Simultaneous Access to the same queue

Question

Could some one please clarify how does the AQ mechanism handle requests in the following scenario:
Two applications are both talking to the same Queue using the same consumer for dequeuing messages. The dequeuing happens under a main transaction and the dequeue mode is set to REMOVE. The messages is completely removed from the queue when the transaction is committed at the end of processing the message. Also, the dequeue API is through stored procedures.
I would like to know if, while one application has dequeued a message and is still processing it as a part of the transaction, what happens when the second application tries to access the same queue and try to dequeue a message. Would it get the same message that the first application is still processing and is not yet been completely dequeued.
Let me know.
Thanks,
-NS.

Answer 1

--- create queue type
CREATE TYPE mesaj_t AS OBJECT
message varchar2(100)
--- create queue table , queue and start queue
BEGIN
dbms_aqadm.create_queue_table(queue_table => 'mesaj_qt',
queue_payload_type => 'mesaj_t',
sort_list => 'PRIORITY,ENQ_TIME',
multiple_consumers => FALSE);
dbms_aqadm.create_queue(queue_name => 'mesaj_q',
queue_table => 'mesaj_qt');
dbms_aqadm.start_queue(queue_name => 'mesaj_q');
END;
--enqueue message
DECLARE
r_enqueue_options dbms_aq.enqueue_options_t;
r_message_properties dbms_aq.message_properties_t;
v_message_handle RAW(16);
BEGIN
dbms_aq.enqueue(queue_name => 'mesaj_Q',
enqueue_options => r_enqueue_options,
message_properties => r_message_properties,
payload => mesaj_t(message => 'selam'),
msgid => v_message_handle);
END;
--dequeue message from session 1 and dont comit
DECLARE
r_dequeue_options dbms_aq.dequeue_options_t;
r_message_properties dbms_aq.message_properties_t;
v_message_handle RAW(16);
l_corr_identifier VARCHAR2(128);
l_recipient_list dbms_aq.aq$_recipient_list_t;
v_payload_handle mesaj_t;
BEGIN
dbms_aq.dequeue(queue_name => 'mesaj_Q',
dequeue_options => r_dequeue_options,
message_properties => r_message_properties,
payload => v_payload_handle,
msgid => v_message_handle);
END;
--and try to dequeue from second session same message with using msg_id property
DECLARE
r_dequeue_options dbms_aq.dequeue_options_t;
r_message_properties dbms_aq.message_properties_t;
v_message_handle RAW(16);
l_corr_identifier VARCHAR2(128);
l_recipient_list dbms_aq.aq$_recipient_list_t;
v_payload_handle mesaj_t;
BEGIN
r_dequeue_options.wait := DBMS_AQ.NO_WAIT; -- if i change to DBMS_AQ.FOREVER then it will wait for new message for dequeuing, with no_wait you will get an exception immediately if there is no message for you in queue
r_dequeue_options.msgid := 'BBAB7BA403CD4B259A86B55783862912'; --replace this with your msg_id of your enqueued message
dbms_aq.dequeue(queue_name => 'mesaj_Q',
dequeue_options => r_dequeue_options,
message_properties => r_message_properties,
payload => v_payload_handle,
msgid => v_message_handle);
END;
conclusion :
with remove mode other session can not dequeue message when first session processing message and not committed yet