2.8.2 XML (eXtensible Markup Language) as a data format

XML (eXtensible Markup Language) is a standard from the world wide web consortium ⁸ and still evolving. There are many introductions to XML and many books are written about XML. XML in 10 points is an XML introduction which is used here and can also be read at http://www.w3.org/XML/1999/XML-in-10-points

1. XML is for structuring data
   XML has basically two parts: the definition of a structure and one or many instances of it. First the structure is defined. An address for example has typically one street name, one street address, one country, one zip code, one area code etc. An instance address which conforms to the structure is created next. XML Schema provides data type definition and inheritance.
2. XML looks a bit like HTML
   XML also uses tags like in HTML. The advantage is, that the tags can be defined. A tag can have attributes. A tag typically has attributes and content and/or sub tags (called child elements).
3. XML is text, but isn't meant to be read
   It is very convenient to have XML human readable. But XML is mainly used as application to application data format. At both ends (application) XML parsers parse the XML document for further processing its content.
4. XML is verbose by design
   As mentioned above, XML is text based. The XML document can be read. To transfer the XML document (file) it can be compressed to save bandwidth.
5. XML is a family of technologies
   There are many further technologies in connection with XML. XSLT adds a style-sheet with rules to an XML document. With a XSLT style-sheet XML can be converted to HTML by simple rules. XPointer can point to parts within an XML document. XLink is used to link/reference other XML documents.
6. XML is new, but not that new
   XML is a subset of SGML which was developed in the early '80s. SGML is too complicated for general usage and XML provides a good subset.
7. XML leads HTML to XHTML
   HTML is a subset of XML or a defined vocabulary with elements like $<p>$ for paragraph etc. XHTML is XML-based but has more or less the same elements as general HTML.
8. XML is modular
   An XML element can have multiple child elements. And each child element can have further child elements. When an XML document uses more than one XML definition it might happen, that there are elements with the same name. The solution to avoid name collision is to use a namespace mechanism.
9. XML is the basis for RDF and the Semantic Web
   The Semantic Web Group wants to add semantic to the content of the web. A browser does not know what an address is. If the browser could understand that the content of the address element is a geographical address the browser/computer could understand the content of web sites. RDF (Resource Description Framework) will address the semantic meaning of web.
10. XML is license-free, platform-independent and well-supported
    XML is an open standard. Because of its nature to define data structure it is platform and programming language independent. There are free available parsers in almost any programming language on almost any platform.

It is possible to visit the w3c.org web site for the specifications of XML and related XML technologies. XML technologies are used by ebXML. These are typically XML, XSLT, XLink, XPointer, XML namespaces, SOAP, and XML Encryption.

Footnotes

... consortium⁸

http://www.w3c.org