Integrating XML and Relational Database Technologies:
A Position Paper

Introduction
XML is an open standard for defining data elements on a Web page and business documents. In contrast to the HTML markup language, which defines how elements on a Web page are displayed, XML defines the structured information those elements contain.

XML was originally developed as an application profile of SGML to use over the Internet. But the ease of both writing applications that process XML document and creating XML documents has made XML an instant success for a variety of other application domains, too. In fact, a truly amazing number of applications based on XML are currently under development. One area where the benefits of XML have become immediately apparent is data exchange. For instance, XML has been combined with EDI (Electronic Data Interchange).

There are clear benefits to using XML. First, the design of XML is formal and concise, so it's relatively easy to write data-exchange protocol processors that rely on standard XML components. Second, since XML documents are human-readable, a developer can figure out what the content means by simply inspecting the XML document. Third, momentum is building to create standardized XML protocols-also referred to as XML Schemas-for almost any type of business. So there's an excellent chance that in the near future a majority of transactions will be carried out using XML as the underlying infrastructure protocol.

The shift from binary protocols to XML-based protocols resembles the computing industry's move from mainframe-centric terminals and central processing units to the client/server paradigm of distributed computing. Like client/server before it, XML requires new ways of storing data. In the shift from mainframe-centric to client/server computing, a major side effect was the development of relational database servers. Similarly, in the case of the move to XML protocols, there is a noticeable trend toward not only exchanging XML data, but also storing it directly as XML data. To support this practice, some relational database vendors and independent middleware vendors have released native XML database products and extended relational database features to support XML.

Before discussing the integration of XML and relational database technologies, let's look briefly at native XML database technology. In many instances, native XML database technology seems derived from the object-oriented database technology of the early '90s-especially in its indexing techniques. Compared with object-database technology and its lackluster track record, native XML databases benefit from an application's need to retrieve and store data directly as XML data. While this is a tremendous advantage for XML data that is content or structured text rather than data, it becomes cumbersome when the same XML content has to be mapped to corporate data, such as that stored in relational tables. Given the existing RDBMS installed base, unless the XML middleware industry can seamlessly integrate native-XML-database and relational-database technologies, it will be extremely hard for the native-XML technology to thrive.

Looking ahead, we believe an increasing number of developers will shift to an XML-based development paradigm. But this shift will not bring about a revolution in the underlying database technology. On the contrary, we expect that relational technology will remain the dominant storage technology-albeit, with important extensions to XML-and that it will coexist with the new native-XML database technology. Of course, native-XML database technology arose due to the intrinsic differences between the relational-database paradigm and the XML paradigm, the so-called XML-relational impedance mismatch. But we also believe that just as the object-relational mapping technique simplifies object persistency on relational repositories, XML mapping technology can bridge the gap to properly retrieve XML data from relational databases and persistent XML data on relational databases.

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