Middleware and XML Application Servers

Another approach to accessing existing data in XML is through a gateway server or middleware layer. The concept of Message Oriented Middleware (MOM) is not new. Client/server applications were depending on messaging layers long before the first Web-based applications appeared. In this case, a middleware layer or server acts as a bridge between the older, relational model and the new XML model.

One simple way to implement an XML gateway is to configure a normal Web server to retrieve relational data and return it in a predefined XML document format. Figure 9.3 shows how a dedicated application server can be used to hide XML data conversions from client applications.

Using an intermediate server to perform conversions now simplifies the transition to fully XML-aware databases later. If the capabilities of the back-end database change, only the gateway application itself needs to be modified. Abstracting the XML-to-DB linkage isolates client applications from changes in database technology.