Extensible Markup Language (XML) is an open-standard language that defines rules for encoding data, that is machine-readable but also relatively easy for humans to read. XML separates document content, structure, and presentation, enabling the repurposing of data in a wide variety of ways. You can use XML to define a structure for documents and for data, and also use it to validate and enforce the structure. XML is essentially an agreement among people to store and share textual data using standard methods.
Where It Fits
XML is a platform-independent application development language, and as such, can be used in just about any area of Information Technology. You’ll probably encounter XML often in any web-development career.
Example of Usage
Suppose I gave you the following information:
Alex Chilton 52 Arkansas
Given just this information, you wouldn’t know what it was for or what it meant. XML provides a way to mark up this data so that it can be interpreted by other people, as well as by computer programs. So, the data above might be marked up like this:
<PLAYERINFO> <PLAYERNAME>Alex Chilton</PLAYERNAME> <PLAYERNUMBER>52</PLAYERNUMBER> <PLAYERUNIVERSITY>Arkansas</PLAYERUNIVERSITY> </PLAYERINFO>
Now you know a lot more about the meaning of this information. After the XML tags are added, you can tell that Alex Chilton is some kind of sports player, that his number is 52, and that he plays for the University of Arkansas. It might look a lot like HTML to you, but the markup above doesn’t work exactly like HTML. XML doesn’t tell us how content should look; instead, it gives it context and meaning.
O’Reilly School supports the use of XML, and all of our courses are written in XML. Separating the document structure from its presentation is important for us. Since our courses are written in XML, our course development staff doesn’t have to worry about formatting course content for the web, print, or any other particular technology. The presentation is left up to design professionals. Our authors don’t have to spend time adding unnecessary tags to their documents. And design professionals don’t have to spend time changing multiple individual documents so they have a uniform appearance. If we decide to change the look of the HTML representation of our tutorials, we only have to edit one XSL file. Or, if we decide to make our tutorials available in plain text format, we only have to create one XSL file. Good stuff!
Other companies have found powerful ways to use XML too. The internet has demonstrated that open technologies and formats are much more useful than proprietary formats. A file created on a word processor that saves documents in XML format can be read by another word processor that reads XML files. No conversion is necessary. That same file could also be opened and modified by a database system.
Finally, XML can be used as an intermediary between users. For example, chemists can use XML to share information with each other in a simple (yet powerful) format. Or banks can distribute financial information to their customers in an XML format that all programs can read.
XML is a product of the Generalized Markup Language (GML) that IBM created in the 1960s. GML was created to provide a standard way to mark up textual documents for portability across different platforms. This proved to be a difficult task, but after nearly 20 years of research, the International Organization for Standards adopted IBM’s GML and called it SGML, or Standard Generalized Markup Language. SGML was the basis for complex documentation systems. Unfortunately SGML is a little too complex for general use.</P>
HTML was created in the early 1990s as a subset of SGML. It was designed to be a simple markup language to facilitate the transfer of web-based documents. But HTML’s simplicity can be limiting. Consider the different HTML that is sometimes required to display similar content on different browsers.
In 1996, XML was created to resolve the problems of HTML and the complexities of SGML. XML’s strength was in its organization. Document structure and presentation were finally separated, while the benefits of standard markup procedure were maintained.
XML at the O’Reilly School of Technology
You would probably consider learning XML as part of a larger course of study for a career in application development. At the O’Reilly School of Technology, you can learn the fundamentals of XML for use with XML-enabled applications or general web use. We provide a beginner-level introduction to XML that prepares you with a foundation in one of the key elements of web programming. For a full description of the course or the certificate program that includes it, click one of the links below. If you have any questions or would like additional guidance, don’t hesitate to contact us—we’re here to help!