Selective dequeue of messages unrelated to those already in a transaction.

Hello
In an AQ queue, I need to ensure that messages which are related to each other are processed sequentially.
For example, assume the queue is seeded with the four messages that have a business-related field called transaction reference (txn_ref) and two of the messages (1,3) belong to the same transaction (000001):
id | txn_ref |
1 | 000001 |
2 | 000002 |
3 | 000001 |
4 | 000003 |
Assume also that I am running 4 threads/processes that wish to dequeue from this queue. The following should occur:
1. thread 1 dequeues message #1
2. thread 2 dequeues message #2
3. thread 3 dequeues message #4 (because message #3 is related to #1 and #1 has not yet completed).
4. thread 4 blocks waiting for a message
5. thread 1 commits its work for message #1
6. thread 4 (or perhaps thread 1) dequeues message #3.
My initial thought was that I could achieve this with a dequeue condition where the ENQ_TIME (enqueue time) is not later than any other ENQ_TIME of all the messages that have the same TXN_REF. But my problem is how to reference the TXN_REF of a message that I have not yet selected, in order to select it. e.g.
// Java API
String condition = "ENQ_TIME = (select min(ENQ_TIME) from AQ_TABLE1 where ??"; // where what?
dequeueOption.setCondition(condition);
Is it possible to achieve what I want here?
Edited by: user8264946 on Oct 19, 2010 9:03 PM
Edited by: user8264946 on Oct 19, 2010 9:04 PM

It depends on how acceptable it is for your commit to fail. By far the easiest and safest thing to do is use Optimistic Locking. This way if the server tries to commit changes to something that has already been committed, an Optmistic Lock exception will be thrown. It's up to your application to handle this, and depending on your domain and the likelyhood of this happeneing, you could just tell the user "try again". The other option is pessemistic locking, which is very, very, dangerous (in my opinion). The main issue is that it's resource intensive, and different databases have different semantics on how it works.
IIWY, I'd look at Optimistic Locking in the docs and go from there.
- Don

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Hi all,
I've implemented Advanced Queuing in one Schema(Say "temp") which has AQ privileges. My requirement is Asynchronous Communication between two programs.
In One program i've DBMS_AQ.ENQUEUE to put the message onto the queue. The user "temp" enqueued the message.
I've a anonymous PL/SQL callback procedure registered to dequeue the message. but while dequeuing the message, the user is "SYS". Immediately after dequeuing the message, i've a call to the second program which uses some views. In my application we have dba policies applied to these views. So only temp user can access those views where SYS user doesnt have any access to these views.
we already have one solution i.e., giving access to SYS user also but this will result in some other security concerns for the application
Another solution could be dequeuing the message with "temp" user (NON sys user).
So i would like to know whether it is possible to dequeue the message with NON-SYS user ? i've tried searching the dequeue options and message properties but couldn't get anything..
can anyone help me in solving this problem??

Hi paul,
Thanks for the response :). I've tried this already but still the dequeue user id "SYS". i'll explain you in detail with the code.
I've create queue table and queue with the following code :
create or replace TYPE temp_msg_type as object (seq_no NUMBER(10),
req_type varchar2(6))
begin
dbms_aqadm.create_queue_table(
queue_table => 'test_queue_tab',
queue_payload_type => 'temp_msg_type',
multiple_consumers => true);
dbms_aqadm.create_queue(
queue_name => 'test_queue',
queue_table => 'test_queue_tab');
dbms_aqadm.start_queue(
queue_name => 'test_queue');
end;
After this an Agent is subscribed to this queue.
begin
dbms_aqadm.add_subscriber(
queue_name => 'test_queue',
subscriber => sys.aq$_agent('recipient', null, null));
end;
Then registered an PL/SQL callback procedure to this queue.
begin
dbms_aq.register(aq$_reg_info_list(
aq$_reg_info('test_queue:RECIPIENT',
DBMS_aq.NAMESPACE_AQ,
'plsql://notifyCB_prd',
HEXTORAW('FF')) ) ,
1);
end;
The follwing procedure will enqueue the message. in the below code before i enqueue it i am inserting the current sessions' user, it got printed as "SCOTT".
create or replace procedure enqueue_msg_prd( O_status_code IN OUT varchar2 )
as
enqueue_options dbms_aq.enqueue_options_t;
message_properties dbms_aq.message_properties_t;
message_handle RAW(16);
message temp_msg_type;
begin
--enqueue_options.visibility := DBMS_AQ.IMMEDIATE;
message := temp_msg_type(112, user);
O_status_code := 'S';
dbms_aq.enqueue(queue_name => 'test_queue',
enqueue_options => enqueue_options,
message_properties => message_properties,
payload => message,
msgid => message_handle);
end;
The following procedure will dequeue the message. this procedure will be automatically triggered when i commit the enqueue transaction since there is anonymous callback procedure registered for this queue. In the below code after i dequeue it i am inserting the sessions' user, it got printed as "SYS".
create or replace
procedure notifyCB_prd( context raw,
reginfo aq$_reg_info,
descr aq$_descriptor,
payload raw,
payloadl number)
as
dequeue_options dbms_aq.dequeue_options_t;
message_properties dbms_aq.message_properties_t;
message_handle raw(16);
message temp_msg_type;
begin
dequeue_options.msgid := descr.msg_id;
dequeue_options.consumer_name := descr.consumer_name;
--execute immediate 'conn session set current_user= scott';
DBMS_AQ.DEQUEUE(
queue_name => descr.queue_name,
dequeue_options => dequeue_options,
message_properties => message_properties,
payload => message,
msgid => message_handle);
insert into temp values(message.seq_no,user);
commit;
end;
Even i queried the aq$test_queue_Tab view. in this view, value in deq_user_id is "SYSS"
is there any way to get the users session as 'SCOTT' ??

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FROM AQ$PROCESSASSETS_Q;
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Have you looked at
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   queue_name          IN      VARCHAR2,
   dequeue_options     IN      dequeue_options_t,
   message_properties OUT     message_properties_t,
   payload             OUT     "type_name",
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If there is some data falls in the queue table.How to dequeue a message automatically by getting a notification in my application side .
Below is the link i am following for enqueuing the message into a queue table, its happening successfully.
http://www.oratechinfo.co.uk/aq.html
I can see the message i enqueued in the queue table with the following query at the scheduled time. select user_data from queue_table;
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http://docs.oracle.com/cd/A83908_02/NT816EE/DOC/nt.816/a99999/o4c00069.htm

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I added a table lock to prevent parallel execution of the Call Back, but when this resource lock is freed, many oracle processes are started in parallel to process the rest of the notification pending.
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If you do a dequeue with dequeue_options.wait=dbms_aq.forever it will wait until the message is enqueued. It is not consuming (much) cpu-time. AQ solves this under water for you by polling the queue-table.
If you schedule this job with an interval of say 10secs (using dbms_job or dbms_scheduler) the job will come up after database startup. The only other option is to start the process using a sqlplus or similar tool. But since the dequeueu will wait for the next incomming message the sqlplus session will halt. Using the jobscheduler in the database causes the job to be run in a session with in the database (so you get your session freed) and after a database restart, the job will come up.
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. not use wait=dbms_aq.forever, but a reasonable amount of wait time and then let your job run to complete. Then the job scheduler can reschedule the process in an interval of for example 1 - 10 sec.
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Regards,
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Selective dequeue of messages unrelated to those already in a transaction.

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