XML Demystified

CHAPTER 1 XML: An Inside Look
For example, you’d use a parser to retrieve book information from the books
XML document that you saw earlier in this chapter. The extracted book information
is then combined with HTML code to create a dynamic web page that displays the
information about the book on the screen.
Developers use one of two basic parsers. These are the Document Object Model
(DOM) parser and the Simple API for XML (SAX).
DOM reads the entire XML document into memory and then creates a tree
structure of elements (see Figure 1-1). Various techniques are used to traverse the
tree to local information contained in the XML document. You can also use DOM
to write data to the XML document, but it’s limited to working with small XML
documents because the entire XML document is placed in memory.
SAX reads the XML document, noting the locations of markup tags. The SAX
parser makes a one-time pass through the XML from start to finish. As it encounters
tags and data, it calls events that you define in your code. SAX is ideal for reading
large XML documents because there aren’t any memory constraints; only a chunk
of the XML document is ever in memory at one time. A drawback of SAX is that it
cannot traverse an XML document. That is, SAX makes one pass through the
document. If you want to return to a previous part of the document, then the
document needs to be read from the beginning of the documents.
Both DOM and SAX validate the contents of an XML document against a DTD.
You’ll learn more about DOM and SAX later in this book.
Figure 1-1 DOM transforms elements of an XML document into a tree structure,
enabling the parser to traverse elements.
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