Connecting and disconnecting subsystems

IMS can only connect to another subsystem if that subsystem is identified in the subsystem member in IMS.PROCLIB. You must specify the subsystem member name in the SSM EXEC parameter before an IMS subsystem can access databases in an external subsystem (another program executing in a z/OS® address space).

When specified to and accessed by IMS, the subsystem member name cannot be changed without stopping IMS.

Connections between an IMS subsystem and another subsystem can occur with or without operator intervention, and without regard to which subsystem is available first.

• Automatic connection: When the SSM EXEC parameter is specified, IMS automatically establishes the connection when it processes the parameter.
• Operator-controlled connection: When the SSM EXEC parameter is not specified, the connection is established when the /START SUBSYS SSM command is entered. The /START SUBSYS SSM command specifies the subsystem member in IMS.PROCLIB that IMS uses in connecting to subsystems.

Disconnecting IMS from another subsystem is initiated with the /STOP SUBSYS command, and normally completes without operator intervention. IMS will quiesce all dependent region external subsystem activity prior to stopping the subsystem.

If an external subsystem fails, IMS might not be able to disconnect properly from the failed subsystem. In this case, IMS might appear to hang in the disconnect process and the external subsystem might not be able to reconnect to IMS until the threads that were open at the time of the failure are properly closed.

To prevent this problem, issue the /STOP SUBSYS command before the failing external subsystem is shut down. After the external subsystem is brought back up, reconnecting to IMS is automatic.

If it is not possible to issue the /STOP SUBSYS command before the external subsystem is shut down, issue the /DISPLAY ACTIVE or /DISPLAY ALL command. These commands enable you to display the external subsystem threads that are still connected to IMS. Each thread has to be canceled individually. After all threads are cancelled and the external subsystem is brought back up, reconnecting to IMS is automatic.