If the application puts or gets the messages as part of a unit of work, and the application then decides to back out the unit of work, the group and segment information is restored to the value that it had previously:
Queues which were updated by the application after the unit of work had started, but outside the scope of the unit of work, do not have their group and segment information restored if the unit of work is backed out.
Restoring the group and segment information to its previous value when a unit of work is backed out allows the application to spread a large message group or large logical message consisting of many segments across several units of work, and to restart at the correct point in the message group or logical message if one of the units of work fails. Using several units of work may be advantageous if the local queue manager has only limited queue storage. However, the application must maintain sufficient information to be able to restart putting or getting messages at the correct point in the event that a system failure occurs. For details of how to restart at the correct point after a system failure, see the PMLOGO option described in MQPMO - Put-message options, and the GMLOGO option described in MQGMO - Get-message options.
The remaining usage notes apply only when the queue manager coordinates the units of work:
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