2014-10-31 06:20:28 -04:00
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<?xml version="1.0" encoding="UTF-8"?>
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<!-- ============================================================================= -->
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<chapter id="pre-acknowledge">
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<title>预先通知模式（pre-acknowledge）</title>
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<para>JMS 规定了三种消息通知方式</para>
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<itemizedlist>
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<listitem>
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<para><literal>AUTO_ACKNOWLEDGE</literal></para>
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</listitem>
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<listitem>
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<para><literal>CLIENT_ACKNOWLEDGE</literal></para>
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</listitem>
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<listitem>
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<para><literal>DUPS_OK_ACKNOWLEDGE</literal></para>
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</listitem>
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</itemizedlist>
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<para>还有一种情况JMS不支持：应用程序在出现故障时可以容忍消息丢失，这样可以在消息在传递给客户
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端<emphasis>之前</emphasis>就通知服务器。</para>
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2014-11-19 03:44:57 -05:00
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<para>ActiveMQ支持这种模式，称为<emphasis>pre-acknowledge</emphasis>。</para>
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2014-10-31 06:20:28 -04:00
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<para>这种模式的缺点是消息在通知后，如果系统出现故障时，消息可能丢失。并且在系统重启后该消息
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不能恢复。</para>
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<para>使用<literal>pre-acknowledgement</literal>模式可以节省网络传输和CPU处理资源。</para>
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<para>股票价格更新是一个适用于此模式的例子。如果因为服务器故障丢失了一些消息，等服务器重启后新的
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股票更新消息很快到达，以前丢失的过时的股票消息即使丢失也无关紧要。 </para>
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<note>
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<para>注意如果你使用pre-acknowledge模式，在接收消息端不能支持事务。因为这个模式不是在提交时
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通知消息，是在消息在传递之前就通知了。</para>
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</note>
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<section id="pre-acknowledge.configure">
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<title>使用PRE_ACKNOWLEDGE</title>
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2014-11-19 14:58:44 -05:00
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<para>这个模式在<literal>activemq-jms.xml</literal>文件中
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的<literal>connection factory</literal>下配置：</para>
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<programlisting><connection-factory name="ConnectionFactory">
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<connectors>
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<connector-ref connector-name="netty-connector"/>
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</connectors>
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<entries>
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<entry name="ConnectionFactory"/>
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</entries>
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<pre-acknowledge>true</pre-acknowledge>
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</connection-factory></programlisting>
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<para>另一个选择是使用JMS接口来设置pre-acknowledgement模式。只需要在创建JMS会话（session）
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时使用<literal>ActiveMQSession.PRE_ACKNOWLEDGE</literal>常数即可。</para>
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<programlisting>
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// messages will be acknowledge on the server *before* being delivered to the client
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Session session = connection.createSession(false, ActiveMQSession.PRE_ACKNOWLEDGE);
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2014-10-31 06:20:28 -04:00
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</programlisting>
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2014-11-19 03:44:57 -05:00
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<para>你还可以直接在<literal>ActiveMQConnectionFactory</literal>实例上设置该模式。</para>
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2014-10-31 06:20:28 -04:00
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<para>另外，如果使用核心接口，则在<literal>ClientSessionFactory</literal>实例上直接
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设置该模式。</para>
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</section>
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<section>
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<title>例子</title>
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<para>参见<xref linkend="examples.pre-acknowledge"/>。这是一个使用JMS的例子。</para>
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</section>
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</chapter>
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