LTERM users (subpools) and components

IMS user blocks are sets of IMS logical terminals (LTERMs) defined by the SUBPOOL macro during IMS system definition or dynamically created from ETO user descriptors.

Subpools defined at system definition cannot be used with ETO LU 6.1 terminals. Users (subpools) defined for ISC are separate from the users defined for dial-type terminals and are permitted only in conjunction with ISC parallel session support.

Definition: The collection of all static ISC users is known as the VTAMPOOL. LTERMs defined within the VTAMPOOL are reassignable only between users within the VTAMPOOL. LTERMs not defined within the VTAMPOOL cannot be assigned into the VTAMPOOL.

ISC users are dynamically assigned to an ISC session instance as a result of session initiation. That is, these parameters define the user that is available to the given session instance. This user remains allocated to the named parallel session instance even across session and subsystem failures until released through normal termination by mutual agreement of IMS and the other subsystem. For a single nonparallel session, the allocation of a set of LTERMs is fixed during system definition.

Each IMS LTERM is associated with one input and one output IMS component. The input and output components can be the same component, or different components can be specified. Conversely, IMS does not prevent multiple input or output LTERMs from being associated with a single component. However, doing so can cause problems with input component determination or output presentation.

IMS uses the input component ID to identify the LTERM that is to be associated with the input message. For other terminal support, IMS assumes that all input is from the first LTERM in the list that passes the necessary operational and security checks. However, for input from an ISC node, the input component is determined based on the component that is indicated in the ATTACH FM header. If no FM header is received, IMS assumes the input is to be associated with the LTERM for input component (ICOMPT) one. After the component value is determined, if the associated LTERM cannot be found, is stopped, or is not ready, or if that LTERM cannot pass security checks, the message is rejected.

When output is sent, it is sent to the component (COMPT) identified with the output LTERM. Message switches, broadcast messages for specific LTERMs, and data replies from transactions are directed to the component that is associated with the specified output LTERM. The user-written MPP can insert to the I/O PCB and default to the output component that is associated with the selected input LTERM. It can also address specific components through the appropriate LTERM name by an insert to an alternate PCB.

You can establish proper relationships between input and output components through the NAME macro during IMS system definition or LTERM keyword on the ETO user descriptor. This enables a logical unit to indicate its input component and causes output to be returned to the associated output component that was indicated during IMS system definition or on an ETO user descriptor. Proper definition and use of components can reduce or eliminate the need for LTERM naming conventions, DL/I CHNG calls, and inserts to alternate PCBs.

Recommendation: Incorrectly specifying the message delete system definition parameter, MSGDEL, or the ETO user descriptor while defining ISC users (subpools) can prevent a session from being initiated. An ISC session can be initiated only if the MSGDEL specifications on the TERMINAL or ETO user descriptor and SUBPOOL macros match.

Also, when using the ISC message switch support or an alternate PCB insert from a local transaction in order to route messages to another subsystem and return replies to a source terminal operator, MSGDEL=SYSINFO must be specified on the IMS system definition TERMINAL macro or ETO logon descriptor associated with both the ISC session and the source terminal. Specifying MSGDEL=NONIOPCB for the ISC session prevents ISC-message routing to other subsystems. Specifying MSGDEL=NONIOPCB for the source terminal prevents message replies from being routed to the source terminal operator.

The output bracket and send and receive protocols used by IMS depend on a combination of the system definition output component specifications, the protocol used to input the message to IMS, and the type of IMS message received. For example, IMS synchronous message types have a predefined protocol for output replies. Associated with the output component is a definition of send and receive protocols for asynchronous output.