Terminal interruptibility
When a conversational task is running, CICS allows nothing else to send messages to that task’s terminal. This has advantages and disadvantages. The advantage is that unexpected messages (for example, broadcasts) cannot interrupt the user-machine dialogue and, worse, corrupt the formatted screen. The disadvantage is that the end user cannot then be informed of important information, such as the intention of the control operator to shut down CICS after 10 minutes. More importantly, the unwitting failure of the end user to terminate the conversation may in fact prevent or delay a normal CICS shutdown.
Pseudoconversational applications can allow messages to come through between message pairs of a conversation. This means that notices like shutdown warnings can be delivered. This might disturb the display screen contents, and can sometimes interfere with transaction sequences controlled by the EXEC CICS
RETURN command with the TRANSID option. However, this can be prevented by forcing the terminal into NOATI status during the middle of a linked sequence of interactions (like building one order in the example above), or by judiciously allowing space at the top or bottom of the screen for use by any message being sent to the screen.
The main problem is that CICS shutdown could occur in mid sequence—in our example, when an order is only partly built. This is because CICS cannot distinguish between the last CICS task of a user transaction and any other. You can guard against this by ensuring that users are warned of any intended shutdown sufficiently far in advance, so they do not start work that they might not complete in time.
Summary of pseudoconversational and conversational design
Conversational tasks may be easier to write, and may require less processor time, but they have serious disadvantages in their need for more memory and in their effect on the overall operability of the CICS systems containing them.
Using resources effectively
This section gives advice about designing application programs to minimize the use of resources.
When you have decided which services you want an application to provide, you need to consider what resources you must conserve while providing these services.
These are: v
Processor storage v
Processor time v
Exclusive-use resources (such as terminals, file records, and scratch-pad areas) v Line transmission capacity
There will occasionally be conflict when trying to save one resource at the cost of another. The appropriate compromises will vary from one application to the next.
Processor storage
Your applications use up processor storage in two ways. First, there are the CICS control blocks associated with any transaction being processed. Second, there are the programs being executed to run the transaction. The programs, in turn, take up space both for executable code and for working storage areas. In an online system,
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CICS for iSeries Application Programming Guide V5
