ActiveMQ的消息重发策略和DLQ处理

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在以下三种情况中,ActiveMQ消息会被重发给客户端/消费者:
1.使用一个事务session,并且调用了rollback()方法;
2.一个事务session,关闭之前调用了commit;
3.在session中使用CLIENT_ACKNOWLEDGE签收模式,并且调用了Session.recover()方法。

Broker根据自己的规则,通过BrokerInfo命令包和客户端建立连接,向客户端传送缺省发送策略。但是客户端可以使用ActiveMQConnection.getRedeliveryPolicy()方法覆盖override这个策略设置。
RedeliveryPolicy policy = connection.getRedeliveryPolicy();policy.setInitialRedeliveryDelay(500);policy.setBackOffMultiplier(2);policy.setUseExponentialBackOff(true);policy.setMaximumRedeliveries(2);

一旦消息重发尝试超过重发策略中配置的maximumRedeliveries(缺省为6次)时,会给broker发送一个"Poison ack",通知它,这个消息被认为是一个毒丸(a poison pill),接着broker会将这个消息发送到DLQ(Dead Letter Queue),以便后续分析处理。

缺省死信队列(Dead Letter Queue)叫做ActiveMQ.DLQ;所有的未送达消息都会被发送到这个队列,以致会非常难于管理。你可以设置activemq.xml文件中的destination policy map的"individualDeadLetterStrategy"属性来修改
<broker...>  <destinationPolicy>    <policyMap>      <policyEntries>        <!-- Set the following policy on all queues using the '>' wildcard -->        <policyEntry queue=">">          <deadLetterStrategy>            <!--              Use the prefix 'DLQ.' for the destination name, and make              the DLQ a queue rather than a topic            -->            <individualDeadLetterStrategy queuePrefix="DLQ." useQueueForQueueMessages="true" />          </deadLetterStrategy>        </policyEntry>      </policyEntries>    </policyMap>  </destinationPolicy>  ...</broker>

自动丢弃过期消息(Expired Messages)
一些应用可能只是简单的丢弃过期消息,而不想将它们放到DLQ中,完全跳过了DLQ。在dead letter strategy死信策略上配置processExpired属性为false,可以实现这个功能。
<broker...>  <destinationPolicy>   <policyMap>     <policyEntries>       <!-- Set the following policy on all queues using the '>' wildcard -->       <policyEntry queue=">">         <!--           Tell the dead letter strategy not to process expired messages           so that they will just be discarded instead of being sent to           the DLQ         -->         <deadLetterStrategy>           <sharedDeadLetterStrategy processExpired="false" />         </deadLetterStrategy>       </policyEntry>     </policyEntries>   </policyMap>  </destinationPolicy>...</broker>

将非持久消息(non-persistent messages)放入死信队列
ActiveMQ缺省不会将未发到的非持久消息放入死信队列。如果一个应用程序并不想将消息message设置为持久的,那么记录下来那些未发送到的消息对它来说往往也是没有价值的。不过如果想实现这个功能,可以在dead-letter strategy死信策略上设置processNonPersistent="true"
<broker...>  <destinationPolicy>   <policyMap>     <policyEntries>       <!-- Set the following policy on all queues using the '>' wildcard -->       <policyEntry queue=">">         <!--           Tell the dead letter strategy to also place non-persisted messages           onto the dead-letter queue if they can't be delivered.         -->         <deadLetterStrategy>           <sharedDeadLetterStrategy processNonPersistent="true" />         </deadLetterStrategy>       </policyEntry>     </policyEntries>   </policyMap>  </destinationPolicy>...</broker>

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