<div class="wiki-content maincontent"><h2>Mirrored Queues</h2>

<p>Queues provide an excellent reliable and high performance <link><page ri:content-title="How does a Queue compare to a Topic"></page><link-body>load balancing mechanism</link-body></link>. Each message placed on a queue can only be successfully processed by a single consumer. This is a good thing! <emoticon ac:name="smile"></emoticon>. However sometimes you want to monitor what messages flow between the producers and consumers on a queue. </p>

<p>To do this you can use <link><page ri:content-title="Virtual Destinations"></page></link> to setup a virtual Queue which forwards the message to multiple physical queues. However enabling this for every single queue in your system can be painful.</p>

<p>So to make it easy to monitor queues, we have added a feature we call <em>Mirrored Queues</em>. Mirrored Queues are kinda like a zero-configuration <a shape="rect" href="http://activemq.apache.org/camel/wire-tap.html">Wire Tap</a> on all of your queues inside your Message Broker.</p>

<h3>Example</h3>

<p>For example imagine we have a number of producers sending to queue <strong>Foo.Bar</strong> and consumers consuming from queue <strong>Foo.Bar</strong> and we want to monitor or view activity. </p>

<p>If you enable Mirrored Queues then by default you can subscribe to the topic <strong>VirtualTopic.Mirror.Foo.Bar</strong> and receive all the messages that are sent to the queue <strong>Foo.Bar</strong>. Since its a topic as many consumers can subscribe to this topic as are required.</p>

<p>If you want you can use this feature with <link><page ri:content-title="Virtual Destinations"></page><link-body>Virtual Topics</link-body></link>; so that you can define a logical consumer; say called A. Then you can subscribe to the queue <strong>Consumer.A.VirtualTopic.Mirror.Foo.Bar</strong> to receive all the messages sent to queue <strong>Foo.Bar</strong> for the consumer A. You can then run multiple instances of this consumer who can then load balance among each other.</p>

<p>This combination of Mirrored Queues and <link><page ri:content-title="Virtual Destinations"></page></link> can be extremely useful for monitoring transaction flows; for example with <a shape="rect" href="http://activemq.apache.org/camel/bam.html">Business Activity Monitoring (BAM)</a>.</p>

<h3>How Mirrored Queues work</h3>

<p>When enabled, mirrored queues causes every message sent to a queue to also be sent to a topic of a similar name; so that folks who are interested in watching message exchanges on a queue can consume from the mirrored queue topic.</p>

<p>When coupled with <link><page ri:content-title="Virtual Destinations"></page><link-body>Virtual Topics</link-body></link> on this topic as described in the above example, you can actually end up creating new queues which are mirrors of a given queue dynamically at runtime!</p>

<h3>Enabling Mirrored Queues</h3>

<p>By default Mirrored Queues is disabled; as enabling it will cause a virtual topic to be created for each queue you use.</p>

<p>To enable Mirrored Queues, set the <strong>useMirroredQueues</strong> property on <a shape="rect" href="http://activemq.apache.org/maven/activemq-core/apidocs/org/apache/activemq/broker/BrokerService.html">BrokerService</a> or add the following inside the &lt;broker&gt; element in the <link><page ri:content-title="Xml Configuration"></page></link>:</p>

<structured-macro ac:macro-id="e151574b-2698-4924-882f-aebcb96befd4" ac:name="code" ac:schema-version="1"><parameter ac:name="">xml</parameter><plain-text-body>
    &lt;destinationInterceptors&gt;
        &lt;mirroredQueue copyMessage = "true" postfix=".qmirror" prefix=""/&gt;
    &lt;/destinationInterceptors&gt;
</plain-text-body></structured-macro>

<p>This would make a topic named "*.qmirror" for each queue on your Broker.</p>
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