Why Is XML Important?

Why Is XML Important?

There are a number of reasons for XML's surging acceptance. This section lists a few of the most prominent.

Plain Text

Because XML is not a binary format, you can create and edit files using anything from a standard text editor to a visual development environment. That makes it easy to debug your programs, and it makes XML useful for storing small amounts of data. At the other end of the spectrum, an XML front end to a database makes it possible to efficiently store large amounts of XML data as well. So XML provides scalability for anything from small configuration files to a company wide data repository.

Data Identification

XML tells you what kind of data you have, not how to display it. Because the markup tags identify the information and break the data into parts, an email program can process it, a search program can look for messages sent to particular people, and an address book can extract the address information from the rest of the message. In short, because the different parts of the information have been identified, they can be used in different ways by different applications.

Stylability

When display is important, the stylesheet standard, XSL, lets you dictate how to portray the data. For example, consider this XML:

you@yourAddress.com  

The stylesheet for this data can say

Start a new line.
Display "To:" in bold, followed by a space
Display the destination data.

This set of instructions produces:

To: you@yourAddress  

Of course, you could have done the same thing in HTML, but you wouldn't be able to process the data with search programs and address-extraction programs and the like. More importantly, because XML is inherently style-free, you can use a completely different stylesheet to produce output in Postscript, TEX, PDF, or some new format that hasn't even been invented. That flexibility amounts to what one author described as "future proofing" your information. The XML documents you author today can be used in future document-delivery systems that haven't even been imagined.

Inline Reusability

One of the nicer aspects of XML documents is that they can be composed from separate entities. You can do that with HTML, but only by linking to other documents. Unlike HTML, XML entities can be included "inline" in a document. The included sections look like a normal part of the document: you can search the whole document at one time or download it in one piece. That lets you modularize your documents without resorting to links. You can single-source a section so that an edit to it is reflected everywhere the section is used, and yet a document composed from such pieces looks for all the world like a one-piece document.

Linkability

Thanks to HTML, the ability to define links between documents is now regarded as a necessity. Appendix B discusses the link-specification initiative. This initiative lets you define two-way links, multiple-target links, expanding links (where clicking a link causes the targeted information to appear inline), and links between two existing documents that are defined in a third.

Easily Processed

As mentioned earlier, regular and consistent notation makes it easier to build a program to process XML data. For example, in HTML a

tag can be delimited by

, another

,

, or . That makes for some difficult programming. But in XML, the

tag must always have a

terminator, or it must be an empty tag such as

. That restriction is a critical part of the constraints that make an XML document well formed. (Otherwise, the XML parser won't be able to read the data.) And because XML is a vendor-neutral standard, you can choose among several XML parsers, any one of which takes the work out of processing XML data.

Hierarchical

Finally, XML documents benefit from their hierarchical structure. Hierarchical document structures are, in general, faster to access because you can drill down to the part you need, as if you were stepping through a table of contents. They are also easier to rearrange, because each piece is delimited. In a document, for example, you could move a heading to a new location and drag everything under it along with the heading, instead of having to page down to make a selection, cut, and then paste the selection into a new location.