XML 101 by Jeff Jones
Whether you're discussing e-commerce, knowledge management, or the Internet in
general, you've likely seen or heard reference to eXtensible Markup Language
(XML). XML is, without a doubt, one of the most heralded technologies to come
across the wire in recent years (pun intended). What is XML? Why is it creating
such a deluge of interest? What should you know about XML, and perhaps more
importantly, why should you even care about it? In this article, I will provide
a high-level description of what XML is (and what it's not), discuss the key
components of an XML document, and provide a compelling argument for why it's
well worth your while to learn more about XML.
To understand XML, it's helpful to compare and contrast it with another
technology with which a great many of us are familiar - HyperText Markup
Language (HTML).
If you've used or read about HTML, you know that it was created so that users
could format and display information on the Web. HTML uses a fixed and finite
set of tags, elements, and attributes that allow it to communicate to a user's
browser how that browser should display the document. We see HTML everywhere,
and it has for some time served as the lingua franca for displaying information
on the Web. It is a proven technology that well serves its purpose in most
scenarios. What if, however, the current version of HTML doesn't allow me to do
something that I want to do? I have two choices: I can either write my own
browser that understands my tags (bad idea) or I can put my project on hold for
a year or so and hope that the next version of HTML includes the functionality
that I need (even worse idea). Try selling either of these options to your boss
or client and see if you still have a job by the time you end your discussion.
So concludes our one-paragraph, in-depth investigation of HTML.
Now, if I may lapse into my days of standardized test taking, HTML is to
displaying information as XML is to defining information. They both are
text-based, and they both consist of tags, elements, and attributes. Unlike
HTML, however, XML allows users to structure and define the information in
their documents. While technically it is a markup language (it allows you to
use tags to "mark-up" the contents of your document), it more appropriately is
a meta-language. By meta-language, I mean that it allows users to create their
own collection of tags, elements, and attributes as needed and in so doing to
accurately describe the physical contents of a document. Unlike HTML with its
finite collection of tags, XML allows users to create their own to meet their
own requirements (thus the eXtensibility).
I've made several references to tags, elements, and attributes. These are the
core building blocks of an XML document. Consider the following HTML fragment.
It should be painfully familiar to anyone who's ever looked at an HTML document
and will prove useful in understanding XML syntax.
<table border="0" cellpadding="0" cellspacing="0"> <tr> <td
width="50%">Here is the first group of text< d> <td
width="50%">Here is the second group of text< d> < r> <
able>
This document contains a table element ("<table>") with a table row
element ("<tr>"). The table row element, in turn contains two table cell
elements ("<td>"). Each of these elements has both an opening tag
("<table>") and a closing tag ("< able>"). While this is fairly
straightforward, it also is somewhat inflexible. What if, for example, I need
to create a document that describes my company's employee roster for the Annual
InfoStrat softball tournament? With XML, it's as easy as replacing the element
and attribute names from the previous HTML document with my own custom tags
that describe my company and its employees. Here is what such a document might
look like:
<?xml version="1.0"?> <company name="Information Strategies">
<employees> <employee id="1">Hank Aaron</employee>
<employee id="2">Babe Ruth</employee> </employees>
</company>
With this XML document, I have defined my company and two of its employees and
have described the relationship between company (parent) and employees
(children). I have shown that my company has two employees, but I easily could
add new employee elements to reflect new hires that we bring on to ensure that
we don't lose this year's tournament:
<employee id="3">Mickey Mantle</employee> <employee id="4">Ty
Cobb</employee>
After creating my XML document, I can display its contents in my format of
choice. The same XML document could easily be displayed as HTML, a Microsoft
Word document, an Adobe .pdf file, or even as text in the body of an e-mail
message. As long as the XML document is well formed (meaning that it follows
the appropriate XML format and syntax), you can choose your method of
preference (or necessity) for displaying its content.
Let's dissect the pieces of my company roster XML document to see each piece's
role and responsibility.
Header:
The header tells the document's user that this is an XML document - using
version 1.0 of the XML specification in this case.
<?xml version="1.0"?> <company name="Information Strategies">
<employees> <employee id="1">Hank Aaron</employee>
<employee id="2">Babe Ruth</employee> </employees>
</company>
Tags (brackets, greater than, less than):
Just like in HTML, you use greater than (">") and less than ("<") signs
called tags to indicate the opening and closing of an element.
<?xml version="1.0"?> <company name="Information Strategies">
<employees> <employee id="1">Hank Aaron</employee>
<employee id="2">Babe Ruth</employee> </employees>
</company>
Elements:
Elements are the basic building blocks of XML. They may contain text, comments,
or other elements, and consist of a start tag and an end tag. Typically, XML
elements are akin to nouns in the real world. They represent people, places, or
things.
<?xml version="1.0"?> <company name="Information Strategies">
<employees> <employee id="1">Hank Aaron</employee>
<employee id="2">Babe Ruth</employee> </employees>
</company>
Note that in XML, every opening element (i.e. "<company>") must also
contain a closing element (i.e. "</company>"). The closing element
consists of the name of the opening element, prefixed with a slash ("/"). XML
is case-sensitive. While "<company ></company>" is well-formed,
"<COMPANY></company >" and "<Company></cOMPANY >" are
not. Also, if the element does not contain text or other elements, you may
abbreviate the closing tag by simply adding a slash ("/") before the closing
bracket in your element (i.e. "<company></company>" can be
abbreviated as "<company />"). In addition to the rules defining opening
and closing tags, it is important to note that in order to create a well-formed
XML document, you must properly nest all elements. The previous document
properly nests the "<employee>" elements within the "<employees>"
element, but the following would not be acceptable in XML because the second
"<employee>" element exists outside of the "<employees>" element:
<employees> <employee id="1">Hank Aaron</employee>
</employees> <employee id="2">Babe Ruth</employee>
Attributes:
Where elements represent the nouns contained in an XML document, attributes
represent the adjectives that describe the elements. The following document
tells me that Hank Aaron's id is "1" and that Babe Ruth's is "2". This helps to
describe these two employees.
<?xml version="1.0"?> <company name="Information Strategies">
<employees> <employee id="1">Hank Aaron</employee>
<employee id="2">Babe Ruth</employee> </employees>
</company>
Note that in order to be well formed, all attribute values must be contained
within quotation marks. id="1" is correct, while id=1 is not acceptable. This
is a marked difference from standard HTML formatting that places much looser
restrictions on what is acceptable.
Text/Content:
Elements contain contents that give critical information about them. This
information represents that entity itself in an XML document. In the following
document, Hank Aaron is the employee; Babe Ruth is the employee. <?xml
version="1.0"?> <company name="Information Strategies">
<employees> <employee id="1">Hank Aaron</employee>
<employee id="2">Babe Ruth</employee> </employees>
</company>
As you can see, XML and HTML are practically identical with the exception that
XML is far less lenient when it comes to case-sensitivity, using closing tags,
and properly nesting parent/child elements. This is excellent news for Web
developers everywhere as it ensures that if you write well-formed HTML, you'll
find the transition to XML virtually seamless.
To summarize, XML is a text-based meta-language that uses tags, elements, and
attributes to add structure and definition to documents. It is similar to HTML
in syntax and implementation, but different with regard to functionality. Where
HTML allows users to control how documents are displayed, XML allows them to
describe the actual contents of the documents. It is a markup language because
it uses tags to mark-up documents and it is a meta-language because it uses
these tags to give structure to documents that it in turn uses as a means of
communication. XML is extensible because it enables users to create their own
collection of tags (unlike HTML).
Now, why should you care about XML? If for now other reason, consider that the
World Wide Web Consortium (W3C), the Internet's governing body, is considering
a proposal to rewrite the HTML 4 language in XML 1.0. As of the time this
article was written, XHTML had received endorsement by the director of the W3C
as a recommendation. This proposal, known as XHTML will require well formedness
in all HTML documents. The W3C is a neutral standards body responsible for
defining the future of the Internet. They do not support every new idea that
comes along, and we should view their full support of XML (or any technology),
as a harbinger of where tomorrow's Internet will take us. Ignore XML if you
will, but know that it is most definitely a legitimate technology that will
revolutionize the way that we program applications for the Web
About the Author
Jeff Jones For more information on XHTML, XML, and the W3C, check out the W3C
website at http://www.w3c.org.
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