This is an MDB container question, probably to do with pool size etc. refer to http://wiki.jboss.org/wiki/ConfigJBossMDB on how to configure MDB's.
FYI The JMS spec doesn't require message order to be preserved after rollback.
Yes I know that the spec says nothing about order.
But still this behavior is strange. The pool size is set via annotations:
@ActivationConfigProperty(propertyName="maxMessages", propertyValue="1") })
I just wonder why its always twisting with 2 messages instead of 3 or 5 or any higher number.
If I can force this to 1 consumer its exacly what my customer need. :-)
Like I say, this is an MDB question, if you post on the appropriate forum they will be able to help you. Remember MDB's are JMS Provider agnostic.