Message Scheduling

Scheduling is the loading of the appropriate program into a message processing region. IMS can then pass messages stored on the IMS message queue to this program when it issues the Get Unique (GU) IOPCB call. For more information about application calls, see Chapter 14, "Application Programming Overview," on page 217.

After an input message is available in the message queue, it is eligible for scheduling. Scheduling is the routing of a message in the input queue to its corresponding application program in the message processing region or the Java message processing region.

The linkage between an input message (defined by its transaction code) and an application program (defined by its name) is established at system definition time in the PSB (see Figure 13-5 on page 207). Multiple transaction codes can be linked to a single application program, but only one application program can be linked to a given transaction code.

Figure 13-5. Message Scheduling Based on Information in the PSB

You can specify the class that transaction codes are assigned. This class assignment determines into which message region an application program is loaded. When the IMS message regions are started, they are assigned from one to four message classes. When a message region is assigned more than one class, the scheduling algorithm treats the first class specified as the highest priority class, and each succeeding class is treated as a lower-priority class.

The class in which a transaction code runs is defined in two ways:

• On the APPLCTN macro

• On the MSGTYPE parameter of the TRANSACT macro

If the class is specified on the APPLCTN macro, it need not be defined on the TRANSACT macro. If it is specified on both, then the class on the TRANSACT macro overrides the APPLCTN macro specification. Figure 13-6 illustrates the definition of a transaction.

Figure 13-6. Sample APPLCTN Macro Transaction Definition in IMS Stage 1 Input

 APPLCTN PSB=DFSIVP1,PGMTYPE=TP     TRANSACT CODE=IVTNO,MODE=SNGL,                          X         MSGTYPE=(SNGLSEG,NONRESPONSE,1) APPLCTN PSB=DFSIVP2,PGMTYPE=(TP,1)     TRANSACT CODE=IVTNO2,MODE=SNGL,                         X         MSGTYPE=(SNGLSEG,NONRESPONSE) 

Note the following about the transaction definitions in Figure 13-6 on page 207:

• Transaction DFSIVP1 has the class defined as the third parameter on the MSGTYPE parameter on the TRANSACT macro.

• Transaction DFSIVP2 has the class defined on the APPLCTN macro.

• Both transactions are assigned to class 1.