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Extensible Markup Language
So far we have seen how to store data in Javascript (Chapter 3), text files (Chapter 4), and structured tables (Chapter 5). XML represents yet another way to store data. Although somewhat verbose, an XML document is both human-readable and machine-readable. It may be regarded as something intermediate between a text file and a structured table.Whereas an SQL table is two-dimensional, an XML document is hierarchical. While SQL operates at the server, XML is a client technology as much as a server technology. In general, SQL should be used for very dynamic data because of its storage efficiency, while XML should be used for static content. Where it gives significant advantages in retrieval efficiency, storage space and security, SQL is more appropriate. However, to the human coder, with XML, data can be easily prepared, transformed and transported.
The following is an example of an XML document. Containing different tags and attributes, an XML document resembles a HTML document, except that the names for the XML tags and attributes are defined by the coder.
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error-handling.html:
A well-formed XML document must:
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error-handling.html:
<bookslist>
<book id="123321">
<title>Harry Potter</title>
<author>J. K. Rowling</author>
<ISBN>9780545010221</ISBN>
<price>22.90</price>
<bestseller/>
</book>
<book id="456654">
<title>Fantasy in Death</title>
<author>J. D. Robb</author>
<ISBN>9781101185360</ISBN>
<price>7.81</price>
</book>
</bookslist>- have a root element. In the example above, <booklist> is the root element.
- close all elements. This means that for each opening tag <tag>, there must be a closing tag </tag>. Empty tags can be abbreviated by <tag/>.
- quote all attributes values. In the example above, ‘id’ is an attribute for the element <book>, and its value (123321) must be quoted.
- be case-sensitive in tags.
- escape some special characters like < and & when they are not markup. In XML, there are only five built-in character entities: <(<), >(>), &(&), '(‘), "(“). Additional entities may be defined in a DTD as we shall see later.
- use hexadecimal references for any Unicode character, such as &.
Note that white space is preserved in XML and not truncated as in HTML.
You can use the browsers to view an XML document. We recommend using Altova XMLSpy or OxygenXML to process XML documents in various ways as described later in this chapter. A free plugin for Notepad++ called XML Tools, available at http://sourceforge.net/projects/npp-plugins/files/XML%20Tools/, can be used to format XML documents, perform XSD validation, apply XSLT etc.