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<title>ActiveMQ JMS Load Balanced Clustered Queue Example</title>
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<h1>JMS Load Balanced Clustered Queue Example</h1>
<p>This example demonstrates a JMS queue deployed on two different nodes. The two nodes are configured to form a cluster.</p>
<p>We then create a consumer on the queue on each node, and we create a producer on only one of the nodes.</p>
<p>We then send some messages via the producer, and we verify that <b>both</b> consumers receive the sent messages
in a round-robin fashion.</p>
<p>In other words, ActiveMQ <b>load balances</b> the sent messages across all consumers on the cluster</p>
<p>This example uses JNDI to lookup the JMS Queue and ConnectionFactory objects. If you prefer not to use
JNDI, these could be instantiated directly.</p>
<p>Here's the relevant snippet from the server configuration, which tells the server to form a cluster between the two nodes
and to load balance the messages between the nodes.</p>
<pre class="prettyprint">
<code><cluster-connection name="my-cluster">
<address>jms</address>
<connector-ref>netty-connector</connector-ref>
<retry-interval>500</retry-interval>
<use-duplicate-detection>true</use-duplicate-detection>
<forward-when-no-consumers>true</forward-when-no-consumers>
<max-hops>1</max-hops>
<discovery-group-ref discovery-group-name="my-discovery-group"/>
</cluster-connection>
</code>
</pre>
<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 -Pexample</code> from this directory</i></p>
<ol>
<li> Get an initial context for looking up JNDI from server 0.</li>
<pre class="prettyprint">
<code>
ic0 = getContext(0);
</code>
</pre>
<li>Look-up the JMS Queue object from JNDI</li>
<pre class="prettyprint">
<code>Queue queue = (Queue)ic0.lookup("/queue/exampleQueue");</code>
</pre>
<li>Look-up a JMS Connection Factory object from JNDI on server 0</li>
<pre class="prettyprint">
<code>ConnectionFactory cf0 = (ConnectionFactory)ic0.lookup("/ConnectionFactory");</code>
</pre>
<li>Get an initial context for looking up JNDI from server 1.</li>
<pre class="prettyprint">
<code>ic1 = getContext(1);</code>
</pre>
<li>Look-up a JMS Connection Factory object from JNDI on server 1</li>
<pre class="prettyprint">
<code>ConnectionFactory cf1 = (ConnectionFactory)ic1.lookup("/ConnectionFactory");
</code>
</pre>
<li>We create a JMS Connection connection0 which is a connection to server 0</li>
<pre class="prettyprint">
<code>
connection0 = cf0.createConnection();
</code>
</pre>
<li>We create a JMS Connection connection1 which is a connection to server 1</li>
<pre class="prettyprint">
<code>
connection1 = cf1.createConnection();
</code>
</pre>
<li>We create a JMS Session on server 0</li>
<pre class="prettyprint">
<code>
Session session0 = connection0.createSession(false, Session.AUTO_ACKNOWLEDGE);
</code>
</pre>
<li>We create a JMS Session on server 1</li>
<pre class="prettyprint">
<code>
Session session1 = connection1.createSession(false, Session.AUTO_ACKNOWLEDGE);
</code>
</pre>
<li>We start the connections to ensure delivery occurs on them</li>
<pre class="prettyprint">
<code>
connection0.start();
connection1.start();
</code>
</pre>
<li>We create JMS MessageConsumer objects on server 0 and server 1</li>
<pre class="prettyprint">
<code>
MessageConsumer consumer0 = session0.createConsumer(queue);
MessageConsumer consumer1 = session1.createConsumer(queue);
</code>
</pre>
<li>We create a JMS MessageProducer object on server 0.</li>
<pre class="prettyprint">
<code>
MessageProducer producer = session0.createProducer(queue);</code>
</pre>
<li>We send some messages to server 0.</li>
<pre class="prettyprint">
<code>
final int numMessages = 10;
for (int i = 0; i < numMessages; i++)
{
TextMessage message = session0.createTextMessage("This is text message " + i);
producer.send(message);
System.out.println("Sent message: " + message.getText());
}
</code>
</pre>
<li>We now consume those messages on *both* server 0 and server 1.
We note the messages have been distributed between servers in a round robin fashion.
ActiveMQ has <b>load balanced</b> the messages between the available consumers on the different nodes.
ActiveMQ can be configured to always load balance messages to all nodes, or to only balance messages
to nodes which have consumers with no or matching selectors. See the user manual for more details.</li>
JMS Queues implement point-to-point message where each message is only ever consumed by a
maximum of one consumer.
<pre class="prettyprint">
<code>
for (int i = 0; i < numMessages; i += 2)
{
TextMessage message0 = (TextMessage)consumer0.receive(5000);
System.out.println("Got message: " + message0.getText() + " from node 0");
TextMessage message1 = (TextMessage)consumer1.receive(5000);
System.out.println("Got message: " + message1.getText() + " from node 1");
}
</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 (connection0 != null)
{
connection0.close();
}
if (connection1 != null)
{
connection1.close();
}
}
</code>
</pre>
</ol>
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