Advanced Queuing - Oracle

Dequeue Features
For messages intended for multiple recipients, each message keeps a separate
retry count for each recipient. The message is moved to the exception queue
only when retry counts for all recipients of the message have exceeded the
specified retry limit. The default retry limit is 5 for single-consumer queues and
8.1-compatible multiconsumer queues. No retry limit is not supported for 8.0-
compatible multiconsumer queues.
The statement executed by the client contains a dequeue that succeeded but the
statement itself was undone later due to an exception. To understand this case,
consider a PL/SQL procedure that contains a call to DBMS_AQ.DEQUEUE. If the
dequeue procedure succeeds but the PL/SQL procedure raises an exception,
AQ will attempt to increment the RETRY_COUNT of the message returned by the
dequeue procedure.
The client program successfully dequeued a message but terminated before
committing the transaction.
Messages intended for 8.1-compatible multiconsumer queues cannot be dequeued
by the intended recipients once the messages have been moved to an exception
queue. These messages should instead be dequeued in the REMOVE or BROWSE
mode exactly once by specifying a NULL consumer name in the dequeue options.
The messages can also be dequeued by their message IDs.
Messages intended for single consumer queues, or for 8.0-compatible
multiconsumer queues, can only be dequeued by their message IDs once the
messages have been moved to an exception queue.
Users can associate a RETRY_DELAY with a queue. The default value for this
parameter is 0, meaning that the message will be available for dequeue immediately
after the RETRY_COUNT is incremented. Otherwise the message will be unavailable
for RETRY_DELAY seconds. After RETRY_DELAY seconds, the queue monitor marks
the message as READY.
For a multiconsumer queue, RETRY_DELAY is for each subscriber.
Scenario
In the BooksOnLine application, the business rule for each shipping region is that
an order will be placed in a back order queue if the order cannot be filled
immediately. The back order application will try to fill the order once a day. If the
order cannot be filled within 5 days, it is placed in an exception queue for special
processing. You can implement this process by making use of the retry and
exception handling features in AQ.
8-82 Oracle9i Application Developer’s Guide - Advanced Queuing