IMS transactions and programs

The IMS system administrator defines the transactions. For each transaction that is defined, a program name is specified. When you invoke a transaction by using an IMS node, the IMS Control Region determines which program is configured for that transaction, and queues the data for retrieval by that program.

The diagram shows that the IMS Control Region queue data for the appropriate IMS program.

After the program has prepared the response data for the IMS node in the message flow, it inserts that data onto another queue. This output queue is tied to the socket to which IBM® App Connect Enterprise is connected. Therefore, multiple concurrent message flows that are calling the same transaction each have a separate queue to receive the responses.

The IMS program gets messages by issuing a GU (GetUnique) call and it produces messages by issuing an ISRT (Insert) call. These calls are known as DL/1 calls. DL/1 is the programming interface to IMS. Other common DL/1 calls are PURG (purge) and GN (GetNext).