Connecting to Data

Virtually all corporate IT applications and large-scale Web sites must retrieve, display, and update data stored in a database. But the fact that the desired data is usually located in multiple database management systems (DBMSs) and frequently involves different database products and host operating systems makes writing even a simple application or Web service to manipulate the data a complex programming problem. A database with customer names, for example, might be kept in a DB2 database running on an IBM AS/400, while a products database might be stored in SQL Server running on Windows NT. An application that the company uses to ship a product to a customer needs to efficiently and correctly access both databases.

Application servers fill two roles to ease the task of creating applications that span multiple databases: data integrity and universal data access.

Data integrity. As discussed previously in the section “Transactions Increase Reliability,” application servers must ensure data integrity by coordinating the activities of the components of an application.