What is the syntax for creating a queue subscriber in PL/SQL?

Question

I'm trying to create a queue and a callback that triggers when a message is queued, but I can't get the callback to trigger. What am I doing wrong?

I have a trigger that enqueues a message, and I can see it on the queue message table, and I can dequeue it by hand and process it, I just can't get the callback to fire on enqueue.

BEGIN    
DBMS_AQADM.CREATE_QUEUE_TABLE (
  queue_table        => 'queue_message_table',
  queue_payload_type => 'queue_message_type',
  multiple_consumers => TRUE);

DBMS_AQADM.CREATE_QUEUE (
  queue_name  => 'message_queue',
  queue_table => 'queue_message_table');
DBMS_AQADM.START_QUEUE (queue_name => 'message_queue');
END;

CREATE OR REPLACE PROCEDURE queue_callback(
  context RAW, reginfo  SYS.AQ$_REG_INFO, descr SYS.AQ$_DESCRIPTOR, payload  RAW, payloadl NUMBER) AS

    queue_options       DBMS_AQ.DEQUEUE_OPTIONS_T;
    message_properties  DBMS_AQ.MESSAGE_PROPERTIES_T;
    my_message          queue_message_type;
    ret                 varchar2(200);
    message_id          RAW(16);
BEGIN
    DBMS_OUTPUT.PUT_LINE('Callback');
    queue_options.msgid := descr.msg_id;
    queue_options.consumer_name := descr.consumer_name;

    DBMS_AQ.DEQUEUE(
        queue_name => descr.queue_name,
        dequeue_options => queue_options,
        message_properties => message_properties,
        payload => my_message,
        msgid => message_id );
    ret := handle_message(my_message);
    commit;
END;

BEGIN
  DBMS_AQADM.ADD_SUBSCRIBER (queue_name => 'message_queue',
    subscriber => SYS.AQ$_AGENT('queue_subscriber', 'message_queue',NULL));
  DBMS_AQ.REGISTER (
    SYS.AQ$_REG_INFO_LIST(
      SYS.AQ$_REG_INFO(
        'MESSAGE_QUEUE:QUEUE_SUBSCRIBER',
        DBMS_AQ.NAMESPACE_AQ,
        'plsql://QUEUE_CALLBACK',
        HEXTORAW('FF')
      )
    ), 1
  );
END;

Answer 1

At first glance, it appears you're neither starting the queue (dbms_aqadm.start_queue), neither are you enqueueing anything to it (dbms_aq.enqueue).

I'd recommend following this demo.