1. Software Engineering for the World Wide Web
XML Overview
Jeff Offutt
SWE 642
Software Engineering for the World Wide Web
sources: Professional Java Server Programming, Patzer, Wrox, 2nd edition, Ch 5, 6
Programming the World Wide Web, Sebasta, Ch 8, Addison-Wesley
Michelle Lee and Ye Wu

2. Topics Motivation How does XML work ? Syntax of XML documents
XML and HTML
9/17/2018
© Offutt

3. XML eXtensible Markup Language
Markup languages insert “tags” into text files to describe presentation or other information
SGML : Standard Generalized Markup Language
HTML : Visual presentation
Latex : Document formatting
XML : Data description
W3C standard:
9/17/2018
© Offutt

4. Why XML?
Passing data from one software component to another has always been difficult
The two components must agree on format, types, and organization
Web software applications have unique requirements for data passing:
Very loose coupling
Dynamic integration
9/17/2018
© Offutt

5. Passing Data – 1978
Program P2 needs to use data produced by program P1
Data saved to a file as records (COBOL, Fortran, …)
The file format often not documented
Source for P1 may not be available
Data saved in binary mode – not readable by hand P1 P2
File
Format of file deduced from executing P1 and from trial and error
MSU Computing Services, 1979 : Two weeks of trial and error executions of P1 to understand format of file
9/17/2018
© Offutt

6. Passing Data – 1985
Program P2 needs to use data produced by program P1
Data saved to a file as records (C, Ada, …)
The file format often not documented
Data saved as plain text P1 P2
File WM
Both P1 and P2 access the file through a “wrapper module”
Module needs to repeatedly updated
Module written by development team
Data hard to validate
Mothra, 1985 : ~12 data files shared among 15 to 20 separate programs
9/17/2018
© Offutt

7. Wrapper Method Problems
Slow – everything is a file in plain text
Sharing – Developers of P1 and P2 must agree to share source of WM
Maintenance – Who has control of WM?
Solution – data sharing that is :
Independent of type
Self documenting
Easy to understand format
Especially important for web applications
– XML
9/17/2018
© Offutt

8. Passing Data – 21st Century
Data is passed directly between components
XML allows for self-documenting data
P1, P2 and P3 can see the format, contents, and structure of the data
Free parsers are available to put XML messages into a standard format
Information about type and format is readily available
Schema P1 P2
Parser
XML
File P3
9/17/2018
© Offutt

9. Web Services
A Web Service is a program that offers software services over the Internet to other programs
Internet-based
Uses SOAP and XML
Peer-to-peer communication
Web service components can integrate dynamically
Finding other services during execution
Web services transmit data that are formatted in XML
9/17/2018
© Offutt

10. Topics Motivation How does XML work ? Syntax of XML documents
XML and HTML
9/17/2018
© Offutt

11. Introductory Example Programmers can create their own tags
Tags have been designed for mathematics, formal specifications, resumes, recipes, addresses, …
Pizza Markup Language (PML):
<pizza>
<topping extracheese="yes">Pepperoni</topping>
<price> </price>
<size> large </size>
</pizza>
9/17/2018
© Offutt

12. Markup Language – History
1969 : Charles Goldfarb, Ed Mosher, and Ray Lorie invented GML (Generalized Markup Language)
1974 : Goldfarb proved the concept of a validating parser for GML
: Goldfarb worked on the team that made the transition from GML to ISO 8879, called SGML
9/17/2018
© Offutt

13. Markup Language – History
1989 : Tim Berners-Lee and Anders Berglund developed a markup language for hyperlinked text documents: HTML
HTML does not quite conform to SGML
1993 : A student at University of Illinois, Marc Andreesen, created a graphical Web browser called Mosaic to render HTML
Although Mosaic was not the first web browser, the NCSA ported it to the three most popular computing platforms and gave it away for free.
9/17/2018
© Offutt

14. Markup Language – History
1994 : Jim Clark and Marc Andreesen co-founded Netscape Communications and created the Netscape Navigator web browser
To solve the problems of interoperability and scalability on the web, the W3C began work on a simplified version of SGML, called Extensible Markup Language (XML)
1998 : The XML 1.0 standard was approved and published by the World Wide Web Consortium (W3C)
1994, Jim Clark (founder of Silicon Graphics) and Marc Andreesen co-founded Netscape Communications (originally Mosaic Communications Corp.) and created the Netscape Navigator web browser.
As the web grew in popularity the weaknesses of HTML became apparent in several areas including interactivity, data interchange and precise text formatting.
In response to this, vendors began extending HTML in proprietary ways to address these shortcomings.
To solve the problems of interoperability and scalability on the web, without further extending HTML, the W3C began work on a simplified version of SGML, which is called the Extensible Markup Language XML.
9/17/2018
© Offutt

15. Markup Languages – Typesetting
Markup languages began in typesetting
Documents were marked-up to represent how they would be printed
For example, words can be Bold, italicized, or underlined
Typesetting only effects the printing of specific phrases or words, and not categories of phrases or words
Typesetting only effects the printing of specific phrases or words, and not categories of phrases or words.
For example, if a newspaper wanted all its headlines in boldface, the typesetter had to markup every single headline.
Markup languages were developed so that the typesetter simply could indicate in one command to make all headlines boldface.
9/17/2018
© Offutt

16. Markup Languages – Semantic Tags
Markup languages can be used to logically organize the contents of a document
For example, a document representing a book can contain the following organizational tags :
Title
Chapter headings
Section headings
9/17/2018
© Offutt

17. Markup Languages – Semantic Tags
A markup language can also provide semantic information (meta-data) about the text in a document
Examples : First name, Last name, Phone number
Semantic tags can improve the accuracy of document queries
Documents can be searched using their tag assignments rather than the plain-text contents
9/17/2018
© Offutt

18. Markup Languages – Semantic Tags
Use semantic tags to define the hierarchical structure of the document
Author
First name
Last name
Publisher
Name
Address
9/17/2018
© Offutt

19. Markup Languages – Examples
Typesetting tags
<bold> Chapter 1 </bold>
<italic> Background </italic>
<underline> Important text </underline>
Semantic tags
<first name> Steffi </first name>
<last name> Offutt </last name>
<phone number> </phone number>
9/17/2018
© Offutt

20. SGML SGML — Standard Generalized Markup Language
Set up by the ISO in 1986
Super set of all markup languages – includes all the features of every markup language derived from it
Allows a document to be annotated with text that describes the semantic meanings of portions of the document
The standard of the markup of electronic documents.
9/17/2018
© Offutt

21. SGML Separates the structure of the document from the content
The structure denotes the purpose of the document’s data
Use Document Type Definitions (DTDs) to define the syntax of the annotations used in a document
SGML captures meta-data for a document by marking up the content
Data that describes other data is called meta-data.
9/17/2018
© Offutt

22. Characteristics of XML
XML is extensible
Tags have been designed for mathematics, formal specifications, resumes, recipes, addresses, …
XML has a strict structure
XML is validating
Grammars (DTD & Schema) define XML languages
Documents can be checked against the grammar
Required fields, etc
Allows programs to assume the data is formatted correctly, reducing the amount of checking the program must do
9/17/2018
© Offutt

23. XML Provides Data Independence
Allows data to be used by any application
Requires every document to be in a clear and specific format
Fosters information sharing better than other markup languages
9/17/2018
© Offutt

24. XML Simplifies Data Sharing
Plain text
Create and edit files with any editor
Easy to debug
Scalability : suitable for both small and large scaled data
Data identification
Once different parts of the information have been identified, they can be used in different ways by different applications
Data transference
Very easy to move between XML and form parameters
Very easy to move between XML and databases
Plain Text: Since XML is not a binary format, you can create and edit files with anything from a standard text editor to a visual development environment. Easy to debug your programs, and make it useful for storing small amounts of data. At the other end of the spectrum, an xml front end to a database makes it possible to efficiently store large amounts of xml data as well. So XML provides scalability for anything from small configuration files to a company-wide data repository.
Data Identification: XML tells you what kind of data you have, not how to display. Because the markup tags identify the information and break up the data into parts, an program can process it, a search program can look for messages sent to a particular people, and an address book can extract the address information from the rest of the message. In short, because the different parts of the information have been identified, they can be used in different ways by different applications.
For example, when searching the address book, one can only match a keyword in the last name field.
9/17/2018
© Offutt

25. XML Can Easily Be Displayed
The stylesheet standard, XSL, lets you dictate how to format and display the data
Since XML is inherently style free, you can use different stylesheets to produce different output formats
When display is important, the stylesheet standard, XSL, lets you dictate how to portray the data.
Hierarchical document structures are, in general, faster to access because you can drill down to the part of need, like stepping through a table of contents.
9/17/2018
© Offutt

26. Searching XML
Keywords can be compared to the entire contents of a document
Search can be limited to a single field or a set of fields
9/17/2018
© Offutt

27. XML Example <message> <to> you@yourAddress.com </to>
<from> </from>
<subject> XML Is Really Cool </subject>
<text>
How many ways is XML cool? Let me count the ways ...
</text>
</message>
9/17/2018
© Offutt

28. Topics Motivation How does XML work ? Syntax of XML documents
XML and HTML
9/17/2018
© Offutt

29. XML Structure Containment : Tags can be contained in other tags
Tag names should be meaningful
All tags must have an end tag
Note that HTML does not … meaning HTML is not fully SGML-compliant
9/17/2018
© Offutt

30. XML Can Easily Be Validated
XML messages are described in grammars
Two ways to describe an XML language
Document Type Definitions (DTD) : Older, easier to read and understand, but somewhat limited
Schemas : Grammar plus types and facets
Documents can be checked against the grammar
Grammar can specify that certain fields are required
Allows programs to assume the data is formatted correctly, reducing the amount of checking the program must do
9/17/2018
© Offutt

31. Syntax of XML XML syntax is defined at two levels
General syntax : defines syntax on all XML documents
Correct documents said to be “well formed”
Specific syntax : defines syntax on a specific group of documents
Correct documents said to be “valid”
Statements in an XML document
XML declaration – which version of XML
Data elements – the primary contents of the document
Markup declarations – instructions to XML parser
Processing instructions – instructions to the program
9/17/2018
© Offutt

32. <?xml version="1.0" encoding="ISO-8859-1" standalone="yes“ ?>
XML Declaration
<?xml version="1.0" encoding="ISO " standalone="yes“ ?>
<? .. ?> Prolog declaration
version
Identifies the version of the XML markup language used in the data
This attribute is required
encoding
Identifies the character set used to encode the data
"ISO " is "Latin-1" the Western European and English language character set
Default is compressed Unicode: UTF-8
standalone
Tells whether or not this document references an external entity or an external data type specification
If there are no external references, use “yes”
9/17/2018
© Offutt

33. XML Data Element Names
Must start with a letter or underscore, and can include digits, hyphens, and periods
XML names are case sensitive
lastName, lastname, LASTNAME are all different
9/17/2018
© Offutt

34. XML General Syntax Rules Well-formed
Every XML document has a single root element
Opening tag must be first line of XML
All other elements are nested inside the root element
XML tags are surrounded by pointy brackets “< >”
Every XML tag must have a closing tag
If no content: <empty/>
XML elements must be properly nested
<B><I> … </B></I> is not well formed XML
All attribute values must be enclosed in quotes
9/17/2018
© Offutt

35. Pizza Markup Language (PML)
XML Example
Pizza Markup Language (PML)
root element
data value
attribute
<pizza>
<topping extracheese=“yes”>Pepperoni</topping>
<price>13.00</price>
<size>large</size>
</pizza>
9/17/2018
© Offutt

36. XML Attributes Product XML <products> <product>
<name>Monitor</name>
<!-- Price can be USD, Euro, or Yuan -->
<price currency=“USD”>200</price>
</product>
</products>
9/17/2018
© Offutt

37. Attributes vs. Nested Tags
In PML, “extraCheese” could have been defined as attribute or a nested tag
Images can only be attributes
It is easier to add new tags than attributes
Attributes cannot define structure
<… name=“Yao Ming”>
<name>Yao Ming</name>
<name>
<familyName>Ming</familyName>
<givenName>Yao</givenName>
</name>
9/17/2018
© Offutt

38. Attributes vs. Nested Tags (2)
Attributes are necessary when :
Identifying numbers or names of elements
Values are selected from a finite set
Attributes should be used when :
No substructure
Attribute describes information about the element
9/17/2018
© Offutt

39. XML Entity References (Variables)
Entities are usually used to embed special characters into XML messages
Document Entity : The file that represents the document
Other entities have names
Entity names start with letters, dash, colon
Can also contain digits, periods, underscores
References to entities surround name with &;
&entityName;
Some built-in XML entities: < > & " &apos;
Use entities to avoid malformed XML
<pred> X < Y </pred> … <pred> X < Y </pred>
9/17/2018
© Offutt

40. Topics Motivation How does XML work ? Syntax of XML documents
XML and HTML
9/17/2018
© Offutt

41. XML vs. HTML
Unlike HTML, XML tags tell you what the data means, rather than how to display it
XML elements must be strictly nested, XML can represent data in any level of complexity
Both XML and HTML allow empty tags; in XML an empty tag must be followed by a forward slash: <emptyTag/>
9/17/2018
© Offutt

42. XML vs. HTML
XML attribute values must be surrounded by single or double quotes
HTML does not require quotes for single values
XML tags are case sensitive
HTML tags are not
This is really confusing at first !
9/17/2018
© Offutt

43. XML Summary
XML gives software engineers an incredibly flexible, simple, and powerful way to represent data
Works with all sorts of data
Maps naturally to tables, spreadsheets and databases
Grammatical rules can be defined (next slides …)
Human readable
Performance costs
Plain text files use more space on disk
Takes time to read, write, and reformat XML to and from internal representations
This cost is seldom important and almost never within web applications
9/17/2018
© Offutt