XML. Extensible Markup Language

Size: px

Start display at page:

Download "XML. Extensible Markup Language"

Rodney Ryan
5 years ago
Views:

1 XML Extensible Markup Language Acknowledgements and copyrights: these slides are a result of combination of notes and slides with contributions from: Michael Kiffer, Arthur Bernstein, Philip Lewis, Hanspeter Mφssenbφck, Hanspeter Mφssenbφck, Wolfgang Beer, Dietrich Birngruber, Albrecht Wφss, Mark Sapossnek, Bill Andreopoulos, Divakaran Liginlal, Anestis Toptsis, Addison Wesley, Microsoft AA. They serve for teaching purposes only and only for the students that are registered in CSE4413 and should not be published as a book or in any form of commercial product, unless written permission is obtained from each of the above listed names and/or organizations. 1

2 Why XML? XML can be considered as a knowledge representation language. Can be used to express and organize various (any?) kinds of knowledge, such as: documents. Data (such as numbers, database tables, objects in the OOP sense). Possibly images Possibly software 2

3 Why XML? XML was invented as a generic HTML (HTML is the language used to describe web pages) but soon its scope extended beyond that. Nowadays XML is used for Writing web pages (like HTML, but better ) Exchanging information that facilitates e-commerce. Representing various kids of information, including DB data. 3

4 Information that facilitates e- commerce E-commerce is all infrastructure that enables commercial transactions (buy, sell, advertise, ) over the internet (usually the www). In the heart of this infrastructure is software designed to carry out business activities, across networked computers. Part/examples of a business activity could be: A shopping cart (such as the ones in amazon.com and dell.com). A service (such as am income tax calculator, or a mortgage calculator). A spell checker. A display of the price of a stock symbol. A reservation for a restaurant. Booking of a vacation. A message that informs a mobile subscriber of how many minutes remain for use in his monthly phone plan. (pretty much anything imaginable!) 4

5 Information that facilitates e- commerce Such software is best implemented and deployed as web services. Web services are programs that execute on a server and can be used by clients, over the internet. A web service program can be written in any programming language (such as Java, C#, VB.NET), but the access and use of a web service is done through following certain rules (protocols). 5

6 Information that facilitates e- Such protocols are commerce SOAP simple object access protocol. WSDL web services description language UDDI universal description, discovery and integration. All above protocols involve message/information exchange between computers (client + server). All messages/information exchanged are written in XML!!! 6

7 Information that facilitates e- commerce XML, therefore, is a fundamental component of e-commerce (probably the most fundamental component). 7

8 Why XML? Besides its importance in e-commerce directly, XML is a standard for data exchange that is taking over the world. Most major database products have been enhanced with facilities to store and construct XML documents. SQL-like query languages for such databases have been developed (and currently further being developed). XML is closely related to object-oriented and socalled semistructured data 8

9 Semistructured Data An HTML document to be displayed on the Web: <dt>name: John Doe <dd>id: s <dd>address: <ul> <li>number: 123</li> <li>street: Main</li> </ul> </dt> <dt>name: Joe Public <dd>id: s </dt> HTML does not distinguish between attributes and values 9

10 Semistructured Data (cont d.) To make the previous student list suitable for machine consumption on the Web, it should have these characteristics: Be object-like Beschemaless (not guaranteed to conform exactly to any schema, but different objects have some commonality among themselves). Be self-describing (some schema-like information, like attribute names, is part of data itself) Data with these characteristics are referred to as semistructured. 10

11 What is Self-describing Data? Non-self-describing (relational, object-oriented). Needs a schema to accompany the data, in order to make sense of what is what: Data part: (#123, [ Students, {[ John, s , [123, Main St ]], [ Joe, s , [321, Pine St ]] } ] ) Schema part: PersonList[ ListName: String, Contents: [ Name: String, Id: String, Address: [Number: Integer, Street: String] ] ] 11

12 What is Self-Describing Data? Self-describing describing: (contd.) Attribute names embedded in the data itself, but are distinguished from values Doesn t need schema to figure out what is what (but schema might be useful nonetheless) (#12345, [ListName: Students, Contents: { [ Name: John Doe, Id: s , Address: [Number: 123, Street: Main St. ] ], [Name: Joe Public, Id: s , Address: [Number: 321, Street: Pine St. ] ] } ] ) 12

13 XML The De Facto Standard for Semistructured Data XML: Suitable for semistructured data and has become a standard: Easy to describe object-like data Self-describing Doesn t require a schema (but a schema can be provided optionally) 13

14 Overview of XML XML is like HTML, but any number of different tags can be used (up to the document author) extensible. <Person> Al Gore is a <role> proponent </role> of <technology> Internet technologies </technology> </Person> Unlike HTML, no semantics behind the tags For instance, HTML s <table> </table> means: render contents as a table; in XML: doesn t mean anything special Some semantics can be specified using XML Schema (types); some using stylesheets (browser rendering) Unlike HTML, is intolerant to bugs Browsers will render buggy HTML pages XML processors are not supposed to process buggy XML documents 14

15 Benefits of XML Open W3C standard Representation of data across heterogeneous environments Cross platform Allows for high degree of interoperability Completely text based (nothing in binary format) Strict rules Syntax Structure Case sensitive 15

16 Benefits of XML - XML helps standardize data formats in a community. - Not tied to a vendor - Platform independent Deliver information in a form that allows automatic processing 16

17 Who Uses XML? Microsoft IBM Hewlett-Packard Fujitsu Laboratories Sun Microsystems Netscape (AOL), and others 17

18 XML example Pre-XML representation of data PO-1234, CUST001, X9876, 5, XML representation of the same data: <PURCHASE_ORDER> <PO_NUM> PO-1234 </PO_NUM> <CUST_ID> CUST001 </CUST_ID> <ITEM_NUM> X9876 </ITEM_NUM> <QUANTITY> 5 </QUANTITY> <PRICE> </PRICE> </PURCHASE_ORDER> 18

19 How XML compares to HTML Self Describing Data Unlimited Types of Tags <Person> <Name>Bill Clinton</Name> <Title>Ex-President</Title> <Age>52</Age> </Person> Like HTML, XML can Intermix Tags and Text <Person> Al Gore is a <role> proponent </role> of <technology> Internet technologies </technology> </Person> No Default Presentation for Each Tag XML Tags describe Information Not Presentation No special tags for titles, paragraphs or images. Extensible by defining new tags and their usage 19

20 Example <?xml version= 1.0?> attributes <PersonList Type= Student Date= > <Title Value= Student List /> <Person> </Person> <Person> </Person> </PersonList> elements Empty element Root element Elements are nested Root element contains all others Element (or tag) names 20

21 More Terminology Opening tag Content of Person <Person Name = John Id = s > John is a nice fellow <Address> <Number>21</Number> <Street>Main St.</Street> </Address> </Person> standalone text, not very useful as data, non-uniform Nested element Address, child of Person Child of Address, Descendant of Person Closing tag: What is open must be closed Person Address Person is Parent of Address, Ancestor of number Number Street 21

22 Syntax and Structure Components of an XML Document <?xml version= 1.0?> <?xml-stylesheet type="text/xsl" href= template.xsl"?> <ROOT> <ELEMENT1><SUBELEMENT1 /><SUBELEMENT2 /></ELEMENT1> <ELEMENT2> </ELEMENT2> <ELEMENT3 type= string > </ELEMENT3> <ELEMENT4 type= integer value= 9.3 > </ELEMENT4> </ROOT> Elements with Attributes Elements Prologue (processing instructions) 22

23 Conversion from XML to Objects (but without methods attached (*)) Straightforward: <Person Name= Joe > <Age>44</Age> <Address><Number>22</Number><Street>Main</Street></Address> </Person> Becomes: (#345, [Name: Joe, Age: 44, Address: [Number: 22, Street: Main ] ] ) (*) objects usually have methods associated with them XML does not seem to have this feature (at least not yet). 23

24 Conversion from Objects to XML Also straightforward Non-unique: Always a question if a particular piece (such as Name) should be an element in its own right or an attribute of an element Example: A reverse translation could give <Person> <Person Name= Joe > <Name>Joe</Name> <Age>44</Age> <Address> <Number>22</Number> <Street>Main</Street> </Address> </Person> This or this 24

25 Differences between XML Documents and Objects XML s origin is document processing, not databases Allows things like standalone text (usually useless or at least non-standard for databases) <foo> Hello <moo>123</moo> Bye </foo> XML data is ordered, while database data is not: <something><foo>1</foo><bar>2</bar></something> is different from <something><bar>2</bar><foo>1</foo></something> 25

26 Differences between XML Documents and Objects (cont d) Attributes may just bloat the number of ways to represent the same thing: More concise vs. <foo bar= 12 >ABC</foo> <foobar><foo>abc</foo><bar>12</bar></foobar> More uniform, database-like 26

27 XML Building Block (Element) Start Tag Content EndTag eg.: <name> Brilliant </name> Document Element has a tree structure Customer <Customer> </Customer> <Person> <Name> Bill Dosser </Name> </Person> <Business> <Name>Grainger </Name> </Business> Person Business Name Name Bill Dosser Grainger 27 Divakaran Liginlal

28 Well-formed XML Documents XML 1.0 Recommendation (October 2000) Must have a root element. There must be one, and only one, root element. Every opening tag must have matching closing tag. <TAG_NAME>...</TAG_NAME> <name> Abc </name> Elements can be empty (<TAG_NAME />) Elements must be properly nested <foo><bar></foo></bar> is a no-no 28

29 Syntax and Structure Well-Formed XML? No, CHILD2 and CHILD3 do not nest propertly <xml? Version= 1.0?> <PARENT> <CHILD1>This is element 1</CHILD1> <CHILD2><CHILD3>Number 3</CHILD2></CHILD3> </PARENT> 29

30 Syntax and Structure Well-Formed XML? No, there are two root elements <xml? Version= 1.0?> <PARENT> <CHILD1>This is element 1</CHILD1> </PARENT> <PARENT> <CHILD1>This is another element 1</CHILD1> </PARENT> 30

31 Syntax and Structure Well-Formed XML? Yes <xml? Version= 1.0?> <PARENT> <CHILD1>This is element 1</CHILD1> <CHILD2/> <CHILD3></CHILD3> </PARENT> 31

32 Well-formed XML Documents (cont d) XML is case-sensitive <tag> and <TAG> are not the same type of element Names must begin with a letter, underscore or colon Valid characters are these plus digits, hyphens, or periods 32

Semistructured data, XML, DTDs

Semistructured data, XML, DTDs Introduction to Databases Manos Papagelis Thanks to Ryan Johnson, John Mylopoulos, Arnold Rosenbloom and Renee Miller for material in these slides Structured vs. unstructured