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XML APIs for databases

Blend the power of XML and databases using custom SAX and DOM APIs

By Ramnivas Laddad, JavaWorld.com, 01/01/00

Databases and XML offer complementary functionality for storing data. Databases store data for efficient retrieval, whereas XML offers an easy information exchange that enables interoperability between applications. To take advantage of XML's features we can convert database tables into XML documents. We can then use XML tools with such documents for further processing. For example, XML documents can be presented as HTML pages with XSLT stylesheets, can be searched with XML-based query languages such as XQL, can be used as a data-exchange format, and so on. However, converting a database into an XML document is an expensive approach, one that requires not only the initial cost of conversion but also the subsequent costs of synchronizing both information sources.

TEXTBOX: TEXTBOX_HEAD: Abbreviations in this article API: application programming interface

DOM: Document Object Model

DTD: document type definition

JAXP: Java API for XML Parsing

JDBC: Java Database Connectivity

SAX: Simple API for XML

XML: Extensible Markup Language

XQL: XML Query Language

XSL: Extensible Stylesheet Language

XSLT: XSL Transformations

XT: An XSLT processor

:END_TEXTBOX

For processing XML documents, most XML tools work with the SAX or DOM API. In this article, we'll look at a way to implement the same APIs directly over a database, enabling XML tools to treat databases as if they were XML documents. That way, we can obviate the need of converting a database.

We'll see an implementation of the SAX API for Databases that should work with any database with a JDBC driver. Next, we'll examine an implementation of the DOM API for Databases that uses the SAX API internally. To demonstrate the SAX API for Databases, we'll look at its integration with XT (an XSLT processor). We'll also see an example of how such integration can create HTML pages that incorporate an XSLT stylesheet directly from a database and how it can convert a database into an XML document. Finally, we'll look at how the DOM API for Databases integrates with an XQL processor.

In this article, I make use of existing tools rather than create new tools to illustrate the applications of the SAX and DOM APIs for Databases. I show how to leverage a number of available XML tools to work with a database. All the XML tools I mention are either available for free or free for noncommercial use (though you should, of course, check licensing agreements).

Overview of the SAX and DOM APIs

SAX is an event-based API for XML. With it, the SAX parser reports events such as the start and end of elements to the application as it walks over the document. Because the parser reports events as it visits different parts of the document, it does not have to build any internal structure. That reduces the strain on system resources, which makes the parser attractive for large documents. For XML documents received as continuous streams, an event-based API is the only choice.

The DOM API, on the other hand, follows a treelike construct. Elements have parent-child relations with other elements. With this API, the parser builds an internal structure such that an application can navigate it in a treelike fashion. DOM allows an application to have random access to the tree-structured document at the cost of increased memory usage.

 

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