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INTRODUCTION TO XML

Basics
In this unit you will learn about XML technologies.  Before you look into the specifications of XML it is important to understand why XML exists and its advantages.

 HTML, the most popular markup language, has been designed to display data and specify how that data should look. XML has been designed to describe and structure data. XML itself does not actually describe how to display data or what to do data.  It allows you to structure the data in a standard manner. This enables other systems to interpret XML. In other words it provides a ‘interoperable file format" which can be interpreted using other languages. XSL is the popular language used to transform/interpret XML.

CHARACTERISTICS OF XML

 

XML stands for ‘Extensible Markup Language’. XML can be understood as a general-purpose specification used for creating custom markup languages. It is classified as an extensible language as it allows its users to define their own elements. Thus it enables users to create custom tags that suit their requirement. XML has been primarily developed to information systems share their structured data online. It can be used to encode documents as well as to serialize data so that it can be efficiently used.  Some of the features of XML have been summarized below:

1.  XML can be understood as an extensible language which is freely available.

2.   XML tags are user made tags. They are not predefined tags. In case of HTML predefined tags are used (like ,

, etc.). While working with XML author can define his own tags and develop own document structure.


3.  XML is a Complement to HTML

4.  XML is not a replacement for HTML. Both scripting languages have their own purpose. As web is developing XML is being popularly used to describe the data and HTML will be used to format and display the same data.

5.  XML has been inherited from SGML

SGML stands for Standard Generalized Markup Language. It is an ISO standard that defines an extremely powerful markup language. It is popularly used in the  publishing industry and large manufacturing companies. It can be understood as the meta language that can be used  to create other markup languages such as HTML. It marks the origin of XML.


XML is a markup language like the Hypertext Markup Language (HTML) which is commonly used for scripting web page.  XML is specifically designed to describe data so that it can be effectively stored online. Web today contains such vast information. XML enables structuring of data so that it can then be mined to get suitable information.  In case of XML unlike HTML there are no predefined tags.  XML can also be called as self-descriptive markup language as users need to define their on tags.

For better understanding check out the example below:

Suppose you are storing information about a set of books. You may store the information in html as follows:
 Book.html

< html > 
< head >  <title > Storing Information< /title > 
< body> 
< p >  Linear Programming by A.S. Bajaj < br > 
 < p >  Marketing Research by Kotler </p > 
< /body > 
< /html > 



















In the above example you can see that you can easily define data in an XML file. The file shows that there is a catalogue containing books, each of which contains title and author detail of the book. You can see that XML the file size in XML is more then the other file size. You may feel that XML will loss in efficiency those results from this increased size. However XML makes this loss by speeding up the processing of a well-defined XML file.  The way you interpret an html file is dependent on the pre-defined tags available in html. In contrast XML file tags are user defined and represent a piece of information in a hierarchical manner. Such kind of data which describes is also called metadata. Such data provides great strength to XML as you can easily create your own specifications and structure the data in the way you want it to be interpreted by any other system.

3.2.1 Difference between HTML and XML

It is very important for you to understand that you will not use XML for replacing  HTML. Both XML and HTML have been designed for different goals which can be summarized as follows:

  1. XML is designed specifically to describe data where as HTML is designed to display data.
  2. XML is focused on defining data with its attributes. It basically tells what data is all about. HTML is focused on presentation of data and is used to customize looks of data.
  3.  In case of HTML Document tags to be used and the structure of the documents are predefined. You can only use tags that are defined in the HTML standard. In case of XML you can define your own tags and develop your own document structure.
  4.  An XML document is saved with an extension .XML whereas an HTML document is saved as .HTML.

E.g.    The following example is an  e-mail from Ram to Shyam stored as XML
 
Ram
Shyam 
Hi how are you?
Let’s go for a New Year party


In above example e-mail has been stored using XML markup language. You can see that own tags have been created to store the names of sender and receiver. Similarly different tags have been created to store the subject and content of the web page.

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