:anytype"/> </xs:sequence> </xs:complextype> 53

54 Extensions You can base a complex type on another complex type and add some elements. <xs:complextype name="newtype"> <xs:complexcontent> <xs:extension base="othertype">...new stuff like this... <xs:element > </xs:element> </xs:extension> </xs:complexcontent> </xs:complextype> 54

55 Empty Elements An empty complex element cannot have contents, only attributes. <xs:element name="counter"> <xs:complextype> <xs:attribute name="count" type="xs:integer"/> </xs:complextype> </xs:element> 55

56 Predefined String Types Recall that a simple element is defined as: <xs:element name="name" type="type" /> Here are a few of the possible string types: xs:string -- a string xs:normalizedstring -- a string that doesn t contain tabs, newlines, or carriage returns xs:token -- a string that doesn t contain any whitespace other than single spaces Allowable restrictions on strings: enumeration, length, maxlength, minlength, pattern, whitespace 56

57 Predefined Date and Time Types xs:date -- A date in the format CCYY-MM-DD, for example, xs:time -- A date in the format hh:mm:ss (hours, minutes, seconds) xs:datetime -- Format is CCYY-MM-DDThh:mm:ss The T is part of the syntax Allowable restrictions on dates and times: enumeration, mininclusive, minexclusive, maxinclusive, maxexclusive, pattern, whitespace 57

58 Predefined Numeric Types Here are some of the predefined numeric types: xs:decimal xs:byte xs:short xs:int xs:long xs:positiveinteger xs:negativeinteger xs:nonpositiveinteger xs:nonnegativeinteger Allowable restrictions on numeric types: enumeration, mininclusive, minexclusive, maxinclusive, maxexclusive, fractiondigits, totaldigits, pattern, whitespace 58

59 References

60 Any Question... 60

Markup Languages. Lecture 4. XML Schema

Markup Languages. Lecture 4. XML Schema Markup Languages Lecture 4. XML Schema Introduction to XML Schema XML Schema is an XML-based alternative to DTD. An XML schema describes the structure of an XML document. The XML Schema language is also