Priorities for CICS web support transactions (CWXN, CWXU, CWBA, CW2A)

When CICS® receives a request which is not eligible for being processed by directly attached alias transaction, a web attach task is used to receive requests from web clients and perform initial checks. The default transaction ID for a web attach task is CWXN for the HTTP protocol, or CWXU for the USER protocol. Alias transactions are used to cover subsequent processing for application-generated responses. The default transaction ID for a CICS web support alias transaction is CWBA, and for an Atom feed alias transaction, it is CW2A.

Task structure for CICS web support has more information about the transactions that are used for CICS web support processing, and how they interact with each other.

If you set the priority of the CWXN or CWXU transaction (or its alias) higher than the priority of the alias transactions such as CWBA or CW2A, this can result in a build-up of requests that have been received but not yet processed, which might lead to a short-on-storage situation.

The default priorities for the CWXN and CWXU transactions are set to 1, and the default priorities for the CICS-supplied CWBA and CW2A transactions are also set to 1. You can adjust these priorities depending on your workload. Make sure the priorities of the alias transactions like CWBA or CW2A are higher than the priority of the transactions associated with web attach tasks like CWXN or CWXU.

Consider using TCLASS definitions to limit the number of web attach tasks that can run to less than 50% of the maximum user task limit, in order to give priority to the processing of alias transactions. At peak workload, this means that storage is freed in preference to being acquired and that inflight tasks are processed in preference to new requests.