Setting link priorities for remote transactions

You can assign priorities to remote transactions by using the PRTY= keyword on the TRANSACT macro during system definition in the destination system. You can also assign priorities dynamically by using the type-2 commands CREATE TRAN and UPDATE TRAN.

You might want to assign priorities because a remote input transaction might have a different scheduling priority in the destination system from that defined for it during system definition of the originating system. This means there could be a long wait for a response, even though the remote transaction had a high priority when it was defined with the SYSID= parameter of the TRANSACT macro or the NPRI keyword of the CREATE TRAN command.

For example, in the following figure, Terminal 1 is sending a remote transaction (ASMB1) to System B.

Figure 1. Link priorities for a remote transaction
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In IMS system A, this transaction is enqueued to a control block (CB) and has a priority of 9 (defined with the SYSID= parameter). When ASMB1 is sent to system B, it becomes a local transaction and is processed. When the message router is called to send a response, it enqueues the response on the control block that represents the MSC link. If ASMB1 is defined in system B with a priority of 7, all transactions with priorities of 12-15 (in queue 1) are processed first, and 8-11 (in queue 2) are processed next. The ASMB1 (with a priority of 7 in queue 3) is processed next. However, as shown in the figure, the limit priority can be set at 8 and the limit count can be set at 3. As a result, when the number of message queues to be processed reaches 3, the priority of ASMB1 is changed to 8, and subsequent responses are placed on queue 2. On system B, all messages for BSMB1 are sent before responses are sent. Messages for BSMB2 are sent last in this example.

In a shared-queues environment, however, priority applies only to messages that are received from the MSC link to be processed. Priority does not apply to response messages (messages going back to the input system) because the coupling facility does not have the four queues. Instead, all response messages are sent FIFO.