XML

HTML is good for displaying documents for humans to read, but it is poor for describing semantics or allowing programs to search documents looking for information. Many people have written programs to do "screen scraping", parsing an HTML document to get information. This is hard to do and is usually based on assumptions about how an HTML document is configured.

For example, suppose that you wanted to write a program to do stock market analysis, and so each night you send queries to a server that sends you HTML files which contain the closing price, trading volume, etc for stocks. You have to write a program to extract the information that you want from the HTML, a non-trivial task. If the server changes the format of the HTML pages that it sends, you have to rewrite your program.

XML solves this problem. XML is a cross-platform, software and hardware independent tool for transmitting information. If the server sent an xml file (with a link to a separate style sheet file which had instructions for how to display the information), writing a program to extract the information that you want would be much easier.

XML stands for EXtensible Markup Language

XML is a markup language much like HTML. Both are descended from SGML (Standard Generalized Markup Language), invented at IBM in 1974.

HTML is about displaying information, while XML is about describing information.

XML does not do anything. It is a way of describing information

XML will enable smarter searches. A search for Chip will not get people named chip or chocolate chip cookies if you want computer chips.

Most people think that XML will be everywhere soon.

The syntax rules of XML are very simple and very strict. The rules are very easy to learn, and very easy to use.

<?xml version="1.0" encoding="ISO-8859-1"?>
<library> 
   <book isbn="123456">
        <title>Unix Network Programming</title>
        <author>W. Richard Stevens</author>
        <publisher>Addison Wesley</publisher>
   </book>
   <book isbn="987654">
        <title>Modern Operating Systems</title>
        <author>Andrew S. Tanenbaum</author>
        <publisher>Prentice Hall</publisher>
   </book>
</library>

An xml document has these basic building blocks.

• Element An element starts with the element name enclosed in angle braces (less than and greater than signs) (< element-name >). An element ends with the element name, preceded by a slash, enclosed in angle brackets. These are called the start tag and the end tag respectively. In the example above, library, book, and title are examples of elements. If an element has no children, the start tag and end tag can be combined, like this <tag-name />
• Attribute An attributed is a name = value statement enclosed in the start tag of an element. The value must always be a string in double quotes (or single quotes if the value string contains a double quote). In the example above, isbn="123456" is an attribute.
• Text Text is a string enclosed in an element. In the example above, Unix Network programming is a Text.
• Declaration A statement at the start of the document declaring that this is an xml document. The declaration starts with the tag <?xml and ends with the tag ?>. There may be attributes in the declaration.

We will see other components of an xml document below, but these are the most important.

Differences between XML and HTML

Superficially, XML looks a lot like HTML. This is because they share a common ancestor, SGML. There are some important differences.

• With XML, it is illegal to omit the closing tag.
• XML tags are case sensitive <title> <Title> and <TITLE> are all different
• All XML elements must be properly nested
• All XML documents must have a root element All other elements must be within this root element.
• All elements can have sub elements (child elements). Sub elements must be correctly nested within their parent element:
• Attribute values must always be quoted
• With XML, white space is preserved. In HTML multiple white space are collapsed to a single white space.
• With XML, CR / LF is converted to LF

Comments are the same as in HTML <!-- this is a comment -->

Don't put comments inside tags or before the prolog

An XML file must meet two criteria: it has to be well-formed and valid. Well-formed means that it corresponds to the rules outlined above. Browsers are pretty tolerant of poorly formed HTML, but XML documents have to be perfect.

Valid means that it conforms to a Document Type Definition (DTD). This is a template for the document.

These tell writers how to write documents (Example: the big news services have a DTD for news stories: anyone can write and submit a news story, but it has to conform to the DTD).

Here are some other examples.

A web site that searches the web for hard to find auto parts. They share a DTD with all of their suppliers.

Genealogical Markup Language (GedML) and the Chemical Markup Language (CML) are the examples of XML vocabularies. GedML describe ancestral data and CML describes chemical formulas and molecules.

XBRL (short for eXtensible Business Reporting Language) is an XML-based standard for handling corporate financial information.

XML is the basis for Web services like SOAP (Simple Object Access Protocol)

XHTML - a translation of HTML into XML

• XHTML elements must be properly nested
• XHTML documents must be well-formed
• Tag names must be in lowercase
• All XHTML elements must be closed, even those that we do not normally close, like <p>

<b><i>This text is bold and italic</b></i> works in HTML but not in xhtml

These are OK in HTML, but not in xhtml

This is a break<br>
Here comes a horizontal rule:<hr>
Here's an image <img src=''happy.gif'' alt=''Happy face''>

Here is the correct form for xhtml

This is a break<br />

Here comes a horizontal rule:<hr />

Here's an image <img src=''happy.gif'' alt=''Happy face'' />

There are some mandatory elements defined in the xhtml.dtd The root element is html

An XHTML document consists of three main parts (all required):

• the DOCTYPE
• the head
• the body

<!DOCTYPE html
PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN"
"http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">

There are three types STRICT TRANSITIONAL FRAMESET

The rules for writing a DTD

Here is a sample DTD for the library example

<!ELEMENT library (book*)>
<!ELEMENT book (title, author, publisher)>
<!ELEMENT title (#PCDATA)>
<!ELEMENT author (#PCDATA)>
<!ELEMENT publisher (#PCDATA)>
<!ATTLIST book isbn CDATA #REQUIRED>

A sequence of required elements is indicated by a list of element names delimited by commas. The second line of the example says that a book consists of one title, followed by one author, followed by one publisher.

An element which consists only of text is indicated by the keyword #PCDATA.

Attributes are indicated by an ATTLIST. This consists of the element name for which the attribute applies, the name of the attribute, the type of the attribute, (a string is indicated by the keyword CDATA), and then an optional keyword

The reference to the dtd in the xml document is done with the DOCTYPE element. If the dtd is in the same filesystem as the xml document, the DOCTYPE element has this syntax.

<!DOCTYPE root-element SYSTEM "filename">

<!DOCTYPE library SYSTEM "library.dtd">

The + indicates 1 or more
The ? indicates zero or 1
The | indicates choice (logical or)

here are some other keywords
EMPTY - an empty element
ANY - any content is acceptable

XML Schemas

DTDs have some limitations, for example consider this xml document.

<?xml version="1.0" ?>
<country>
 <name>France</name>
 <population>59.7</population>
</country>

<!ELEMENT country (name, population)>
<!ELEMENT name (#PCDATA)>
<!ELEMENT population (#PCDATA)>

Here is a schema for this, country.xsd

<xs:schema
 xmlns:xs="http://www.w3.org/2001/XMLSchema">
 <xs:element name="country">
  <xs:complexType>
   <xs:sequence>
    <xs:element name="name" type="xs:string"/>
    <xs:element name="population" type="xs:decimal"/>
   </xs:sequence>
  </xs:complexType>
 </xs:element>
</xs:schema>

Here are some of the built-in data types

• xs:string
• xs:decimal
• xs:integer
• xs:boolean
• xs:date
• xs:time

<xs:attribute name="lang" type="xs:string" use="required"/>

In schemas you can set minimum and maximum values

<xs:element name="age">
<xs:simpleType>
  <xs:restriction base="xs:integer">
    <xs:minInclusive value="0"/>
    <xs:maxInclusive value="100"/>
  </xs:restriction>
</xs:simpleType>
</xs:element> 

You can do enumerated types

<xs:element name="car">
<xs:simpleType>
  <xs:restriction base="xs:string">
    <xs:enumeration value="Audi"/>
    <xs:enumeration value="Golf"/>
    <xs:enumeration value="BMW"/>
  </xs:restriction>
</xs:simpleType>
</xs:element> 

Name Spaces

In large documents there may be name clashes.

Consider these two xml fragments

<table>
   <tr>
   <td>Apples</td>
   <td>Bananas</td>
   </tr>
</table>

<table>
   <name>African Coffee Table</name>
   <width>80</width>
   <length>120</length>
</table>

There are two elements named table with different structures and different meanings. To solve this problem, use the namespace declaration.

<h:table xmlns:h="http://www.w3.org/TR/html4/">
   <h:tr>
   <h:td>Apples</h:td>
   <h:td>Bananas</h:td>
   <h/:tr>
<h/:table>
<f:table xmlns:f="http://www.w3schools.com/furniture">
   <f:name>African Coffee Table</f:name>
   <f:width>80</f:width>
   <f:length>120</f:length>
</f:table>

Two name spaces are defined, h and f. Each element name is preceded by its namespace delimited by a colon. This ensures that f:table and h:table are treated as different elements by the xml processor.

Required Reading

The W3School has some wonderful tutorials Here is their XML tutorial Go through the entire Basic Tutorial and the Advanced Tutorial on Namespaces.

Also read their DTD tutorial.

This is not required reading, but Sun Microsystems has a good xml tutorial as well