Initialization phase of the XRF process

The initialization phase begins when an operator brings up AVM as a z/OS® started task, either through the START AVM command or through the /NRESTART or /ERESTART command.

The START command loads modules and control blocks and, optionally, opens a session on the ISC link that connects the active and the alternate IMS systems.

The following figure shows the XRF initialization phase.

Figure 1. XRF initialization phase
Two boxes show two systems. Each box has an OS, IMS, and VTAM region. Three cylinders show databases, a log, and an RDS. Arrows between the IMS systems and cylinders show the work flow.

An operator at the active IMS uses z/OS START commands to bring up the active IMS, as in a normal or emergency restart. When it is up, the operator at the alternate IMS can bring up the alternate IMS at any time. The commands are similar for both systems except that the operator at the alternate IMS adds the option AUTO=N to the START IMS command and issues the IMS command, /ERESTART BACKUP.

Attention: An operator error can result in losing system or database integrity.

The alternate IMS is in a state similar to emergency restart until a takeover occurs. To be ready for a takeover, the alternate IMS shares the databases with the active IMS. The alternate IMS opens the databases, but does not access them until after a takeover.

In addition to the normal IMS initialization functions, IMS does the following to set up the XRF complex:
  1. Processes the new DFSHSBxx IMS PROCLIB member. For more information, see XRF parameters in DFSHSBxx.
  2. Loads the new XRF/IMS modules, and obtains the new pools and work areas. Most of the new modules reside in the control region private below the 16 MB line.
  3. Identifies both subsystems to the z/OS availability manager using the RSENAME keyword, described in Defining IMS.PROCLIB members for XRF.
  4. Establishes the DBRC environment on both the active and the alternate subsystems using the RSE name. If a takeover occurs, authorizations of the active IMS are automatically passed to the alternate IMS.
  5. Informs VTAM® of the APPLID of the active IMS system.

Related reading For more information on how the operators bring up the active and alternate IMS systems, see IMS Version 15 Operations and Automation.