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227 lines
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<html>
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<head>
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<title>ActiveMQ Artemis Dead Letter Example</title>
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<link rel="stylesheet" type="text/css" href="../common/common.css" />
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<link rel="stylesheet" type="text/css" href="../common/prettify.css" />
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<body onload="prettyPrint()">
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<h1>Dead Letter Example</h1>
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<p>This example shows you how to define and deal with dead letter messages.</p>
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<p>Messages can be delivered unsuccessfully (e.g. if the transacted session used to consume them is rolled back).
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Such a message goes back to the JMS destination ready to be redelivered.
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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>
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<p>To prevent this, messaging systems define dead letter messages: after a specified unsuccessful delivery attempts, the message is removed from the destination
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and instead routed to a <em>dead letter address</em> where they can be consumed for further investigation.
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<p>
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The example will show how to configure ActiveMQ Artemis to route a message to a dead letter address after 3 unsuccessful delivery attempts.<br />
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The example will send 1 message to a queue. We will deliver the message 3 times and rollback the session every time.<br />
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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 />
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We will then consume this dead letter message.
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</p>
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<h2>Example setup</h2>
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<p><em>Dead letter addresses</em> and <em>maximum delivery attempts</em> are defined in the configuration file <a href="src/main/resources/activemq/server0/broker.xml">broker.xml</a>:</p>
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<pre class="prettyprint">
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<code><address-setting match="jms.queue.exampleQueue">
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<dead-letter-address>jms.queue.deadLetterQueue</dead-letter-address>
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<max-delivery-attempts>3</max-delivery-attempts>
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</address-setting>
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</code>
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</pre>
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<p>This configuration will moved dead letter messages from <code>exampleQueue</code> to the <code>deadLetterQueue</code>.</p>
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<p>ActiveMQ Artemis allows to specify either a <code>Queue</code> by prefixing the <code>dead-letter-address</code> with <code>jms.queue.</code>
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or a <code>Topic</code> by prefixing with <code>jms.topic.</code>.<br />
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In this example, we will use a <code>Queue</code> to hold the dead letter messages.</p>
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<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
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the <code>deadLetterQueue</code>.
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<p>Since we want to consume messages from this deadLetterQueue, we also need to add a JNDI binding to perform a lookup.
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This is configured in <a href="src/main/resources/activemq/server0/activemq-jms.xml">activemq-jms.xml</a></p>
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<pre class="prettyprint">
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<code><queue name="deadLetterQueue">
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<entry name="/queue/deadLetterQueue"/>
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</queue></code>
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</pre>
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</p>
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<h2>Example step-by-step</h2>
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<p><i>To run the example, simply type <code>mvn verify -Pexample</code> from this directory</i></p>
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<ol>
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<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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<pre class="prettyprint">
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<code>InitialContext initialContext = getContext();</code>
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</pre>
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<li>We look up the JMS queue object from JNDI</li>
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<pre class="prettyprint">
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<code>Queue queue = (Queue) initialContext.lookup("/queue/exampleQueue");</code>
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</pre>
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<li>We look up the JMS connection factory object from JNDI</li>
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<pre class="prettyprint">
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<code>ConnectionFactory cf = (ConnectionFactory) initialContext.lookup("/ConnectionFactory");</code>
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</pre>
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<li>We create a JMS connection</li>
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<pre class="prettyprint">
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<code>connection = cf.createConnection();</code>
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</pre>
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<li>We create a JMS <em>transacted</em> session
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<pre class="prettyprint">
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<code>Session session = connection.createSession(true, 0);</code>
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</pre>
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<li>We create a JMS message producer on the session. This will be used to send the messages</li>
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<pre class="prettyprint">
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<code>MessageProducer messageProducer = session.createProducer(topic);</code>
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</pre>
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<li>We create a text messages</li>
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<pre class="prettyprint">
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<code>TextMessage message = session.createTextMessage("this is a text message");</code>
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</pre>
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<li>We send the message to the queue</li>
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<pre class="prettyprint">
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<code>producer.send(message);</code>
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</pre>
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<li>We commit the session to effectively send the message to the queue</li>
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<pre class="prettyprint">
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<code>session.commit();</code>
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</pre>
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<p>We will now consume the message from the queue 3 times and roll back the session every time</p>
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<li>We create a JMS message consumer on the queue</li>
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<pre class="prettyprint">
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<code>MessageConsumer messageConsumer = session.createConsumer(queue);</code>
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</pre>
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<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>
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<pre class="prettyprint">
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<code>connection.start();</code>
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</pre>
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<li>We receive the message a 1<sup>st</sup> time</li>
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<pre class="prettyprint">
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<code>TextMessage messageReceived = (TextMessage)messageConsumer.receive(5000);
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System.out.println("1st delivery from " + queue.getQueueName() + ": " + messageReceived.getText());</code>
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</pre>
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<li>We roll back the session. The message we received is undelivered and goes back to the queue</li>
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<pre class="prettyprint">
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<code>session.rollback();</code>
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</pre>
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<li>We receive a message and roll back the session a 2<sup>nd</sup> time
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<pre class="prettyprint">
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<code>messageReceived = (TextMessage)messageConsumer.receive(5000);
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System.out.println("2nd delivery from " + queue.getQueueName() + ": " + messageReceived.getText());
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session.rollback();</code>
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</pre>
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<li>We do it againt a 3<sup>rd</sup> time
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<pre class="prettyprint">
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<code>messageReceived = (TextMessage)messageConsumer.receive(5000);
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System.out.println("3rd delivery from " + queue.getQueueName() + ": " + messageReceived.getText());
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session.rollback();</code>
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</pre>
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<p>Since the queue was configured to move messages to the <code>deadLetterQueue</code> after <code>3</code> unsuccessful delivery attempts,
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the message won't be in the <code>queue</code> anymore</p>
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<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>
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<pre class="prettyprint">
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<code>messageReceived = (TextMessage)messageConsumer.receive(5000);
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System.out.println("4th delivery from " + queue.getQueueName() + ": " + messageReceived);</code>
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</pre>
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<p>We have configured ActiveMQ Artemis to send any dead letter messages to the <code>deadLetterQueue</code>.
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We will now consume messages from this queue and receives the <em>dead letter messages</em>.</p>
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<li>We look up the JMS <em>dead letter queue</em> object from JNDI</li>
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<pre class="prettyprint">
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<code>Queue deadLetterQueue = (Queue)initialContext.lookup("/queue/deadLetterQueue");</code>
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</pre>
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<li>We create a JMS message consumer on the dead letter queue</li>
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<pre class="prettyprint">
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<code>MessageConsumer deadLetterConsumer = session.createConsumer(expiryQueue);</code>
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</pre>
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<li>We consume a message from the dead letter queue:</li>
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<pre class="prettyprint">
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<code>messageReceived = (TextMessage)deadLetterConsumer.receive(5000);</code>
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</pre>
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<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>
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<pre class="prettyprint">
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<code>System.out.println("Received message from " + deadLetterQueue.getQueueName() + ": " + messageReceived.getText());</code>
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</pre>
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<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
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is a <strong>different</strong> message than the message removed from the queue after the unsuccessful delivery attempts:
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the messages have the same content (properties and body) but their JMS headers differ.<br />
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ActiveMQ Artemis defines additional properties for messages received from a dead letter destination</p>
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<li>The message's destination is the dead letter queue</li>
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<pre class="prettyprint">
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<code>System.out.println("Destination of the message: " + ((Queue)messageReceived.getJMSDestination()).getQueueName());</code>
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</pre>
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<li>The <strong>origin destination</strong> is stored in the <code>_AMQ_ORIG_ADDRESS</code> property
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<pre class="prettyprint">
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<code>System.out.println("*Origin destination* of the message: " + messageReceived.getStringProperty("_AMQ_ORIG_ADDRESS"));</code>
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</pre>
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<li>We do not forget to commit the session to acknowledge that we have received the message from the dead letter queue</li>
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<pre class="prettyprint">
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<code>session.commit();</code>
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</pre>
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</p>
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<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>
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<pre class="prettyprint">
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<code>finally
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{
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if (initialContext != null)
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{
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initialContext.close();
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}
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if (connection != null)
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{
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connection.close();
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}
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}</code>
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</pre>
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</ol>
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<h2>More information</h2>
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<ul>
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<li>User Manual's <a href="../../../docs/user-manual/en/html_single/index.html#undelivered-messages">Undelivered Messages chapter</a></li>
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</ul>
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</body>
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</html>
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