OpenEdge Development: Working with XML - Product ...

Developing with XML in OpenEdge
Developing XML-enabled ABL applications with SAX
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The Simple API for XML (SAX) is an application programming interface (API) for XML
documents. It was developed by a group of programmers, and even though it is not a W3C
recommendation, it is widely used industry standard.
SAX is a streaming model that processes one element at a time and provides mechanisms for
you to respond to the current element before it is flushed and the parser moves on to the next
element. Contrast this with the memory-resident model of DOM.
When an XML document is accessed by a SAX application, as the XML parser encounters an
XML element, it parses that element and provides its information to the application
immediately, through a callback procedure. The callback procedure allows you to provide
custom programming on how to handle each type of information that the parser provides to your
program. On the other hand, if you do not have any custom code, you do not need to provide
callback procedures. The parser’s default behavior is to simply read the whole document.
So, while the DOM API decomposes an XML document into a set of nodes in a hierarchical
tree, the SAX API decomposes the document into a series of procedure calls. Your application
must act on the information presented as it is provided to the application. SAX can only stream
forward during the parse. At the conclusion of the parse, nothing is left in memory.
As an example, if you were parsing a list of address changes intended for your database, your
application would need to validate and update each change as you retrieved it from the parse.
The SAX technology built into OpenEdge also allows you to write an XML document in a
forward-streaming way.
SAX advantages
The general advantages of SAX include:
• The nature of a streaming model means that you need far less memory to process large
XML documents.
• You do not have to process the entire document. Use callback procedures to identify and
respond to only the XML elements you are interested in.
• You can halt the parse at any time.
• You can parse an XML document that is not well formed.
• SAX provides callback procedures that let you to provide more error handling.
SAX limits
The general limits of SAX include:
• You cannot back up in the parse.
• You must control the context. In other words, you must be able to grab the data you need
as it goes by, while ignoring the data you don’t need.
• There is no structure in memory to do in-place updates.