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<h1>JMS Multiple Failover using Replication Example</h1>
<p>This example demonstrates three servers coupled as a live-backup-backup group for high availability (HA) using replication, and a client
connection failing over from live to backup when the live server is crashed and then to the second backup once the new live fails.</p>
<p>For more information on ActiveMQ failover and HA, and clustering in general, please see the clustering
section of the user manual.</p>
<h2>Example step-by-step</h2>
<p><i>To run the example, simply type <code>mvn verify</code> from this directory</i></p>
<p>In this example, the live server is server 1, and the backup server is server 0</p>
<p>The connection will initially be created to server1, server 1 will crash, and the client will carry on
seamlessly on server 0, the backup server.</p>
<ol>
<li>Get an initial context for looking up JNDI from server #0.</li>
<pre class="prettyprint">
initialContext = getContext(0);
</pre>
<li>Look up the JMS resources from JNDI on server #0.</li>
<pre class="prettyprint">
Queue queue = (Queue)initialContext.lookup("/queue/exampleQueue");
ConnectionFactory connectionFactory = (ConnectionFactory)initialContext.lookup("/ConnectionFactory");
</pre>
<li>Create a JMS Connection</li>
<pre class="prettyprint">
connection = connectionFactory.createConnection();
</pre>
<li>Create a client acknowledged Session</li>
<pre class="prettyprint">
Session session = connection.createSession(false, Session.CLIENT_ACKNOWLEDGE);
</pre>
<li>Start the connection to ensure delivery occurs</li>
<pre class="prettyprint">
connection.start();
</pre>
<li>Create a JMS MessageProducer</li>
<pre class="prettyprint">
MessageProducer producer = session.createProducer(queue);
</pre>
<li>Create a JMS MessageConsumer</li>
<pre class="prettyprint">
MessageConsumer consumer = session.createConsumer(queue);
</pre>
<li>Send some messages to server #0, the live server</li>
<pre class="prettyprint">
for (int i = 0; i < numMessages; i++)
{
TextMessage message = session.createTextMessage("This is text message " + i);
producer.send(message);
System.out.println("Sent message: " + message.getText());
}
</pre>
<li>Receive and acknowledge a third of the sent messages</li>
<pre class="prettyprint">
TextMessage message0 = null;
for (int i = 0; i < numMessages / 3; i++)
{
message0 = (TextMessage)consumer.receive(5000);
System.out.println("Got message: " + message0.getText());
}
message0.acknowledge();
</pre>
<p>When server #0 crashes, the client automatically detects the failure and automatically
fails over from server #0 to server #1 (in your real program you wouldn't need to sleep).
</p>
<li>Acknowledging the 2nd third of the sent messages will fail as failover to the backup server has occurred</li>
<pre class="prettyprint">
try
{
message0.acknowledge();
}
catch (JMSException e)
{
System.err.println("Got exception while acknowledging message: " + e.getMessage());
}
</pre>
<li>Consume again the 2nd third of the messages again. Note that they are not considered as redelivered.</li>
<pre class="prettyprint">
for (int i = numMessages / 3; i < (numMessages / 3) * 2; i++)
{
message0 = (TextMessage)consumer.receive(5000);
System.out.printf("Got message: %s (redelivered?: %s)\n", message0.getText(), message0.getJMSRedelivered());
}
message0.acknowledge();
</pre>
<p>now we Kill server #1 and wait for the server #2 to take over the responsibilities.</p>
<li>Acknowledging the 3rd third of the sent messages will fail as failover to the backup server has occurred</li>
<pre class="prettyprint">
try
{
message0.acknowledge();
}
catch (JMSException e)
{
System.err.println("Got exception while acknowledging message: " + e.getMessage());
}
</pre>
<li>Consume again the 3rd third of the messages again. Note that they are not considered as redelivered</li>
<pre class="prettyprint">
for (int i = (numMessages / 3) * 2; i < numMessages; i++)
{
message0 = (TextMessage)consumer.receive(5000);
System.out.printf("Got message: %s (redelivered?: %s)\n", message0.getText(), message0.getJMSRedelivered());
}
message0.acknowledge();
</pre>
<li>And finally, <strong>always</strong> remember to close your 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">
finally
{
if (connection != null)
{
connection.close();
}
if (initialContext != null)
{
initialContext.close();
}
}
</pre>
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