Advanced Queuing - Oracle

Dequeue Features
If a message is browsed, it remains available for further processing. Similarly if a
message is locked, it remains available for further processing after the lock is
released by performing a transaction commit or rollback. After a message is
consumed, using either of the remove modes, it is no longer available for dequeue
requests.
When a message is dequeued using REMOVE_NODATA mode, the payload of the
message is not retrieved. This mode can be useful when the user has already
examined the message payload, possibly by means of a previous BROWSE dequeue.
In this way, you can avoid the overhead of payload retrieval that can be substantial
for large payloads
A message is retained in the queue table after it has been consumed only if a
retention time is specified for a queue. Messages cannot be retained in exception
queues (refer to the section on exceptions for further information). Removing a
message with no data is generally used if the payload is known (from a previous
browse/locked mode dequeue call), or the message will not be used.
Note that after a message has been browsed, there is no guarantee that the message
can be dequeued again since a dequeue call from a concurrent user might have
removed the message. To prevent a viewed message from being dequeued by a
concurrent user, you should view the message in the locked mode.
In general, use care while using the browse mode. The dequeue position is
automatically changed to the beginning of the queue if a nonzero wait time is
specified and the navigating position reaches the end of the queue. Hence repeating
a dequeue call in the browse mode with the "next message" navigation option and a
nonzero wait time can dequeue the same message over and over again. We
recommend that you use a nonzero wait time for the first dequeue call on a queue
in a session, and then use a zero wait time with the next message navigation option
for subsequent dequeue calls. If a dequeue call gets an "end of queue" error
message, the dequeue position can be explicitly set by the dequeue call to the
beginning of the queue using the "first message" navigation option, following which
the messages in the queue can be browsed again.
Scenario
In the following scenario from the BooksOnLine example, international orders
destined to Mexico and Canada are to be processed separately due to trade policies
and carrier discounts. Hence, a message is viewed in the locked mode (so no other
concurrent user removes the message) and the customer country (message payload)
is checked. If the customer country is Mexico or Canada, the message is consumed
(deleted from the queue) using REMOVE_NODATA (since the payload is already
known). Otherwise, the lock on the message is released by the commit call. Note
8-70 Oracle9i Application Developer’s Guide - Advanced Queuing