Advanced Queuing - Oracle

General Features of Advanced Queuing
Subscriber applications may receive messages in the following manner:
A dequeue call retrieves messages that match the subscription criteria.
A listen call may be used to monitor multiple queues for subscriptions on
different queues. This is a more scalable solution in cases where a
subscriber application has subscribed to many queues and wishes to receive
messages that arrive in any of the queues.
Use the OCI notification mechanism. This allows a push mode of message
delivery. The subscriber application registers the queues (and subscriptions
specified as subscribing agent) from which to receive messages. This
registers a callback to be invoked when messages matching the
subscriptions arrive.
Scenario
The BooksOnLine application illustrates the use of a publish-subscribe model for
communicating between applications. The following subsections give some
examples.
Defining queues The Order Entry application defines a queue (OE_booked_
orders_que) to communicate orders that are booked to various applications. The
Order Entry application is not aware of the various subscriber applications and
thus, a new subscriber application can be added without disrupting any setup or
logic in the Order Entry (publisher) application.
Setting Up Subscriptions The various shipping applications and the customer service
application (that is, Eastern region shipping, Western region shipping, Overseas
shipping and Customer Service) are defined as subscribers to the booked_orders
queue of the Order Entry application. Rules are used to route messages of interest to
the various subscribers. Thus, Eastern Region shipping, which handles shipment of
all orders for the East coast and all rush U.S. orders, expresses the subscription rule
as follows:
rule => 'tab.user_data.orderregion = ''EASTERN'' OR
(tab.user_data.ordertype = ''RUSH'' AND
tab.user_data.customer.country = ''USA'') '
Each subscriber can specify a local queue where messages are to be delivered. The
Eastern region shipping application specifies a local queue (ES_booked_orders_
que) for message delivery by specifying the subscriber address as follows:
subscriber := aq$_agent('East_Shipping', 'ES.ES_bookedorders_que', null);
A Sample Application Using AQ 8-29