21.4 Integrating with Oracle Advanced Queuing

Oracle Advanced Queuing is a means for building an asynchronous request/response mechanism around a so-called queue and two processes: ENQUEUE, which puts MESSAGES into a queue, and DEQUEUE, which reads the queue.

Advanced queuing provides sophisticated mechanisms for distributing messages across queues and for queue subscription. These mechanisms are all built on top of these basic elements (ENQUEUE, DEQUEUE, and MESSAGES).

With the Event-Driven Publishing API you can use these queues to store and transmit report jobs. You can even build your own queuing mechanism if the one provided with Oracle Reports Services does not fit your needs.

21.4.1 Creating a Queue That Holds Messages of Type SRW_PARAMLIST

A queue is a table in the database that holds, along with several administrative columns, an object column that represents a message. In our case the message is the parameter list.

The dbms_AQadm package, provided with Advanced Queuing, contains all the administrative functions required for setting up an advanced queuing system.

Use dbms_AQadm.Create_Queue_Table to create the physical table in the database. You must pass it a name for the table and a name for the object type that will define the message for this queue.

For example:

…
execute dbms_AQadm.Create_Queue_Table

(queue_Table=>'queuename._tab',
queue_Payload_Type=>'SRW_PARAMLIST_OBJECT',
compatible=>'9.0');

In earlier examples, we created the object type SRW_PARAMLIST_OBJECT that encapsulates the SRW_PARAMLIST type in object notation so it can be used as a message.

After creating the queue table, you must create the queue with dbms_AQadm.Create_Queue and start the queue with dbms_AQadm.Start_Queue.

For example:

…
execute dbms_AQadm.Create_Queue

(Queue_Name=>'queuename',Queue_Table=>'queuename._tab');

prompt … starting queue
execute dbms_AQadm.Start_Queue

(Queue_Name=>'queuename');

…

Note:

You'll find a complete example for setting up, creating, and starting a simple queue in the demo file srwAQsetup.sql, included with your Oracle Reports Services installation.

Having created and started the queue, what you need now is a procedure that creates a message in this queue and a procedure that reads out the queue and submits the job to the server. These are discussed in the following sections.

21.4.2 Creating the Enqueuing Procedure

The enqueuing procedure is responsible for putting a message into the queue. This procedure can be part of your application, called by a database-trigger, or provided through an external mechanism. In this section, we will provide an example of creating a stored procedure that puts a simple message in this queue.

Because our message is the parameter list itself, the procedure is fairly easy. We use the same code we used in earlier sections to create a parameter list. In addition to the variables we used, we define an object variable to hold the message we will put into the queue.

…
  plist_object SRW_ParamList_Object;
…

After creating the parameter list we create the actual message object using the object constructor.

…
plist_object := SRW_PARAMLIST_OBJECT(plist);
…

Then we enqueue the message using the enqueue procedure provided by Advanced Queuing.

…
dbms_aq.enqueue(queue_name => 'myQueue',

enqueue_options => enqueue_options,
message_properties => message_properties,
payload => PList_Object,
msgid => message_handle);

…

The message is put into the queue. Because we did not set up any message distribution, the message will stay in the queue until it is fetched by a dequeuing-procedure, which is discussed in the next section.

Note:

For the exact syntax of dbms_aq.enqueue refer to the Advanced Queuing API Reference document.

You'll find additional examples in the srwAQsetup.sql file included with your Oracle Reports Services installation.

21.4.3 Creating the Dequeuing Procedure

A dequeuing procedure reads out all available messages in a queue and processes them. In our case, we want to read out the message and submit a job to the server using the parameter list that was attached to the message.

To accomplish this, we follow this example:

BEGIN

dequeue_options.wait := 1; 

loop

DBMS_AQ.DEQUEUE(queue_name => 'myQueue',

dequeue_options => dequeue_options,
message_properties => message_properties,
payload => PList_Object,
msgid => message_handle);

COMMIT;
plist := plist_object.params;   
r_jid := SRW.RUN_REPORT(plist); 

end loop;
exception when aq_timeout then

begin

NULL;

end;

END;

This code example will read out the queue until all messages have been processed. Time allowed for processing is determined by the timeout defined in the second line of code. This timeout defines the amount of seconds the dequeue procedure should wait for a message before creating a timeout exception.

The DBMS_AQ.DEQUEUE built-in is provided by Advanced Queuing for reading out messages. It puts the payload of the message, the object that holds the information, into the object defined by the payload parameter.

Using plist, we extract the information from the payload object. As mentioned before, our object holds a parameter list. It is stored in the attribute PARAMS inside the object. The extracted parameter list is then handed over to SRW.RUN_REPORT for submitting the job.

If you want to avoid the need for invoking this dequeuing procedure by hand, you can run it as a job inside the database.