What is a Dynamic Subsystem?

Dynamic subsystems are those subsystems that can be defined in one of the following ways:

Processing the keyword format IEFSSNxx parmlib member during IPL
Issuing the IEFSSI macro
Issuing the SETSSI system command.

Subsystems have the choice of being dynamic. Subsystems that are not dynamic can be defined only at IPL using the positional form of the IEFSSNxx parmlib member, in which case, they cannot use dynamic SSI services.

In addition to its role in communicating with subsystems, the SSI provides a set of authorized system services that are available only to dynamic subsystems that installations, applications and subsystems can invoke to:

Define (add) a new subsystem dynamically (without requiring an IPL)
Activate a subsystem that is already defined
Deactivate a subsystem that is already defined
Store and retrieve subsystem-dependent information
Define subsystem options, which include deciding:
- If a subsystem can respond to the SSI commands
- Which subsystem a subsystem should start under.
Query subsystem information
Define and modify the response of a subsystem to function requests.

Defining or adding a subsystem is primarily a way of making the subsystem's unique name known to the system. A subsystem is active when it is ready to process requests that the SSI directs to it. To deactivate a subsystem, the subsystem informs the SSI that it is no longer accepting requests. For example, a subsystem may request that it be deactivated to update the list of function requests that it supports, or to respond to a problem.

The dynamic SSI services reject any requests to manipulate subsystems that were not defined dynamically.

The services that allow installations, applications and subsystems to define and modify the response of a subsystem to function requests replace and enhance the existing IEFJSVEC service. You can still use the existing IEFJSVEC service, which is described in Using IEFJSVEC with Your Subsystem, however IBM® recommends that you use the services described in Services for Building and Using Your Subsystem instead of IEFJSVEC. These services provide an easier way to define or change the functional response of a subsystem.