Client-Side Conversion
The most rapid (and least flexible) way to integrate XML into an existing system is through the use of a client-side XML driver. This can be done either by using an XML-enabled database API provider or by inserting an extra component between the client application and the database API provider. Figure 9.2 shows how a relational-data-to-XML mapping layer could be added to an existing system.
Figure 9.2. Adding a client-side conversion layer.
For example, Microsoft's ADO library contains native support for serializing a query's result set into XML. The resulting XML data can then be programmatically manipulated using XML APIs such as the DOM. It can also be transformed using XSLT for display or for input into another XML-based system.
Converting relational database queries into XML using client libraries is the least intrusive approach to implementing XML. In most cases, only the client application itself needs to be modified.
Unfortunately, the types of XML documents that can be constructed from a tabular result set are severely limited. Although more sophisticated commercial products allow nested queries to be executed to construct complex XML documents, for the most part, applications are limited to accessing the same basic tabular data via XML APIs instead of data-access APIs.
Client XML support is, for the most part, read-only. Currently, few client access tools support modifying RDBMS database contents using XML documents.