Advanced Queuing and Transactional Event Queues

Oracle Database Advanced Queuing (AQ) and Oracle Transactional Event Queues (TxEventQ) provide database-integrated message queuing functionality. AQ leverages the functions of Oracle Database so that messages can be stored persistently, propagated between queues on different computers and databases, and transmitted using Oracle Net Services and HTTP(S). TxEventQ provides robust real-time messaging, streaming events, and publish/subscribe messaging with multiple publishers and multiple consumers. It is a high performance partitioned implementation with multiple event streams per queue, while AQ is a disk-based implementation for simpler workflow use cases.

ODP.NET uses the same APIs when executing operations against either AQ or TxEventQ. From a .NET application level, both queue types have the same functionality. Thus, ODP.NET developers can use the same code for working with either AQ or TxEventQ. Where the two queue types differ is in their implementations and administration.

Due to the commonality at the .NET app level, this book will use the term "AQ" to refer synonomously to both AQ and TxEventQ. When differences exist, the book will refer to each queue type separately.

ODP.NET queuing APIs execute operational functions, such as enqueuing and dequeuing messages. Administrative functions, such as creating and managing queues, are not available in ODP.NET. Use Oracle Developer Tools for Visual Studio or DBMS_AQADM PL/SQL package to perform these administrative functions.

As Oracle Database AQ is implemented in database tables, all operational benefits of high availability, scalability, and reliability are also applicable to queue data. Oracle Database AQ supports standard database features such as recovery, restart, and security.

The following items discuss Oracle Database AQ concepts:

  • Queues and Queue Tables

    Messages enqueued in a queue are stored in a queue table. A queue table must be created before creating a queue based on it. Use the DBMS_AQADM PL/SQL package or Oracle Developer Tools for Visual Studio to create and administer queue tables and queues.

    Queues are represented by OracleAQQueue objects.

  • Single-Consumer and Multiple-Consumer Queues

    A single-consumer queue is created based on a single consumer queue table. Messages enqueued in a single-consumer queue can be dequeued by only a single consumer.

    A multiple-consumer queue is based on a multiple-consumer queue table. This queue supports queue subscribers and message recipients.

  • Message Recipients

    A message producer can submit a list of recipients when enqueuing a message. This allows for a unique set of recipients for each message in the queue. The recipient list associated with the message overrides the subscriber list, if any, associated with the queue. The recipients need not be in the subscriber list. However, recipients can be selected from among the subscribers.The Recipients property of an OracleAQMessage can be used to specify the recipients to a specific message in terms of OracleAQAgent objects.

  • Enqueue

    Messages are enqueued when producer applications push the messages into a queue. This is accomplished by calling the Enqueue method on an OracleAQQueue object. Multiple messages can be enqueued using the EnqueueArray method.

  • Dequeue

    Messages are dequeued when consumer applications pull the messages from a queue. This is accomplished by calling the Dequeue method on an OracleAQQueue object. Multiple messages can be dequeued using the DequeueArray method.

  • Listen

    Subscriber applications can use a Listen call to monitor multiple queues for subscriptions on different queues. This is a more scalable solution for cases where a subscriber application has subscribed to many queues and wishes to receive messages that arrive in any of the queues.This is accomplished by calling the Listen method of the OracleAQQueue class, passing the list of subscriptions in form of an array.

  • Notification

    Subscriber applications can utilize the notification mechanism to get notifications about message availability in a queue. The applications can decide to skip or dequeue the message from the queue based on the information received.

    A subscriber application must register for event notification on the queues from which it wants to receive notifications. This is represented by the MessageAvailable event on OracleAQQueue. The event is triggered when messages matching the subscriptions arrive.

    Notifications can be registered as regular or grouping notifications. A time out value for these notifications can also be specified. Various notification options can be set using the OracleAQQueue.Notification property. Notifications set on an OracleAQQueue object gets cancelled automatically when the object gets disposed.

  • Buffered Messaging

    In buffered messaging, messages reside in a shared memory area. This makes it faster than persistent messaging. The messages are written to disk only when the total memory consumption of buffered messages approaches the available shared memory limit. Buffered messaging is ideal for applications that do not require the reliability and transaction support of Oracle Database AQ persistent messaging.

    Buffered and persistent messages use the same single-consumer or multi-consumer queues, and the same administrative and operational interfaces. They are distinguished from each other by a delivery mode parameter. When an application enqueues a message to an Oracle Database AQ queue, it sets the delivery mode parameter as well.

  • Messages

    Messages consist of control information (metadata) and a payload (data). The control information represents message properties AQ uses to manage messages. The payload data is the information stored in the queue and is transparent to Oracle AQ. A message can reside in only one queue. A message is created by the enqueue call and consumed by the dequeue call.

    The possible message payload data types are:

    • JSON

    • RAW

    • User-Defined Type

    • XML

    Unmanaged ODP.NET does not support JSON payloads.

Using ODP.NET for Advanced Queuing

.NET applications can use ODP.NET to access all the operational features of AQ such as Enqueuing, Dequeuing, Listen, and Notification.

Table 3-36 maps the AQ features to their corresponding ODP.NET implementation.

Table 3-36 Mapping AQ Features with their ODP.NET Implementation

Functionality ODP.NET Implementation

Create a Message

Create an OracleAQMessage object

Enqueue a single message

Specify the message as OracleAQMessage, queue as OracleAQQueue and enqueue options on OracleAQQueue, call OracleAQQueue.Enqueue

Enqueue multiple messages

Specify the messages as an OracleAQMessage array in OracleAQQueue.EnqueueArray

Dequeue a single message

Specify dequeue options on OracleAQQueue and call OracleAQQueue.Dequeue

Dequeue multiple messages

Call OracleAQQueue.DequeueArray

Listen for messages on Queue(s)

Call OracleAQQueue.Listen.To listen on multiple queues use static Listen method of OracleAQQueue

Message Notifications

Use OracleAQQueue.MessageAvailable Event along with the NotificationConsumers property

Enqueuing and Dequeuing Example

The following example demonstrates enqueuing and dequeuing messages using a single consumer queue. The first part of the example performs the requisite database setup for the database user, SCOTT. The second part of the example demonstrates enqueuing and dequeuing messages.

-- Part I: Database setup required for this demo
-- SQL to grant appropriate privilege to database user, SCOTT
User altered.
-- PL/SQL to create queue-table and queue and start queue for SCOTT
-- PL/SQL to stop queue and drop queue & queue-table from SCOTT
    queue_name => 'scott.test_q', 
    auto_commit => TRUE);
    queue_table => 'scott.test_q_tab',
    force => FALSE, 
    auto_commit => TRUE);
-- End of Part I, database setup.

//Part II: Enqueuing and dequeuing messages
using System;
using System.Text;
using Oracle.DataAccess.Client;
using Oracle.DataAccess.Types;
namespace ODPSample
  /// <summary>
  /// Demonstrates Enqueuing and Dequeuing raw message 
  /// using a single consumer queue
  /// </summary>
  class EnqueueDequeue
    static void Main(string[] args)
      // Create connection
      string constr = "user id=scott;password=Pwd4Sct;data source=oracle";
      OracleConnection con = new OracleConnection(constr);
      // Create queue
      OracleAQQueue queue = new OracleAQQueue("scott.test_q", con);
        // Open connection
        // Begin txn for enqueue
        OracleTransaction txn = con.BeginTransaction();
        // Set message type for the queue
        queue.MessageType = OracleAQMessageType.Raw;
        // Prepare message and RAW payload
        OracleAQMessage enqMsg = new OracleAQMessage();
        byte[] bytePayload = { 0, 1, 2, 3, 4, 5, 6, 7, 8, 9 };
        enqMsg.Payload = bytePayload;
        // Prepare to Enqueue
        queue.EnqueueOptions.Visibility = OracleAQVisibilityMode.OnCommit;
        // Enqueue message
        Console.WriteLine("Enqueued Message Payload      : "
          + ByteArrayToString(enqMsg.Payload as byte[]));
        Console.WriteLine("MessageId of Enqueued Message : "
          + ByteArrayToString(enqMsg.MessageId));
        // Enqueue txn commit
        // Begin txn for Dequeue
        txn = con.BeginTransaction();
        // Prepare to Dequeue
        queue.DequeueOptions.Visibility = OracleAQVisibilityMode.OnCommit;
        queue.DequeueOptions.Wait = 10;
        // Dequeue message
        OracleAQMessage deqMsg = queue.Dequeue();
        Console.WriteLine("Dequeued Message Payload      : "
          + ByteArrayToString(deqMsg.Payload as byte[]));
        Console.WriteLine("MessageId of Dequeued Message : "
          + ByteArrayToString(deqMsg.MessageId));
        // Dequeue txn commit
      catch (Exception e)
        Console.WriteLine("Error: {0}", e.Message);
        // Close/Dispose objects
    // Function to convert byte[] to string
    static private string ByteArrayToString(byte[] byteArray)
      StringBuilder sb = new StringBuilder();
      for (int n = 0; n < byteArray.Length; n++)
      return sb.ToString();