Extensible Mark-up Language (XML) has very rapidly established itself as a viable technology with a huge range of real-world applications. One of the main reasons for its growing importance and wide acceptance is the fact that it offers a working solution to one of the key problems faced by software developers and computer users alike: the exchange of incompatible data. Each software program creates its own unique type of binary file which only it can understand. When data is exported in XML format, it becomes a known, clearly defined quantity, independent of the environment in which it was originated.
Adobe’s PDF format is another example of a platform-independent format which has gained worldwide acceptance. When a document is saved in PDF format, its format is set in stone, it can viewed and printed with its layout and formatting intact, without the need for the software which created the original document. However, whereas the PDF format concerns itself primarily with the presentation of information, XML is used to describe and encapsulate the information itself.
Though XML itself is still fairly new, the idea behind is over thirty years old. In the 1970s, Standard Generalized Markup Language (SGML) was developed in an attempt to create an application-independent method of describing and storing data. SGML is a text-based language which relies on the concept of adding mark-up to data which describes the data itself. An SGML document contains both the original data and a lexicon of rules defining the structure of that data. SGML is a fairly complex language and, unlike XML, has never gained wide popularity. In the early 1990s, SGML was used to develop and specify the rules of HyperText Markup Language (HTML) and in the late 1990s, SGML was again called upon, this time as the basis for the creation of XML. In a lot of ways XML is basically a restricted form of SGML.
XML has already proved itself an excellent medium for storing, describing and transporting data, particularly over the web. It offers flexibility, clarity and simplicity. An XML document looks similar to an HTML document and consists of the same human-readable tags. However, the tags used to markup an HTML document are pre-defined: only a limited set of tags can legitimately be used. XML allows you to create a markup language and define the tags which are legitimate for your data. It does this using a schema document, which can itself be an XML document. The schema document specifies the vocabulary and grammar which may be used within the XML document which contains your data.
The fact that, when creating and generating XML documents, you can invent all the rules, means that you never have to force your data into a container which was not designed to hold it. You design tags which reflect the nature of your data; you create a schema document which defines the hierarchical structure of your information; and you specify the type of information each element within your document is permitted to contain. In short, if you end up with an XML documents which is unsuitable for holding your information, you have only yourself to blame!
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