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     Apache ActiveMQ &#8482; -- ActiveMQ Retroactive Consumers
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 <div class="wiki-content maincontent"><h2 id="ActiveMQRetroactiveConsumers-Background">Background</h2>

 <p>A retroactive consumer is just a regular NMS consumer who indicates that at the start of a subscription every attempt should be used to go back in time and send any old messages (or the last message sent on that topic) that the consumer may have missed.</p>

 <p>See the <a shape="rect" href="../subscription-recovery-policy.html">Subscription Recovery Policy</a> for more detail.</p>

 <h3 id="ActiveMQRetroactiveConsumers-Example">Example</h3>

 <p>You mark a Consumer as being retroactive as follows:</p>

 <div class="code panel pdl" style="border-width: 1px;"><div class="codeContent panelContent pdl">
 <pre class="brush: java; gutter: false; theme: Default" style="font-size:12px;">
 var topic = session.GetTopic("TEST.Topic?consumer.retroactive=true");
 var consumer = session.CreateConsumer(topic);
 </pre>
 </div></div>

 <h2 id="ActiveMQRetroactiveConsumers-Limitations">Limitations</h2>

 <p>Retroactive consumer will not consistently work when used across a network of brokers.</p>

 <p>In the case of topics, when we connect broker A to broker B, broker B will send broker A all the subscriptions that it has received. Since Broker A replicates messages to each subscription, and we want to avoid receiving duplicates at broker B, we can't send broker A more than 1 subscription for the same topic. So our network bridges keep track of the subscriptions sent across and only send the first subscription to a topic. Subsequent subscriptions increment the reference count of the subscription, but the subscription is not actually sent.</p>

 <p>This in essence is the problem. If the first subscription to a topic is not retroactive, it is sent B to A, and the B broker will not get the retroactive messages. Then subsequent subscriptions could be (retroactive), but it will not cause retroactive message to sent from broker A to broker B since the subsequent subscription will not be sent to broker A.</p></div>
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	<div class="wiki-content maincontent"><h2 id="ActiveMQRetroactiveConsumers-Background">Background</h2>

	<p>A retroactive consumer is just a regular NMS consumer who indicates that at the start of a subscription every attempt should be used to go back in time and send any old messages (or the last message sent on that topic) that the consumer may have missed.</p>

	<p>See the <a shape="rect" href="../subscription-recovery-policy.html">Subscription Recovery Policy</a> for more detail.</p>

	<h3 id="ActiveMQRetroactiveConsumers-Example">Example</h3>

	<p>You mark a Consumer as being retroactive as follows:</p>

	<div class="code panel pdl" style="border-width: 1px;"><div class="codeContent panelContent pdl">
	<pre class="brush: java; gutter: false; theme: Default" style="font-size:12px;">
	var topic = session.GetTopic("TEST.Topic?consumer.retroactive=true");
	var consumer = session.CreateConsumer(topic);
	</pre>
	</div></div>

	<h2 id="ActiveMQRetroactiveConsumers-Limitations">Limitations</h2>

	<p>Retroactive consumer will not consistently work when used across a network of brokers.</p>

	<p>In the case of topics, when we connect broker A to broker B, broker B will send broker A all the subscriptions that it has received. Since Broker A replicates messages to each subscription, and we want to avoid receiving duplicates at broker B, we can't send broker A more than 1 subscription for the same topic. So our network bridges keep track of the subscriptions sent across and only send the first subscription to a topic. Subsequent subscriptions increment the reference count of the subscription, but the subscription is not actually sent.</p>

	<p>This in essence is the problem. If the first subscription to a topic is not retroactive, it is sent B to A, and the B broker will not get the retroactive messages. Then subsequent subscriptions could be (retroactive), but it will not cause retroactive message to sent from broker A to broker B since the subsequent subscription will not be sent to broker A.</p></div>
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