XML Data Access Architecture

SQL Server and Microsoft’s data access libraries enable an application to get query results in XML and update databases with XML messages. Shown is a standard three-tier Web application architecture with data access, business logic, and presentation layers. The following steps take place within the data access layer: To set up a database for XML, the developer defines a mapping schema—an XML schema with annotations that map XML elements and attributes in the XML schema to the corresponding tables and columns in the database. The data access libraries use the mapping schema to access the database. In a typical transaction, the application connects to and queries the database, gets XML data back, and disconnects from the database. The application caches the retrieved data either as an XML document object in memory or as an XML file on disk. The application can then merge the XML data returned from SQL Server with other XML data sources, such as XML files produced by hand, XML messages received from Web services, or data from other XML-capable database products like Oracle 9i and IBM DB2. When the application makes changes to SQL Server data, it can apply those changes to its cached XML data while disconnected, then reconnect to the database and submit the changes in XML form through the data access libraries.