Advanced Queuing - Oracle

Elements of Advanced Queuing
Queue
Queue Table
Agent
The control information represents message properties used by AQ to manage
messages. The payload data is the information stored in the queue and is
transparent to Oracle AQ. A message can reside in only one queue. A message is
created by the enqueue call and consumed by the dequeue call.
A queue is a repository for messages. There are two types of queues: user queues,
also known as normal queues, and exception queues. The user queue is for normal
message processing. Messages are transferred to an exception queue if they cannot
be retrieved and processed for some reason. Queues can be created, altered, started,
stopped, and dropped by using the Oracle AQ administrative interfaces. Refer to
Chapter 9, "Administrative Interface" for more information.
User queues can be persistent (the default) or nonpersistent queues. Persistent
queues store messages in database tables. These queues provide all the reliability
and availability features of database tables. Nonpersistent queues store messages in
memory. They are generally used to provide an asynchronous mechanism to send
notifications to all users that are currently connected.
Queues are stored in queue tables. Each queue table is a database table and contains
one or more queues. Each queue table contains a default exception queue.
Figure 7–1, "Basic Queues" on page 7-2 shows the relationship between messages,
queues, and queue tables.
An agent is a queue user. This can be an end user or an application. There are two
types of agents:
Producers who place messages in a queue (enqueuing)
Consumers who retrieve messages (dequeuing)
Any number of producers and consumers may be accessing the queue at a given
time. Agents insert messages into a queue and retrieve messages from the queue by
using the Oracle AQ operational interfaces. Refer to Chapter 11, "Operational
Interface: Basic Operations" for more information.
An agent is identified by its name, address and protocol. Refer to "Agent Type
(aq$_agent)" on page 2-3 in Chapter 2, "Basic Components" for a formal description
of this data structure.
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