Whether you attach a new member to the JES-defined group or to an entirely different (installation-defined) group depends upon your XCF configuration. That is, consider the number of signalling paths and the additional message traffic you anticipate flowing across those paths. For JES-specific recommendations, refer to either z/OS JES2 Initialization and Tuning Guide or z/OS JES3 Initialization and Tuning Guide.

To create an installation-defined attachment to JES XCF, issue macro IXZXIXAT. IBM recommends that you issue IXZXIXAT from exit routine IXZXIT03. For a description of IXZXIT03, see IXZXIT03 - Attach/Detach Exit. Only programs executing in the JES2 or JES3 main task environment or in the JES2 or JES3 subtask environment can issue IXZXIXAT.

When the system returns control it provides:

1. A return code that indicates whether it completed its function successfully.
2. The name of a default mailbox to collect system event data.

   Initially, JES XCF supplies a default name for a mailbox that collects system event data. (System event data includes any XCF events on any JES member of the JES XCF group and events on the MVS system under which it runs.) You must build this mailbox, named SYSJES$DEFAULT, at the time a member attaches to a group. The default mailbox will not collect system event data until the IXZXIXMB macro has been issued to build the mailbox. IBM recommends that you continue to use this mailbox for the collection of system event data after attach processing completes and that you build a separate mailbox to collect only messages and acknowledgements sent from JDU to JDU. Then use the appropriate mailbox name for future member communications. If you neither require the default mailbox nor intend to use it after attach processing completes, you must still build it (using the IXZXIXMB macro) and then delete it (using the IXZXIXMD macro). Be aware that you must process and acknowledge any messages sent to the default mailbox. You cannot ignore them. If you do so, JES XCF will not be notified to clean up resources held for each message sent to it. This will eventually cause an ‘out of buffer’ condition.

IBM recommends keeping the JES-defined attachment active for the life of the JES address space.

Each additional attachment requires its own resources (such as storage) within the JES XCF address space. Initially be cautious and be certain to closely monitor each additional attachment for potential system performance degradation.