# Dead Letter Example

This example shows you how to define and deal with dead letter messages.

Messages can be delivered unsuccessfully (e.g. if the transacted session used to consume them is rolled back). Such a message goes back to the JMS destination ready to be redelivered. However, this means it is possible for a message to be delivered again and again without any success and remain in the destination, clogging the system.

To prevent this, messaging systems define dead letter messages: after a specified unsuccessful delivery attempts, the message is removed from the destination and instead routed to a dead letter address where they can be consumed for further investigation.

The example will show how to configure HornetQ to route a message to a dead letter address after 3 unsuccessful delivery attempts.
The example will send 1 message to a queue. We will deliver the message 3 times and rollback the session every time.
On the 4th attempt, there won't be any message to consume: it will have been moved to a dead letter address.
We will then consume this dead letter message.

## Example setup

Dead letter addresses and maximum delivery attempts are defined in the configuration file hornetq-configuration.xml:

             <address-setting match="jms.queue.exampleQueue">
                <dead-letter-address>jms.queue.deadLetterQueue</dead-letter-address>
                <max-delivery-attempts>3</max-delivery-attempts>
             </address-setting>

This configuration will moved dead letter messages from `exampleQueue` to the `deadLetterQueue`.

HornetQ allows to specify either a `Queue` by prefixing the `dead-letter-address` with `jms.queue.` or a `Topic` by prefixing with `jms.topic.`.
In this example, we will use a `Queue` to hold the dead letter messages.

The maximum attempts of delivery is `3`. Once this figure is reached, a message is considered a dead letter message and is moved to the `deadLetterQueue`.

Since we want to consume messages from this deadLetterQueue, we also need to add a JNDI binding to perform a lookup. This is configured in hornetq-jms.xml

             <queue name="deadLetterQueue">
                <entry name="/queue/deadLetterQueue"/>
             </queue>

## Example step-by-step

To run the example, simply type `./build.sh` (or `build.bat` on windows) from this directory

1. First we need to get an initial context so we can look-up the JMS connection factory and destination objects from JNDI. This initial context will get it's properties from the `client-jndi.properties` file in the directory `../common/config`

               InitialContext initialContext = getContext();

2. We look up the JMS queue object from JNDI

               Queue queue = (Queue) initialContext.lookup("/queue/exampleQueue");

3. We look up the JMS connection factory object from JNDI

               ConnectionFactory cf = (ConnectionFactory) initialContext.lookup("/ConnectionFactory");

4. We create a JMS connection

               connection = cf.createConnection();

5. We create a JMS transacted session

                  Session session = connection.createSession(true, 0);

6. We create a JMS message producer on the session. This will be used to send the messages

              MessageProducer messageProducer = session.createProducer(topic);

7. We create a text messages

                TextMessage message = session.createTextMessage("this is a text message");

8. We send the message to the queue

                producer.send(message);

9. We commit the session to effectively send the message to the queue

                session.commit();

We will now consume the message from the queue 3 times and roll back the session every time

10. We create a JMS message consumer on the queue

                MessageConsumer messageConsumer = session.createConsumer(queue);

11. We start the connection. In order for delivery to occur on any consumers or subscribers on a connection, the connection must be started

               connection.start();

12. We receive the message a 1st time

                TextMessage messageReceived = (TextMessage)messageConsumer.receive(5000);
                System.out.println("1st delivery from " + queue.getQueueName() + ": " + messageReceived.getText());

13. We roll back the session. The message we received is undelivered and goes back to the queue

                session.rollback();

14. We receive a message and roll back the session a 2nd time

                    messageReceived = (TextMessage)messageConsumer.receive(5000);
                    System.out.println("2nd delivery from " + queue.getQueueName() + ": " + messageReceived.getText());
                    session.rollback();

15. We do it againt a 3rd time

                   messageReceived = (TextMessage)messageConsumer.receive(5000);
                   System.out.println("3rd delivery from " + queue.getQueueName() + ": " + messageReceived.getText());
                   session.rollback();

    Since the queue was configured to move messages to the `deadLetterQueue` after `3` unsuccessful delivery attempts, the message won't be in the `queue` anymore

16. We try to receive a message from the queue for a 4th. Since there is none, the call will timeout after 5000ms and `messageReceived` will be `null`

                   messageReceived = (TextMessage)messageConsumer.receive(5000);
                   System.out.println("4th delivery from " + queue.getQueueName() + ": " + messageReceived);

    We have configured HornetQ to send any dead letter messages to the `deadLetterQueue`. We will now consume messages from this queue and receives the dead letter messages.

17. We look up the JMS dead letter queue object from JNDI

               Queue deadLetterQueue = (Queue)initialContext.lookup("/queue/deadLetterQueue");

18. We create a JMS message consumer on the dead letter queue

                MessageConsumer deadLetterConsumer = session.createConsumer(expiryQueue);

19. We consume a message from the dead letter queue:

                messageReceived = (TextMessage)deadLetterConsumer.receive(5000);

20. The message consumed from the dead letter queue has the same content than the message which was sent to the queue

                    System.out.println("Received message from " + deadLetterQueue.getQueueName() + ": " + messageReceived.getText());

    JMS does not specify the notion of dead letter destinations and messages. From JMS point of view, the message received from the dead letter queue is a different message than the message removed from the queue after the unsuccessful delivery attempts: the messages have the same content (properties and body) but their JMS headers differ.
    HornetQ defines additional properties for messages received from a dead letter destination

21. The message's destination is the dead letter queue

                System.out.println("Destination of the message: " + ((Queue)messageReceived.getJMSDestination()).getQueueName());

22. The origin destination is stored in the `_HQ_ORIG_ADDRESS` property

                    System.out.println("*Origin destination* of the message: " + messageReceived.getStringProperty("_HQ_ORIG_ADDRESS"));

23. We do not forget to commit the session to acknowledge that we have received the message from the dead letter queue

                session.commit();

24. And finally, always remember to close your JMS connections and resources after use, in a `finally` block. Closing a JMS connection will automatically close all of its sessions, consumers, producer and browser objects

               finally
               {
                  if (initialContext != null)
                  {
                    initialContext.close();
                  }
                  if (connection != null)
                  {
                     connection.close();
                  }
               }

## More information

- User Manual's Undelivered Messages chapter