We will investigate how we can build a business application that’s completely XML-enabled. We will look at each tier of the three tier application model and discuss our options to leverage XML in the most efficient way; not only in terms of performance, but also with respect to the development effort and code maintenance.

The data tier is where we encapsulate all the details of data access. In our case we architect it to interface with the rest of the application only through XML. Our data layer is going to return query results in XML and accepts XML documents to insert them into a SQL Server 2000 database to avoid any unnecessary conversions in the business layer. But before we start studying different approaches to architect a data access layer we give a brief overview of SQL Server’s built-in XML support, the SQLXML library and the managed .Net classes to access SQLXML.

The release of SQL Server 2000 started adding features to integrate XML with the relational world. The goal was to simplify development of XML-enabled applications and data exchange over the internet (what else?). Microsoft has been adding more features to this list, but will look at those separately. Initially SQL Server 2000 shipped with the flowing XML related features:

• SQL language extensions to retrieve XML formatted data directly from the database
• SQL language extensions to execute SQL queries against XML documents.
• Support for exposing data in SQL Server through IIS. You can return data from in XML format as a response to HTTP requests, sort of as the forerunner of SOAP web services.
• A SQLOLEDB OLE DB provider to improve performance when executing XML queries
• XML Views of relational data defined by schemas in the XDR format.
• XPath queries to retrieve XML formatted data from the database.
• Execution of XML query templates with optional XSL transformations.

Discussing all of these features in depth is beyond the scope of this book. We will explore how we can retrieve XML directly from SQL Server and insert data from XML documents in the following sections, but we have before we move on to the SQLXML web release and the managed classes to access data in SQL Server. We have to skip features enabling SQL Server access over HTTP because ADO.NET and the managed classes in SQLXML provide more flexible and re-usable means to access a database. We will also not discuss XML views defined by XDR schemas, since they are superceded by views defined by XSD schemas. Check the SQL Server Books online documentation on the MSDN web site, or install it from your SQL Server CD if you are interested in these features.