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<h1>JMS Failover With Transaction Example</h1>
<p>This example demonstrates two servers coupled as a live-backup pair for high availability (HA), and a client
connection failing over from live to backup when the live server is crashed.</p>
<p>Failover behavior differs whether the JMS session is transacter or not.</p>
<p>When a <em>transacted</em> JMS session is used, once-and-only once delivery is guaranteed.</p>
<ul>
<li>if the failover occurs while there is an in-flight transaction, the transaction will be flagged as <em>rollback only</em>. In that case, the JMS client
will need to retry the transaction work.</li>
<li>if the failover occurs while there is <em>no</em> in-flight transaction, the failover will be transparent to the user.</li>
</ul>
<p>ActiveMQ also provides an example for <a href="../non-transactional-failover/readme.html">non-transaction failover</a>.</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 #1.</li>
<pre class="prettyprint">
initialContext = getContext(1);
</pre>
<li>Look up the JMS resources from JNDI on server #1.</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 JMS <em>transacted</em> Session</li>
<pre class="prettyprint">
Session session = connection.createSession(true, 0);
</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 half of the messages, kill the live server and send the remaining messages</li>
<pre class="prettyprint">
sendMessages(session, producer, numMessages, true);
</pre>
<p>When server #1 crashes, the client automatically detects the failure and automatically
fails over from server #1 to server #0 (in your real program you wouldn't need to sleep).
</p>
<li>As failover occurred during transaction, the session has been marked for <em>rollback only</em> and commit will fail</li>
<pre class="prettyprint">
try
{
session.commit();
} catch (TransactionRolledBackException e)
{
System.err.println("transaction has been rolled back: " + e.getMessage());
}
</pre>
<li>We resend all the messages</li>
<pre class="prettyprint">
sendMessages(session, producer, numMessages, false);
</pre>
<li>We commit the session successfully: the messages will be all delivered to the activated backup server</li>
<pre class="prettyprint">
session.commit();
</pre>
<li>We are now transparently reconnected to server #0, the backup server.
We consume the messages sent before the crash of the live server, commit the session, and check there are no other message on the queue</li>
<pre class="prettyprint">
for (int i = 0; i < numMessages; i++)
{
TextMessage message0 = (TextMessage)consumer.receive(5000);
System.out.println("Got message: " + message0.getText());
}
session.commit();
System.out.println("Other message on the server? " + consumer.receive(5000));
</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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