A Constellation Standards of XML

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  Many people agree that spending money is generally more fun than saving it. Sure, you can get a little thrill looking at your bank statement and seeing the dividend from the 3% interest on your savings account, but it isn't as exciting as buying a new plasma screen television. So it is with XML. It contains information like a safe holds money, but the real fun comes from using that information. Whether you're publishing an XHTML document to the Web or generating an image from SVG, the results are much more gratifying than staring at markup.

XML's extended family provides many ways to squeeze usefulness out of XML documents. They are extensions and applications of XML that build bridges to other formats or make it easier to work with data. All the names and acronyms may be a little overwhelming at first, but it's worth getting to know this growing family.

Let's look at these categories in more detail.

Core syntax

These are the minimal standards required to understand XML. They include the core recommendation and its extension, Namespaces in XML. The latter piece allows you to classify markup in different groups. One use of this is to combine markup from different XML applications in the same document.

Human documents

This category has markup languages for documents you'll actually read, as opposed to raw data. XHTML, the XML-friendly upgrade to the Hypertext Markup Language, is used to encode web pages. DocBook is for technical manuals which are heavy in technical terms and complex structures like tables, lists, and sidebars. The Wireless Markup Language (WML) is somewhat like XHTML but specializes in packaging documents for tiny screens on cellular phones.

Modeling

In this group are all the technologies developed to create models of documents that formalize a markup language and can be used to test document instances against standard grammars.

Locating and linking

Data is only as useful as it is easy to access it. That's why there is a slew of protocols available for getting to data deep inside documents. XPath provides a language for specifying the path to a piece of data. XPointer and XLink use these paths to create a link from one document to another. XInclude imports files into a document. The XML Query Language (XQuery), still in drafts, creates an XML interface for non-XML data sources, essentially turning databases into XML documents.

Presentation

XML isn't very pretty to look at directly. If you want to make it presentable, you need to use a stylesheet. The two most popular are Cascading Style Sheets (CSS) and the Extensible Style Language (XSL). The former is very simple and fine for most online documents. The latter is highly detailed and better for print-quality documents.

Media

Not all data is meant to be read. The Scalable Vector Graphics language (SVG) creates images and animations. The Synchronized Multimedia Integration Language (SMIL) scripts graphic, sound, and text events in a timeline-based multimedia presentation. VoiceML describes how to turn text into speech and script interactions with humans.

Science

Scientific applications have been early adopters of XML. The Chemical Markup Language (CML) represents molecules in XML, while MathML builds equations. Software turns instances of these markup languages into the nicely rendered visual representations that scientists are accustomed to viewing.

Resource description

With so many documents now online, we need ways to sort through them all to find just the information we need. Resource description is a way of summarizing and showing relationships between documents. The Resource Description Framework (RDF) is a language for describing resources.

Communication

XML is an excellent way for different systems to communicate with each other. Interhost system calls are being standardized through applications like XML-RPC, SOAP, WSDL, and UDDI. XML Signatures ensures security in identification by encoding unique, verifiable signatures for documents of any kind. SyncML is a way to transfer data from a personal computer to a smaller device like a cellular phone, giving you a fast and dependable way to update address lists and calendars.

Transformation

Converting between one format and another is a necessary fact of life. If you've ever had to import a document from one software application into another, you know that it can sometimes be a messy task. Extensible Style Language Transformations (XSLT) can automate the task for you. It turns one form of XML into another in a process called transformation. It is essentially a programming language, but optimized for traversing and building XML trees.

Development

When all else fails, you can always fall back on programming. Most programming languages have support for parsing and navigating XML. They frequently make use of two standard interfaces. The Simple API for XML (SAX) is very popular for its simplicity and efficiency. The Document Object Model (DOM) outlines an interface for moving around an object tree of a document for more complex processing.

This list demonstrates that XML has worked well as a basis for information exchange and application in a variety of fields.

 

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