External Subsystem Attach Facility (ESAF)

To enable access to the data resources of an external subsystem (ESS) product from IMS applications, the ESS must provide functions necessary for it to attach to the IMS subsystem and to, jointly with IMS, coordinate data access. The IMS Attach Facility presents a programming interface to the external subsystem product. Certain steps are required to install ESAF, which are described in the following information. Other IMS publications also contain ESAF information and references are included, where applicable.

Multiple external subsystems can only be attached by an online IMS system. These subsystems can be of the same, or of different, product types. The installation defines the external subsystems to IMS. A given IMS dependent region can have access to all external subsystems defined to the IMS system, to just a subset, or to none according to installation specifications. An application program executing in a dependent region can access more than one different subsystem. The installation defines a unique token for each subsystem, which IMS uses in routing application calls for external resources. Application program access to more than one subsystem of the same type is supported by IMS, but might not be supported by the external subsystem.

The facility provides for synchronization of external subsystem data resources with IMS data resources. For synchronization processing, IMS is the recovery coordinator and is responsible for directing commit or abort actions on behalf of its application programs. External subsystems are participants in the process and commit or abort data updates by IMS applications according to direction given by IMS. When resources are to be committed, IMS polls the participants as to whether or not they are ready to commit before giving final commit (or abort) direction.

You can also configure a Fast Database Recovery (FDBR) region to recover work on the external subsystem. When a FDBR region is monitoring an IMS system that fails, it receives information about indoubt work on the external subsystem from the ESAF indoubt notification exit routine (DFSFIDN0).

For IMS batch, IMS is allowed to attach only to one external subsystem. IMS expects this external subsystem to be the Recovery Coordinator. This external subsystem has no way of coordinating with any other external subsystem that IMS attaches to, so IMS is restricted to only one external subsystem attachment in batch.

The External Subsystem Attach Facility is different from the coordinator controller (CCTL) associated with DBCTL.

JMP and JBP regions can access control-region-defined Db2 for z/OS® subsystems that use the COORD=RRS parameter in the IMS.PROCLIB member. If this connection method is chosen for the JMP or JBP region, you must add a DD statement (DFSDB2AF) in the DFSJMP or DFSJBP procedure that points to the Db2 for z/OS libraries.