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<title>ActiveMQ JMS Client-Side Load-Balancing Example</title>
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<h1>JMS Client-Side Load-Balancing Example</h1>
<p>This example demonstrates how connnections created from a single JMS Connection factory can be created
to different nodes of the cluster. In other words it demonstrates how ActiveMQ does <b>client side load balancing</b> of
connections across the cluster.</p>
<p>The particular load-balancing policy can be chosen to be random, round-robin or user-defined. Please see the user
guide for more details of how to configure the specific load-balancing policy. In this example we will use
the default round-robin load balancing policy.</p>
<p>The list of servers over which ActiveMQ will round-robin the connections can either be specified explicitly
in the connection factory when instantiating it directly, when configuring it on the server or configured
to use UDP discovery to discover the list of servers over which to round-robin. This example will use UDP
discovery to obtain the list.</p>
<p>This example starts three servers which all broadcast their location using UDP discovery. The UDP broadcast configuration
can be seen in the <code>activemq-configuration.xml</code> file.</p>
<p>A JMS ConnectionFactory is deployed on each server specifying the discovery group that will be used by that
connection factory.</p>
<p>For more information on ActiveMQ load balancing, 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>
<ol>
<li> Get an initial context for looking up JNDI from server 0.</li>
<pre class="prettyprint">
<code>initialContext = getContext(0);</code>
</pre>
<li>Look-up the JMS Queue object from JNDI</li>
<pre class="prettyprint">
<code>Queue queue = (Queue)initialContext.lookup("/queue/exampleQueue");</code>
</pre>
<li>Look-up a JMS Connection Factory object from JNDI on server 0</li>
<pre class="prettyprint">
<code>ConnectionFactory connectionFactory = (ConnectionFactory)initialContext.lookup("/ConnectionFactory");</code>
</pre>
<li> We create 3 JMS connections from the same connection factory. Since we are using round-robin
load-balancing this should result in each sessions being connected to a different node of the cluster</li>
<pre class="prettyprint">
<code>
Connection conn = connectionFactory.createConnection();
connectionA = connectionFactory.createConnection();
connectionB = connectionFactory.createConnection();
connectionC = connectionFactory.createConnection();
conn.close();
</code>
</pre>
<li>We create JMS Sessions</li>
<pre class="prettyprint">
<code>
Session sessionA = connectionA.createSession(false, Session.AUTO_ACKNOWLEDGE);
Session sessionB = connectionB.createSession(false, Session.AUTO_ACKNOWLEDGE);
Session sessionC = connectionC.createSession(false, Session.AUTO_ACKNOWLEDGE);
</code>
</pre>
<li>We create JMS MessageProducer objects on the sessions</li>
<pre class="prettyprint">
<code>
MessageProducer producerA = sessionA.createProducer(queue);
MessageProducer producerB = sessionB.createProducer(queue);
MessageProducer producerC = sessionC.createProducer(queue);
</code>
</pre>
<li>We send some messages on each producer</li>
<pre class="prettyprint">
<code>
final int numMessages = 10;
for (int i = 0; i < numMessages; i++)
{
TextMessage messageA = sessionA.createTextMessage("A:This is text message " + i);
producerA.send(messageA);
System.out.println("Sent message: " + messageA.getText());
TextMessage messageB = sessionB.createTextMessage("B:This is text message " + i);
producerB.send(messageB);
System.out.println("Sent message: " + messageB.getText());
TextMessage messageC = sessionC.createTextMessage("C:This is text message " + i);
producerC.send(messageC);
System.out.println("Sent message: " + messageC.getText());
}
</code>
</pre>
<li>We start the connection to consume messages</li>
<pre class="prettyprint">
<code>
connectionA.start();
connectionB.start();
connectionC.start();
</code>
</pre>
<li>We consume messages from the 3 session, one at a time.<br>
We try to consume one more message than expected from each session. If
the session were not properly load-balanced, we would be missing a
message from one of the sessions at the end.</li>
<pre class="prettyprint">
<code>
consume(sessionA, queue, numMessages, "A");
consume(sessionB, queue, numMessages, "B");
consume(sessionC, queue, numMessages, "C");
</code>
</pre>
<li>And finally (no pun intended), <b>always</b> remember to close your JMS 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 (connectionA != null)
{
connectionA.close();
}
if (connectionB != null)
{
connectionB.close();
}
if (connectionC != null)
{
connectionC.close();
}
if (initialContext != null)
{
initialContext.close();
}
}
</code>
</pre>
</ol>
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