Avoiding affinities using the POST and CANCEL REQID commands

The CICS POST command is used to request notification that a specified time has expired. Another transaction (TRN2) can force notification, as if the specified time has expired, by issuing a CANCEL of the POST request.

The time limit is signalled (posted) by CICS by setting a bit pattern in the event control block (ECB). To determine whether notification has been received, the requesting transaction (TRN1) either tests the ECB periodically, or issues a WAIT command on the ECB.

A TS storage queue can be used to pass the REQID of the POST request from task to task.

Figure 1 illustrates this technique.

Figure 1. Illustration of the use of the POST command

TRN1 executes in AOR1. It issues a POST command and stores the REQId of the POST in a TS queue for TRN2. TRN1 then checks periodically whether notified or resumes when time expired or POST canceled. TRN2 executes in AOR1 and CANCELs the POST when processing complete.

If this technique is used, the dynamic or distributed routing program has to ensure that TRN2 executes in the same CICS region as TRN1 to preserve the application design.

The CANCEL command that notifies the task that issued the POST must:

Be issued in same target region that the POST command was executed in, or
Specify the SYSID of the target region where the POST command was executed, or
Specify a TRANSID (TRN1 in our example) that is defined as a remote transaction residing on the target region where the POST command was 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.

Clearly, there is no problem if the CANCEL is issued by the same task that issued the POST. If a different task cancels the POST command, it must specify REQID to identify the particular instance of that command. Hence the CANCEL command with REQID is indicative of an inter-transaction affinity problem. However, REQID need not be specified on the POST command because CICS automatically generates a REQID and pass it to the application in EIBREQID.

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Last updated: Thursday, 27 June 2019