Tech. Board

Introduction to XML
Supreeth P (Final Year ECE)

Objectives:
ð* Introduction to XML?
ð* Why not SGML
ð* What is XML

ð* Why XML?

Markup is not a concept that evolved with HTML or the World Wide Web. It was there, right from the time the print industry evolved. Every time a reporter submitted an article, the Editor or the Sub-editor used to go through the matter and then they used to make certain markings on the text. These markups used to indicate to the typesetter which part of the text will be printed in bold, which part will be printed in Italics and which part will be made a heading or a sub-heading. In short, markups were used to describe how the information is to be displayed.

When Tim Berners-Lee conceived the World Wide Web, he needed a language that would be useful to display information on the Web. That is why the Hyper Text Markup Language was developed. HyperText Markup Language as we all know is nothing but a set of tags with the help of which content of web pages can be dressed up in such a way that it becomes readable by a browser.

In the olden days, when web pages were few, the limited HTML tags were all right. But as the number of web pages on the web went on increasing it became increasingly difficult to squeeze data within a given set of tags. Also it was very difficult to search for data given the generalized nature of the tags and the large amount of information on diverse topics available on the web.

Every time a browser encounters a tag, it reads it, interprets it and renders the text accordingly on the screen. Now how does a browser understand the meaning, properties, attributes and values of every tag? This is done with the help of a Document Type Definition, which is built into the browser. Every time the browser comes across an HTML tag it refers to this DTD to understand the meaning of the tag and then renders the text accordingly on the screen. The World Wide Web Consortium better known as the W3C has defined the various HTML tags and their meanings. All the browsers recognize these tags. However, there are certain tags, which are proprietary and are not recognized by all the browsers. For example, the <MARQUEE> tag is recognized by Internet Explorer but not by Netscape Navigator. Similarly, the <BLINK> tag is supported only by Netscape Navigator and not by Internet Explorer. These kind of proprietary tags kept causing cross-browser compatibility problems. So how does one take care of such problems?

It was because of all these problems faced that a need was felt for a language that will allow you to create your own set of tags and define their attributes, meaning, behavior. XML helps you to do exactly this. It is a language that helps you to define your own tags. As we go ahead we will get an overview of the various aspects of XML.

ð* What is XML?

Extensible Markup Language (XML) is a meta-markup language, which provides a format for describing structured data. This makes possible precise declaration of content and also makes web content more searchable. XML allows you to create your own set of tags for describing data. HTML is used for displaying data while XML is used for describing data. Let us understand this with an example.

<B>THIS TEXT IS IN BOLD</B>
<I> THIS TEXT IS IN ITALICS</I>

The tags given above are HTML tags. The <B> tag tells the browser that the content of this tag is to be displayed in bold and the <I> tag tells the browser that the text is to be displayed as italicized. In other words, the HTML tags tell the browser how the text is to be displayed. On the other hand XML tags will look something like this.

<PERSONAL>
<NAME> SANKALP </NAME>
<AGE> 22 </AGE>
<COMPANY> CLARIENT </COMPANY >
</PERSONAL>

As you can see here the tag <NAME> tells the browser that the content given within it is the name and the content of the AGE element is the age of that person.

This makes it much easier to search data over the web as tags take on a specific meaning. In the above case, when a search engine comes across the number 22, it knows, because of the tags surrounding it that the number 22 represents age and not someone’s roll number. Given the number of irrelevant results that a search engine returns for every search query entered, this benefit itself should present a strong case in favour of XML.

Also to an extent it would help to solve the problem of cross browser compatibility. This is because the meaning, attributes, possible values etc. of each tag and the structure of the document is defined within a Document Type Definition which the browser can refer to before rendering the text. This makes the page browser independent to an extent because the browser does not have to refer to it’s own Document Type Definition for understanding the tags as is the case while reading HTML files.

XML provides you with one more advantage. It allows you to separate the presentation from the data. The XML file is only used to store the data. The presentation is defined in a separate file called a style sheet, which is stored with the extension ‘.XSL’. This helps us to separate data from the presentation. In the olden days, when the Internet had just started it was only used to transfer text based data. But as the WWW came into the picture it became possible to add multimedia features to the web pages. It was because of this that the World Wide Web started gaining popularity among the masses. Media companies realized the tremendous potential that the web held as a mass medium and started using it as a channel of advertising, entertainment as well as information transfer. As professional media companies got on to the web the presentation gained as much importance as the information. Nobody came to a web-site to read realms and realms of text. Information had to be presented in a very interactive and attractive manner. The look of the site gained importance. Web designing became a professional job, which required a specialist Graphics designer, specialist content writers, web page developers, web programmers etc.. As the amount of work increased it became increasingly necessary to separate content i.e. data from the presentation. XML allows you to do exactly this.

XML also makes data shareable between applications. This is because the structure of the document is stored in a separate file in the form of a Document Type Definition or a Schema. Once this structure is known anybody can write an application to read this data, understand it and can also define his own stylesheet to present this data in a different manner. This feature would make data easily shareable between applications. The benefits of using XML between computer systems and businesses are probably the greatest and easiest to achieve: XML allows businesses to define data formats in XML, and easily build tools which read data, write data and transform data between XML and other formats. This has allowed a number of businesses and industry consortiums to build standard XML data formats. Areas such as Electronic Data Interchange (EDI), inter-bank payments, supply-chain, trading, and document management are all the subject of ongoing XML-based standardization by industry consortiums.