IMS is designed to make the best use of the features of the z/OS operating system. IMS does so by:

• Running in multiple address spaces: IMS subsystems (except for IMS batch applications and utilities) normally consist of a control region address space, separate address spaces for system services, and dependent address spaces for application programs. Running in multiple address spaces provides the following advantages:
  • Maximizes the use of a central processor complex (CPC). Address spaces can be dispatched in parallel.
  • Isolates the application programs from the IMS system code. Doing so reduces outages from application failures.
• Running multiple tasks in each IMS address space: IMS, particularly in the control region, creates multiple z/OS subtasks for the various functions to be performed. Doing so allows other IMS subtasks to be dispatched by z/OS while one IMS subtask waits for system services.
• Using the z/OS cross memory services: IMS uses z/OS cross memory services to communicate between the various address spaces that make up an IMS system. IMS also uses the z/OS CSA and ECSA to store IMS control blocks that are frequently accessed by the address spaces of that IMS system. Doing so minimizes the overhead of running in multiple address spaces.
• Using the z/OS subsystem feature: IMS dynamically registers itself as a z/OS subsystem and uses the z/OS subsystem feature to detect when dependent address spaces fail, thus preventing the cancellation of dependent address spaces through z/OS command entry.
• Using a z/OS sysplex: Multiple IMS subsystems can run on the z/OS systems that make up the sysplex and, therefore, can access the same IMS databases and the same message queue. Doing so provides:
  • High availability: z/OS systems and IMS subsystems can be taken in and out of service without interrupting production.
  • High capacity: multiple IMS subsystems can process far greater volumes than individual IMS subsystems can.

Related Reading: For information about data sharing and shared queues in a sysplex environment, see:

• IMS in the Parallel Sysplex: Volume I: Reviewing the IMSplex Technology
• IMS in the Parallel Sysplex: Volume II: Planning the IMSplex
• IMS in the Parallel Sysplex: Volume III: IMSplex Implementation and Operations