Introduction To Xml For Web Developers

Description

Surely, if you have decided to learn about XML, you are probably

already quite familiar with the concepts behind HTML (HyperText Markup

Language). So let's start from there.

HTML, as its name

implies, is a markup language. As such, it is used to markup text. But

what exactly does it mean to markup text?

Abstractly, marking

up text is a methodology for encoding data with information about

itself. Examples of markups (encoded data) are ubiquitous in the real

world.

For example, back when you were slogging through high

school, you probably used to use a bright yellow highlighter pen to

highlight sentences in your schoolbooks (or at last you knew someone

who did!). You did so because you thought that the highlighted

sentences would be useful to review around exam time and you wanted a

quick way to skim through the important points. Just like you,

thousands of kids around the world did the exact same thing for the

exact same reason.

By highlighting certain bits of text, you

were effectively "marking-up" the data. Essentially, you specified that

certain sentences (data) were important by marking them in yellow.

These sentences became encoded with the fact that they were important.

And

what's more, since everyone followed the same standard of marking up,

you could easily pick up a used text book and get a good idea just from

reading the highlighted sections what were core points of the book.

There

are two crucial points to take away from this example. For markups to

transmit useful information about data to a pool of users...

  
1. a
   
   standard must be in place to define what a valid markup is - In the
   
   example above, markup is defined as a bit of yellow ink atop text. In
   
   HTML a markup is a tag.
2. a standard must be in place to define
   
   what markup means - In the example above, a yellow highlight means the
   
   highlighted text represents an important point. In HTML each tag
   
   communicates its own layout of formatting meaning.

Markups

are also ubiquitous in the world of computers. They are used by word

processors to specify formatting and layout, by communications programs

to express the meaning of data sent over the wires, by database

applications that must associate meaning and relationships with the

data they serve, and by multimedia processing programs which must

express meta-data about images or sound.

As data is sent

through dumb computers and programs, it is essential that the data

carries with it information necessary to communicate what the data

means and/or what the receiver should do with that data.

Data with no context is meaningless just as an unhighlighted book is bad news around exam time!   

HTML

is one of the more famous computer markup systems. HTML defines a set

of tags that associate formatting rules with bits of text. Documents

which have been marked up (which contain plain text as well as the tags

that specify the rules for formatting that text) are read by an HTML

processing application (a web browser for example) that knows how to

display the text according to the rules.

For example, the

tag specifies a rule which instructs an HTML processing

application to bold a specific bit of text. Similarly, the

tag instructs the HTML processing application to center

the text.

Thus

BOLD would be displayed by an HTML processing application as   

  

BOLD  

You might imagine a client contact list which could look like the following bit of HTML code:   

  
Gunther Birznieks  
  
Client ID: 001  
Company: Bob's Fish Store  
Email: gunther@bobsfishstore.com  
Phone: 662-9999  
Street Address: 1234 4th St.  
City: New York  
State: New York  
Zip: 10024  
  
Susan Czigany  
  
Client ID: 002  
Company: Netscape  
Email: susan@eudora.org  
Phone: 555-1234  
Street Address: 9876 Hazen Blvd.  
City: San Jose  
State: California  
Zip: 90034  
  
  

  

The above HTML-encoded data would be displayed by an HTML processing application as:   

  
• Gunther Birznieks   
    
  • Client ID: 001   
  • Company: Bob's Fish Store   
  • Email: gunther@bobsfishstore.com   
  • Phone: 662-9999   
  • Street Address: 1234 4th St.   
  • City: New York   
  • State: New York   
  • Zip: 10024
    
• Susan Czigany   
    
  • Client ID: 002   
  • Company: Netscape   
  • Email: susan@eudora.org   
  • Phone: 555-1234   
  • Street Address: 9876 Hazen Blvd.   
  • City: San Jose   
  • State: California   
  • Zip: 90034

  

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