Avoiding affinities when using the START and CANCEL REQID commands

The CICS START command is used by a transaction to start another transaction. Another transaction can cancel this command using the CANCEL command and specifying the REQID associated with the START command.

With this CICS application programming technique, one transaction issues a START command to start another transaction after a specified interval. Another transaction (not the one requested on the START command) determines that it is no longer necessary to run the requested transaction (which is identified by the REQID parameter) and cancels the START request.

A temporary storage queue is one way that the REQID can be passed from task to task.

Note: The CANCEL command is effective only if the specified interval has not yet expired. To use this technique, the CANCEL command must specify the REQID option, but the START command need not. This is because, provided the NOCHECK option is not specified on the START command, CICS generates a REQID for the request and stores it in the EXEC interface block (EIB) in field EIBREQID.

Figure 1 illustrates this programming technique.

Figure 1. Illustration of the use of the START and CANCEL REQID commands

TRN1 executes in AOR1. It STARTs TRNX and writes the REQID to a TS queue, then terminates. TRN2 executes in AOR1 and reads the REQID. It CANCELs TRNX using REQID.

Using this application programming technique, the CANCEL command that cancels the START request must:

Be issued in same target region that the START command was executed in, or
Specify the SYSID of the target region where the START command was executed, or
Specify a TRANSID (TRNX in our example) that is defined as a remote transaction residing on the target region where the START command was executed.

Note: A START command is not necessarily executed in the same region as the application program issuing the command. It can be function shipped (or, in the case of a non-terminal-related START, routed) and executed in a different CICS region. The previous rules apply to the region where the START command is finally executed.

An application design based on the remote TRANSID technique only works for two target regions. An application design using the SYSID option on the cancel command only works for multiple target regions if all target regions have connections to all other target regions. In either case, the application programs need to be modified: there is no acceptable way to use this programming technique in a dynamic or distributed routing program except by imposing restrictions on the routing program.

In general, this means that the dynamic or distributed routing program has to ensure that TRN2 executes in the same region as TRN1 to preserve the application design, and also that TRNX is defined as a local transaction in the same region.

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Last updated: Thursday, 27 June 2019