One important factor that affects how application programs are preprocessed and executed is whether the database manager is running in single or multiple user mode.
Running a Program in Single User Mode
In single user mode, the system and your application programs run in a single virtual machine. The application or preprocessor starts the database machine, processes the SQL statements, and returns control to CMS. The application server must be restarted for every invocation of an application program or preprocessor. The database machine may have more than one application server defined for it, but only a single application server can be active at any time.
Running a Program in Multiple User Mode
In multiple user mode, one or more applications concurrently access the same application server. The system runs in one virtual machine while one or more DB2 Server for VM application programs or preprocessors operate in other virtual machines. More than one application can access the same application server at the same time, and an application program can access more than one application server. Use the CONNECT statement to switch application servers from within an application. This facility is called switching application servers (see Switching Application Servers).