IBM MQ, Version 8.0Logical and physical ordering
Messages on queues can occur (within each priority level) in physical or logical order.
Physical order is the order in which messages arrive on a queue. Logical order is when all of the messages and segments within a group are in their logical sequence, next to each other, in the position determined by the physical position of the first item belonging to the group.
For a description of groups, messages, and segments, see Message groups. These physical and logical orders can differ because:
Groups can arrive at a destination at similar times from different applications, therefore losing any distinct physical order.
Even within a single group, messages can get out of order because of rerouting or delay of some of the messages in the group. --
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