Editing Simple API for XML (section)

== Drawbacks ==
The event-driven model of SAX is useful for XML parsing, but it does have certain drawbacks.

Virtually any kind of [[XML validation]] requires access to the document in full. The most trivial example is that an attribute declared in the [[Document Type Definition|DTD]] to be of type IDREF, requires that there be only one element in the document that uses the same value for an ID attribute. To validate this in a SAX parser, one must keep track of all ID attributes (any one of them ''might'' end up being referenced by an IDREF attribute at the very end); as well as every IDREF attribute until it is resolved. Similarly, to validate that each element has an acceptable sequence of child elements, information about what child elements have been seen for each parent must be kept until the parent closes.

Additionally, some kinds of XML processing simply require having access to the entire document. [[XSLT]] and [[XPath]], for example, need to be able to access any node at any time in the parsed XML tree. Editors and browsers likewise need to be able to display, modify, and perhaps re-validate at any time. While a SAX parser may well be used to construct such a tree initially, SAX provides no help for such processing as a whole.