Advanced Queuing - Oracle

Dequeue Features
DBMS_AQADM.START_QUEUE procedure. You will get an Oracle error if you try to
enable an exception queue for enqueue.
When a message has expired, it is moved to an exception queue. The exception
queue for a message in multiconsumer queue must also be a multiconsumer queue.
Expired messages from multiconsumer queues cannot be dequeued by the intended
recipients of the message. However, they can be dequeued in the REMOVE mode
exactly once by specifying a NULL consumer name in the dequeue options. Hence,
from a dequeue perspective multiconsumer exception queues behave like
single-consumer queues because each expired message can be dequeued only once
using a NULL consumer name. Messages can also be dequeued from the exception
queue by specifying the message ID.
The exception queue is a message property that can be specified during enqueue
time (see "Enqueuing a Message [Specify Message Properties]" on page 11-9 in
Chapter 11, "Operational Interface: Basic Operations"). In PL/SQL users can use the
exception_queue attribute of the DBMS_AQ.MESSAGE_PROPERTIES_T record to
specify the exception queue. In OCI users can use the LNOCISetAttr procedure to
set the LNOCI_ATTR_EXCEPTION_QUEUE attribute of the
LNOCIAQMsgProperties descriptor.
If an exception queue is not specified, the default exception queue is used. If the
queue is created in a queue table, for example, QTAB, the default exception queue
will be called AQ$_QTAB_E. The default exception queue is automatically created
when the queue table is created. Messages are moved to the exception queues by
AQ under the following conditions:
The message is not being dequeued within the specified expiration interval. For
messages intended for more than one recipient, the message will be moved to
the exception queue if one or more of the intended recipients was not able to
dequeue the message within the specified expiration interval. The default
expiration interval is DBMS_AQ.NEVER, meaning the messages will not expire.
The message is being dequeued successfully. However, because of an error that
arises while processing the message, the application that dequeues the message
chooses to roll back the transaction. In this case, the message is returned to the
queue and will be available for any applications that are waiting to dequeue
from the same queue. A dequeue is considered rolled back or undone if the
application rolls back the entire transaction, or if it rolls back to a save point that
was taken before the dequeue. If the message has been dequeued but rolled
back more than the number of times specified by the retry limit, the message
will be moved to the exception queue.
A Sample Application Using AQ 8-81