Refresher: XML

XML is a author-defined meta markup language with which you create an internal structure for containing data, quite similar to a database. XML has been touted as a "replacement" for HTML but it extends far beyond that -- XML is a data exchange format from which you can manipulate data in any number of ways, from internal processing to external display (the HTML connection).

An XML document is composed of data embedded within markup tags, quite reminiscent of an HTML document, but the tags may be self-defined. These tags define the internal structure of the embedded data, quite analogous to a database containing records and fields, based upon rules set forth in a separate document known as the DTD, or Document Type Definition. Simple XML files with straightforward rulesets do not require an explicitly defined DTD. A very easy way to understand the root concept of XML is to look at a simple XML example, a file we've named "petfolio.xml":

<?xml version="1.0"?> <petfolio> <pet name="ella dibella"> <breed>Domestic Shorthair</breed> <color>black/tan tiger</color> <age units="months">3</age> <weight units="lbs">1.75</weight> <description>Ella is a smart, poised creature full of courage and vinegar. </description> </pet> <pet name="borders mcgee"> <breed>Domestic Shorthair</breed> <color>black/white tuxedo</color> <age units="weeks">10</age> <weight units="lbs">1.5</weight> <description>Borders is a playful, submissive jester. </description> </pet> </petfolio>

The above XML document does not require an external DTD -- it's ruleset can be implied for the basic structure of the document. The root object of this dataset is named petfolio, and we can imagine that it represents a portfolio of pets. Each pet in this portfolio is essentially a record, if you are familiar with database lingo.

A pet "record" is bookended by <pet> tags. One of the great powers of XML is that you can define these tags at will, as long as the structure of the data obeys rules set out in the DTD or which can be implied simply from the XML document itself. One rule in this document, for example, is that the <pet> tag occurs within the <petfolio> tag, and all other tags in the document reside within the <pet> tags. In other words, we've created a database with records which may contain fields.

The "fields" in this case are <breed>, <color>, <age>, <weight>, and <description>. Some of these tags possess attributes, which contain values relevant to the processing of that tag; for example, the <age> tag possess a units attribute indicating the units in which this tag's value ("10") is measured on. The same can be observed for the <weight> tag, and the <pet> tag itself, which assigns an identifier to the record via the name attribute.

Visually speaking, the entire petfolio XML document can be represented as a tree ... literally, that is, the kind of tree which grows in your yard.

Refresher: Object-oriented programming; e.g. JavaScript
XML via the Document Object Model: A Preliminary Course
A Tree Grows in XML