170 lines
6.6 KiB
HTML
170 lines
6.6 KiB
HTML
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<html>
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<head>
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<title>ActiveMQ Artemis Application-Layer Failover Example</title>
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<link rel="stylesheet" type="text/css" href="../../../common/common.css" />
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<link rel="stylesheet" type="text/css" href="../../../common/prettify.css" />
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<script type="text/javascript" src="../../../common/prettify.js"></script>
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</head>
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<body onload="prettyPrint()">
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<h1>Application-Layer Failover Example</h1>
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<pre>To run the example, simply type <b>mvn verify</b> from this directory. This example will always spawn and stop multiple servers.</pre>
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<p>ActiveMQ Artemis implements fully transparent <b>automatic</b> failover of connections from a live node to a backup node which requires
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no special coding. This is described in a different example and requires server replication.</p>
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<p>However, ActiveMQ Artemis also supports <b>Application-Layer</b> failover which is useful in the case where replication is not enabled.</p>
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<p>With Application-Layer failover, it's up to the application to register a JMS ExceptionListener with ActiveMQ Artemis.
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This listener will then be called by ActiveMQ Artemis in the event that connection failure is detected.</p>
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<p>User code in the ExceptionListener can then recreate any JMS Connection, Session, etc on another node and the application
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can continue.</p>
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<p>Application-Layer failover is an alternative approach to High Availability (HA).</p>
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<p>Application-Layer failover differs from automatic failover in that some client side coding is required in order
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to implement this. Also, with Application-Layer failover, since the old Session object dies and a new is created, any uncommitted
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work in the old Session will be lost, and any unacknowledged messages might be redelivered.</p>
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<p>For more information on ActiveMQ Artemis failover and HA, and clustering in general, please see the clustering
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section of the user manual.</p>
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<h2>Example step-by-step</h2>
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<p>In this example, the live server is server 1, which will failover onto server 0.</p>
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<p>The connection will initially be created to server1, server 1 will crash, and the client will carry on
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on server 0, the new server. With Application-Layer failover the node that is failed over onto, does not need to
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be specially configured as a backup server, it can be any node.</p>
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<ol>
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<li> We create our JMS Connection, Session, MessageProducer and MessageConsumer on server 1</li>
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<pre class="prettyprint">
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<code>createJMSObjects(1);</code>
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</pre>
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<li>We set a JMS ExceptionListener on the connection. On failure this will be called and the connection,
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session, etc. will be manually recreated on the backup node.</li>
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<pre class="prettyprint">
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<code>connection.setExceptionListener(new ExampleListener());</code>
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</pre>
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<li>We send some messages to server 1, the live server.</li>
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<pre class="prettyprint">
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<code>
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final int numMessages = 10;
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for (int i = 0; i < numMessages; i++)
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{
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TextMessage message = session.createTextMessage("This is text message " + i);
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producer.send(message);
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System.out.println("Sent message: " + message.getText());
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}
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</code>
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</pre>
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<li>We consume those messages on server 1.</li>
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<pre class="prettyprint">
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<code>
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for (int i = 0; i < numMessages; i++)
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{
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TextMessage message0 = (TextMessage)consumer.receive(5000);
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System.out.println("Got message: " + message0.getText());
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}
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</code>
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</pre>
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<li>We now cause server 1, the live server to crash. After a little while the connection's
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ExceptionListener will register the failure and reconnection will occur.</li>
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<pre class="prettyprint">
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<code>killServer(1);</code>
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</pre>
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<li>The connection's ExceptionListener gets called, and we lookup the JMS objects and
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recreate the connection, session, etc on the other node 0.</li>
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<pre class="prettyprint">
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<code>
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private class ExampleListener implements ExceptionListener
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{
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public void onException(JMSException exception)
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{
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try
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{
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// Close the old resources
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closeResources();
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// Create new JMS objects on the backup server
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createJMSObjects(0);
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failoverLatch.countDown();
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}
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catch (Exception e)
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{
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System.err.println("Failed to handle failover");
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e.printStackTrace();
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}
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}
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}
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</code>
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</pre>
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<li>We are now connected to the other node. We now send some more messages.</li>
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<pre class="prettyprint">
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<code>
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for (int i = numMessages; i < numMessages * 2; i++)
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{
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TextMessage message = session.createTextMessage("This is text message " + i);
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producer.send(message);
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System.out.println("Sent message: " + message.getText());
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}
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</code>
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</pre>
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<li>And consume them.</li>
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<pre class="prettyprint">
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<code>
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for (int i = 0; i < numMessages; i++)
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{
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TextMessage message0 = (TextMessage)consumer.receive(5000);
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System.out.println("Got message: " + message0.getText());
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}
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</code>
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</pre>
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<li>And finally (no pun intended), <b>always</b> 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>
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<pre class="prettyprint">
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<code>
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finally
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{
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closeResources();
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}
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</code>
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</pre>
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</ol>
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</body>
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</html>
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