Initiating the external subsystem connection

IMS automatically connects to the external subsystem during control region initialization processing (for example, when IMS is started) unless the external subsystem chooses to defer the control region identify to a later time.

The external subsystem defers the connection by returning from the control region Initialization exit routine with return code 4 (do not identify), or by not providing an Initialization exit routine for the control region.

If the external subsystem uses the notify message mechanism provided by IMS (and if the external subsystem is not up when IMS activates the Identify exit routine) the connection is automatically established when the external subsystem is started. Return Code 4 from the Identify exit routine causes IMS to wait for the external subsystem to send the notify message passed to the exit routine, and to reactivate the exit routine when the message is received.

If the notify mechanism is not used, the Identify exit routine should return with return code 12, in which case the connection is put in stopped status. Stopped status must be removed by a /START SUBSYS command before IMS will establish a connection.

Once the connection has been established, IMS performs Resolve Indoubt processing to resolve any outstanding recovery tokens with the external subsystem. If outstanding recovery tokens exist and a Resolve Indoubt exit routine was not supplied, IMS terminates and stops the connection; otherwise dependent regions are allowed to connect to the external subsystem.