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<h1>Dead Letter Example</h1>
<p>This example shows you how to define and deal with dead letter messages.</p>
<p>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.</p>
<p>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 <em>dead letter address</em> where they can be consumed for further investigation.
<p>
The example will show how to configure HornetQ to route a message to a dead letter address after 3 unsuccessful delivery attempts.<br />
The example will send 1 message to a queue. We will deliver the message 3 times and rollback the session every time.<br />
On the 4th attempt, there won't be any message to consume: it will have been moved to a <em>dead letter address</em>.<br />
We will then consume this dead letter message.
</p>
<h2>Example setup</h2>
<p><em>Dead letter addresses</em> and <em>maximum delivery attempts</em> are defined in the configuration file <a href="src/main/resources/hornetq/server0/hornetq-configuration.xml">hornetq-configuration.xml</a>:</p>
<pre class="prettyprint">
<code><address-setting match="jms.queue.exampleQueue">
<dead-letter-address>jms.queue.deadLetterQueue</dead-letter-address>
<max-delivery-attempts>3</max-delivery-attempts>
</address-setting>
</code>
</pre>
<p>This configuration will moved dead letter messages from <code>exampleQueue</code> to the <code>deadLetterQueue</code>.</p>
<p>HornetQ allows to specify either a <code>Queue</code> by prefixing the <code>dead-letter-address</code> with <code>jms.queue.</code>
or a <code>Topic</code> by prefixing with <code>jms.topic.</code>.<br />
In this example, we will use a <code>Queue</code> to hold the dead letter messages.</p>
<p>The maximum attempts of delivery is <code>3</code>. Once this figure is reached, a message is considered a dead letter message and is moved to
the <code>deadLetterQueue</code>.
<p>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 <a href="src/main/resources/hornetq/server0/hornetq-jms.xml">hornetq-jms.xml</a></p>
<pre class="prettyprint">
<code><queue name="deadLetterQueue">
<entry name="/queue/deadLetterQueue"/>
</queue></code>
</pre>
</p>
<h2>Example step-by-step</h2>
<p><i>To run the example, simply type <code>mvn verify</code> from this directory</i></p>
<ol>
<li>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 <code>client-jndi.properties</code> file in the directory <code>../common/config</code></li>
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<code>InitialContext initialContext = getContext();</code>
</pre>
<li>We look up the JMS queue object from JNDI</li>
<pre class="prettyprint">
<code>Queue queue = (Queue) initialContext.lookup("/queue/exampleQueue");</code>
</pre>
<li>We look up the JMS connection factory object from JNDI</li>
<pre class="prettyprint">
<code>ConnectionFactory cf = (ConnectionFactory) initialContext.lookup("/ConnectionFactory");</code>
</pre>
<li>We create a JMS connection</li>
<pre class="prettyprint">
<code>connection = cf.createConnection();</code>
</pre>
<li>We create a JMS <em>transacted</em> session
<pre class="prettyprint">
<code>Session session = connection.createSession(true, 0);</code>
</pre>
<li>We create a JMS message producer on the session. This will be used to send the messages</li>
<pre class="prettyprint">
<code>MessageProducer messageProducer = session.createProducer(topic);</code>
</pre>
<li>We create a text messages</li>
<pre class="prettyprint">
<code>TextMessage message = session.createTextMessage("this is a text message");</code>
</pre>
<li>We send the message to the queue</li>
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<code>producer.send(message);</code>
</pre>
<li>We commit the session to effectively send the message to the queue</li>
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<code>session.commit();</code>
</pre>
<p>We will now consume the message from the queue 3 times and roll back the session every time</p>
<li>We create a JMS message consumer on the queue</li>
<pre class="prettyprint">
<code>MessageConsumer messageConsumer = session.createConsumer(queue);</code>
</pre>
<li>We start the connection. In order for delivery to occur on any consumers or subscribers on a connection, the connection must be started</li>
<pre class="prettyprint">
<code>connection.start();</code>
</pre>
<li>We receive the message a 1<sup>st</sup> time</li>
<pre class="prettyprint">
<code>TextMessage messageReceived = (TextMessage)messageConsumer.receive(5000);
System.out.println("1st delivery from " + queue.getQueueName() + ": " + messageReceived.getText());</code>
</pre>
<li>We roll back the session. The message we received is undelivered and goes back to the queue</li>
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<code>session.rollback();</code>
</pre>
<li>We receive a message and roll back the session a 2<sup>nd</sup> time
<pre class="prettyprint">
<code>messageReceived = (TextMessage)messageConsumer.receive(5000);
System.out.println("2nd delivery from " + queue.getQueueName() + ": " + messageReceived.getText());
session.rollback();</code>
</pre>
<li>We do it againt a 3<sup>rd</sup> time
<pre class="prettyprint">
<code>messageReceived = (TextMessage)messageConsumer.receive(5000);
System.out.println("3rd delivery from " + queue.getQueueName() + ": " + messageReceived.getText());
session.rollback();</code>
</pre>
<p>Since the queue was configured to move messages to the <code>deadLetterQueue</code> after <code>3</code> unsuccessful delivery attempts,
the message won't be in the <code>queue</code> anymore</p>
<li>We try to receive a message from the queue for a 4<sup>th</sup>. Since there is none, the call will timeout after 5000ms and <code>messageReceived</code> will be <code>null</code>
<pre class="prettyprint">
<code>messageReceived = (TextMessage)messageConsumer.receive(5000);
System.out.println("4th delivery from " + queue.getQueueName() + ": " + messageReceived);</code>
</pre>
<p>We have configured HornetQ to send any dead letter messages to the <code>deadLetterQueue</code>.
We will now consume messages from this queue and receives the <em>dead letter messages</em>.</p>
<li>We look up the JMS <em>dead letter queue</em> object from JNDI</li>
<pre class="prettyprint">
<code>Queue deadLetterQueue = (Queue)initialContext.lookup("/queue/deadLetterQueue");</code>
</pre>
<li>We create a JMS message consumer on the dead letter queue</li>
<pre class="prettyprint">
<code>MessageConsumer deadLetterConsumer = session.createConsumer(expiryQueue);</code>
</pre>
<li>We consume a message from the dead letter queue:</li>
<pre class="prettyprint">
<code>messageReceived = (TextMessage)deadLetterConsumer.receive(5000);</code>
</pre>
<li>The message consumed from the <em>dead letter queue</em> has the <em>same content</em> than the message which was sent to the <em>queue</em>
<pre class="prettyprint">
<code>System.out.println("Received message from " + deadLetterQueue.getQueueName() + ": " + messageReceived.getText());</code>
</pre>
<p>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 <strong>different</strong> 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.<br />
HornetQ defines additional properties for messages received from a dead letter destination</p>
<li>The message's destination is the dead letter queue</li>
<pre class="prettyprint">
<code>System.out.println("Destination of the message: " + ((Queue)messageReceived.getJMSDestination()).getQueueName());</code>
</pre>
<li>The <strong>origin destination</strong> is stored in the <code>_HQ_ORIG_ADDRESS</code> property
<pre class="prettyprint">
<code>System.out.println("*Origin destination* of the message: " + messageReceived.getStringProperty("_HQ_ORIG_ADDRESS"));</code>
</pre>
<li>We do not forget to commit the session to acknowledge that we have received the message from the dead letter queue</li>
<pre class="prettyprint">
<code>session.commit();</code>
</pre>
</p>
<li>And finally, <b>always</b> remember to close your JMS connections and resources after use, in a <code>finally</code> block. Closing a JMS connection will automatically close all of its sessions, consumers, producer and browser objects</li>
<pre class="prettyprint">
<code>finally
{
if (initialContext != null)
{
initialContext.close();
}
if (connection != null)
{
connection.close();
}
}</code>
</pre>
</ol>
<h2>More information</h2>
<ul>
<li>User Manual's <a href="../../../docs/user-manual/en/html_single/index.html#undelivered-messages">Undelivered Messages chapter</a></li>
</ul>
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