Establishing connections

IMS uses an 'identify' process to establish a connection to the external subsystem.

IMS activates an Identify exit routine contained in the ESAP to identify the control region or dependent region TCB to the external subsystem. The external subsystem can then monitor IMS TCBs in order to respond to IMS abnormal terminations. A connection is established upon successful completion of the identify process, in other words, once the region has been successfully identified to the external subsystem.

IMS provides a notify message mechanism so that if the external subsystem has not been started when IMS attempts to connect the control region, the external subsystem, once started, can notify IMS to establish the connection. If the external subsystem makes use of the notify capability, the order in which IMS and the external subsystem are started is not important.

The connection from the control region is established first before any dependent region connections are established. If the control region connection has not been established when a dependent region is started, the dependent region does not identify itself to the external subsystem. IMS uses a hierarchical relationship between control region and dependent region connections to allow the control region to act as recovery coordinator for dependent regions. If a dependent region fails, the control region takes recovery actions on its behalf.

The external subsystem can optionally provide an Initialization exit routine. IMS activates the Initialization exit routine, if provided, during control region and dependent region initialization before the region identifies itself to the external subsystem. This exit routine allows the external subsystem the chance to do any initialization processing it requires before each connection being established.

IMS can establish the control region connection automatically during control region initialization, provided the external subsystem has been started. However, the connection can be delayed to a later time. If an Initialization exit routine is not provided, or if the exit routine returns the appropriate return code, the control region identify is not done automatically. In this case, the external subsystem can activate the Subsystem Startup Service provided by IMS when it wants the connection established. Or, IMS attempts to establish the connection when a dependent region is ready to identify itself.

IMS also establishes the control region connection in response to a /START SUBSYS operator command.