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<h1>ActiveMQ Clustering with JGroups Example</h1>
<p>This example demonstrates the working of a two node cluster using JGroups as the underlying topology broadcasting/discovery
technique.</p>
<p>We deploy a queue on to the cluster, 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>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>To enable ActiveMQ to use JGroups you need to configure JGroups configuration file and make sure it is on the classpath
by placing in the configuration directory, the file test-jgroups-file_ping.xml is the configuration used in this
exaample</p>
<p>You then configure the jgroups file used by the broadcast and discovery groups in the configuration along with the
channel name which you want this cluster to share.</p>
<pre class="prettyprint">
<code>
<broadcast-groups>
<broadcast-group name="my-broadcast-group">
<broadcast-period>5000</broadcast-period>
<jgroups-file>test-jgroups-file_ping.xml</jgroups-file>
<jgroups-channel>activemq_broadcast_channel</jgroups-channel>
<connector-ref>netty-connector</connector-ref>
</broadcast-group>
</broadcast-groups>
<discovery-groups>
<discovery-group name="my-discovery-group">
<jgroups-file>test-jgroups-file_ping.xml</jgroups-file>
<jgroups-channel>activemq_broadcast_channel</jgroups-channel>
<refresh-timeout>10000</refresh-timeout>
</discovery-group>
</discovery-groups>
</code>
</pre>
<p>For more information on ActiveMQ 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>./build.sh</code> (or <code>build.bat</code> on windows) 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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