Assigning message class and initializing a region in DB/DC and DCCTL

The message class that is assigned to a transaction determines into which message region an application program is loaded.

Each message (transaction code) is assigned a class using the third parameter of the MSGTYPE keyword on the TRANSACT macro. If the class is not specified this way, the value on the PGMTYPE keyword of the APPLCTN macro is applied. 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.

If more than one class is specified, message selection is processed as follows. The first class specified is scanned, in transaction-priority sequence, for waiting messages. If no messages are waiting for the first class, the second and following classes are also scanned in priority sequence. If messages are waiting in the first class, the highest-priority message is selected for scheduling.