Batch message processing: transaction-oriented BMPs

Data a transaction-oriented BMP can access

Unlike a batch-oriented BMP, a transaction-oriented BMP can access the IMS message queue for input and output, and it can only run in the DB/DC and DCCTL environments.

Using a transaction-oriented BMP

Unlike MPPs, transaction-oriented BMPs are not scheduled by IMS. You schedule them as needed and start them with JCL. For example, an MPP, as it processes each message, might send an output message giving details of the transaction to the message queue. A transaction-oriented BMP could then access the message queue to produce a daily activity report.

Typically, you use a transaction-oriented BMP to simulate direct update online: Instead of updating the database while processing its transactions, an MPP sends its updates to the message queue. A transaction-oriented BMP then performs the updates for the MPP. You can run the BMP as needed, depending on the number of updates. This improves response time for the MPP, and it keeps the data current. This can be more efficient than having the MPP process its transactions if the response time of the MPP is very important. One disadvantage in doing this, however, is that it splits the transaction into two parts which is not necessary.

If you have a BMP perform an update for an MPP, design the BMP so that, if the BMP terminates abnormally, you can reenter the last message as input for the BMP when you restart it. For example, suppose an MPP gathers database updates for three BMPs to process, and one of the BMPs terminates abnormally. You would need to reenter the message that the terminating BMP was processing to one of the other BMPs for reprocessing.

BMPs can process transactions defined as wait-for-input (WFI). This means that IMS allows the BMP to remain in virtual storage after it has processed the available input messages. IMS returns a QC status code, indicating that the program should terminate when one of the following occurs:

• The program reaches its time limit.
• The master terminal operator enters a command to stop processing.
• IMS is terminated with a checkpoint shutdown.

You specify WFI for a transaction on the WFI parameter of the TRANSACT macro during IMS system definition.

A batch message processing region (BMP) scheduled against WFI transactions returns a QC status code (no more messages) only for the following commands: /PSTOP REGION, /DBD, /DBR, or /STA.

Like MPPs, BMPs can send output messages to several destinations, including other application programs.

Recovering a transaction-oriented BMP

Like MPPs, with transaction-oriented BMPs, you can choose where commit points occur in the program. You can specify that a transaction-oriented BMP be single or multiple mode, just as you can with an MPP. If the BMP is single mode, issuing checkpoint calls is not as critical as in a multiple mode BMP. In a single mode BMP, a commit point occurs each time the program retrieves a message.