Advanced Queuing - Oracle

Dequeue Features
}
Listen Capability
"(tab.user_data.ordertype = 'RUSH' AND " +
"tab.user_data.customer.country = 'USA')";
bookedorders_q.addSubscriber(agt2, rule);
/* Add a rule-based subscriber for Overseas Shipping
Intl Shipping handles all non-U.S. orders: */
agt3 = new AQAgent("Overseas_Shipping", "OS.OS_bookedorders_que");
rule = "tab.user_data.orderregion = 'INTERNATIONAL'";
bookedorders_q.addSubscriber(agt3, rule);
}
catch (Exception ex)
{
System.out.println("Exception: " + ex);
}
Advanced Queuing can monitor multiple queues for messages with a single call,
LISTEN. An application can use LISTEN to wait for messages for multiple
subscriptions. It can also be used by gateway applications to monitor multiple
queues. If the LISTEN call returns successfully, a dequeue must be used to retrieve
the message (see "Listening to One or More Single-Consumer Queues" on
page 11-23.
Without the LISTEN call, an application which sought to dequeue from a set of
queues would have to continuously poll the queues to determine if there were a
message. Alternatively, you could design your application to have a separate
dequeue process for each queue. However, if there are long periods with no traffic
in any of the queues, these approaches will create unacceptable overhead. The
LISTEN call is well suited for such applications.
Note that when there are messages for multiple agents in the agent list, LISTEN
returns with the first agent for whom there is a message. In that sense LISTEN is not
'fair' in monitoring the queues. The application designer must keep this in mind
when using the call. To prevent one agent from 'starving' other agents for messages,
the application can change the order of the agents in the agent list.
8-90 Oracle9i Application Developer’s Guide - Advanced Queuing