Oracle’s Advanced Queuing (AQ) is an integrated messaging feature in the Oracle database that enables message-based communication between applications. The AQ provides a reliable messaging infrastructure that guarantees the delivery of messages between sender and receiver in a distributed environment, providing a robust solution for message processing and queuing.
The primary purpose of AQ is to facilitate reliable messaging between different components of an application or between different applications. This is achieved by creating a queue where messages can be deposited and retrieved as needed. Messages can be sent to the queue by one component in an application and retrieved by another component in the same or a different application. This facilitates communication and coordination between components or across applications, even when they are running on different servers or different locations.
AQ operates in a publish-subscribe model: publishers create messages and post them to a queue or topic, while subscribers receive the messages from the queue or topic. Unlike traditional messaging systems, AQ provides message persistence and support for transactions, making it suitable for mission-critical applications. The messages are stored in a durable, persistent store and can be retrieved by subscribers even if the server fails or if a network connection is lost.
AQ supports three types of queues: Normal, Exception and Delay queues. Normal queues are the standard queues for storing messages. Messages are stored in the normal queues, and subscribers retrieve messages from the queue as they become available. Exception queues are for handling messages that cannot be processed. If a message fails validation or processing, it is moved to an exception queue, where it can be processed later. Delay queues allow messages to be held for a specified amount of time before being delivered to the subscriber. This is useful for time-sensitive or batch processing applications.
The AQ is built on top of Oracle’s database transactions, so it can provide atomicity, consistency, isolation, and durability (ACID) properties for message transactions. This ensures that message transactions are treated as part of the database transaction and that they are managed in a consistent and reliable manner.
Here is an example of how to use AQ from inside Oracle:
--Create a queue table
EXECUTE DBMS_AQADM.CREATE_QUEUE_TABLE('my_queue_table', 'SYS.AQ$_JMS_TEXT_MESSAGE');
--Create a queue
EXECUTE DBMS_AQADM.CREATE_QUEUE('my_queue', 'my_queue_table');
--Enqueue a message
DECLARE
enqueue_options DBMS_AQ.ENQUEUE_OPTIONS_T;
message_properties DBMS_AQ.MESSAGE_PROPERTIES_T;
message_handle RAW(16);
message_payload VARCHAR2(100) := 'Hello World!';
BEGIN
DBMS_AQ.ENQUEUE(
queue_name => 'my_queue',
enqueue_options => enqueue_options,
message_properties => message_properties,
payload => CAST(message_payload AS SYS.AQ$_JMS_TEXT_MESSAGE),
msgid => message_handle);
END;
/
--Dequeue a message
DECLARE
dequeue_options DBMS_AQ.DEQUEUE_OPTIONS_T;
message_properties DBMS_AQ.MESSAGE_PROPERTIES_T;
message_handle RAW(16);
message_payload VARCHAR2(100);
BEGIN
DBMS_AQ.DEQUEUE(
queue_name => 'my_queue',
dequeue_options => dequeue_options,
message_properties => message_properties,
payload => CAST(message_payload AS SYS.AQ$_JMS_TEXT_MESSAGE),
msgid => message_handle);
DBMS_OUTPUT.PUT_LINE('Dequeued message: ' || message_payload);
END;
/