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<a href="/2020/03/12/RabbitMQ-延时消息的应用与实现/" class="post-title-link" itemprop="url">RabbitMQ 延时消息的应用与实现</a>
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<h2 id="前言"><a href="#前言" class="headerlink" title="前言"></a>前言</h2><p>在实际的业务开发中，经常会碰到一些类似定时的任务，比如每天早上 9 点提醒用户业务进度，或者 30 分钟后锁定某个账单之类的操作。类似这样的业务需求，比较古老的操作就是使用各种定时器来实现，然而面对现在的开发更新速度，以及容器化、微服务架构的流行和普及，原本类似 <code>Crontab</code> 这种简单的东西使用起来会有各种限制，比如在最简单的情况你可以使用 <code>Spring</code> 的 <code>@Scheduled</code> 注解来轻松的实现一些定时需求，但现在你可能就需要考虑多如何将定时任务分配给多个节点，某个时间重新构建容器会不会丢失一些定时任务等等一些问题。这种情况延迟消息能解决掉我们大部分的需求，大概的流程就是丢一个消息给中间件，告诉它什么时候投递出去，投递后再由消费端消费（负责去通知/发邮件/发短信）。由于我们公司一直使用的 <code>Spring Cloud</code> 全家桶，<code>RabbitMQ</code> 也已经使用很多年，这里就讲讲如何在<code>Spring Cloud Stream RabbitMQ</code>下以最小的成本实现延时消息，所以在阅读文章时可能需要有 <code>Spring Cloud Stream</code> 相关的基础知识。</p>
<h2 id="实现"><a href="#实现" class="headerlink" title="实现"></a>实现</h2><h3 id="概述"><a href="#概述" class="headerlink" title="概述"></a>概述</h3><p>实际上 <code>RabbitMQ</code> 的延迟消息是通过 <code>rabbitmq-delayed-message-exchange</code> 这个插件实现的，开启了这个插件之后，只要在 <code>exchange</code> 上定义 <code>x-delayed-message</code> 类型，然后在发送消息的时候 <code>header</code> 带上 <code>x-delay</code> (毫秒数)，就可以实现延迟消息。我们可以利用这一特性，来实现消息的延迟/定时发送。<br><img src="https://jokefaker.oss-cn-hangzhou.aliyuncs.com/article/rabbitmq-delay.png" alt><br>理想的模型是这个样子的，但实际在应用中难免会有一些出入，在文末问题区会做一些相关的讨论。</p>
<h3 id="RabbieMQ-插件安装"><a href="#RabbieMQ-插件安装" class="headerlink" title="RabbieMQ 插件安装"></a>RabbieMQ 插件安装</h3><p>由于我们的 RabbitMQ 服务器都是跑在 <code>Docker</code> 容器里，所以这里就相对简单，新建一个 Dockerfile：</p>
<figure class="highlight plain"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br></pre></td><td class="code"><pre><span class="line">FROM rabbitmq:3.7.8-management-alpine</span><br><span class="line"></span><br><span class="line">RUN cd /plugins \</span><br><span class="line">&& wget https://dl.bintray.com/rabbitmq/community-plugins/3.7.x/rabbitmq_delayed_message_exchange/rabbitmq_delayed_message_exchange-20171201-3.7.x.zip \</span><br><span class="line">&& unzip rabbitmq_delayed_message_exchange-20171201-3.7.x.zip \</span><br><span class="line">&& rm rabbitmq_delayed_message_exchange-20171201-3.7.x.zip \</span><br><span class="line">&& rabbitmq-plugins enable rabbitmq_delayed_message_exchange</span><br></pre></td></tr></table></figure>
<p>如果你使用的是 3.8.x 版本，注意修改对应的版本。另外要是不跑在容器当中，<code>RabbitMQ</code> 官网也有说明，这里就不再赘述。</p>
<h3 id="Sprint-Cloud-Stream-优化"><a href="#Sprint-Cloud-Stream-优化" class="headerlink" title="Sprint Cloud Stream 优化"></a>Sprint Cloud Stream 优化</h3><p><code>stream</code> 配置有以下几个问题：</p>
<ul>
<li>需要在配置中编写 <code>destination</code> 等配置，然而这些配置往往都是模板化的</li>
<li>配置完毕后，代码中还需要将 channel 和 bean 的名称对应上，属于 hard code，如果对不上，或者某一边忘记删除，都会出错</li>
</ul>
<p>实际上在我们整个微服务的使用来看，大部分的配置内容都是一样的，我们先来看一下比较常见的模板：</p>
<figure class="highlight plain"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br></pre></td><td class="code"><pre><span class="line">xxx-events: # 这里是 input</span><br><span class="line"> group: yyy-service</span><br><span class="line"> contentType: application/json # 新的 stream 版本可以提供默认配置，不用每个指定</span><br><span class="line"> durableSubscription: true</span><br><span class="line">yyy-events: # 这里是 output</span><br><span class="line"> destination: yyy-events</span><br><span class="line"> contentType: application/json # 新的 stream 版本可以提供默认配置，不用每个指定</span><br></pre></td></tr></table></figure>
<p>如果一个 service 以领域事件向外输出消息，那么 <code>RabbitMQ</code> 的模板长成这个样子是比较常见的，统一由 yyy-events 发出领域事件，通过 xxx-events（各种领域）来接收领域事件。这样实际我们只需要根据服务的名称去生成对应的配置即可，如果你自己的业务有所不同，则需要根据自己的使用方式来生成配置。</p>
<p>下面给出实现上述配置的代码：</p>
<figure class="highlight plain"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br><span class="line">19</span><br><span class="line">20</span><br><span class="line">21</span><br><span class="line">22</span><br><span class="line">23</span><br><span class="line">24</span><br><span class="line">25</span><br><span class="line">26</span><br><span class="line">27</span><br><span class="line">28</span><br><span class="line">29</span><br><span class="line">30</span><br><span class="line">31</span><br><span class="line">32</span><br><span class="line">33</span><br><span class="line">34</span><br><span class="line">35</span><br><span class="line">36</span><br><span class="line">37</span><br><span class="line">38</span><br><span class="line">39</span><br><span class="line">40</span><br><span class="line">41</span><br><span class="line">42</span><br><span class="line">43</span><br><span class="line">44</span><br><span class="line">45</span><br><span class="line">46</span><br><span class="line">47</span><br><span class="line">48</span><br><span class="line">49</span><br><span class="line">50</span><br><span class="line">51</span><br><span class="line">52</span><br><span class="line">53</span><br><span class="line">54</span><br><span class="line">55</span><br><span class="line">56</span><br><span class="line">57</span><br><span class="line">58</span><br><span class="line">59</span><br><span class="line">60</span><br><span class="line">61</span><br><span class="line">62</span><br><span class="line">63</span><br><span class="line">64</span><br><span class="line">65</span><br><span class="line">66</span><br><span class="line">67</span><br><span class="line">68</span><br><span class="line">69</span><br><span class="line">70</span><br><span class="line">71</span><br><span class="line">72</span><br><span class="line">73</span><br><span class="line">74</span><br><span class="line">75</span><br><span class="line">76</span><br><span class="line">77</span><br><span class="line">78</span><br><span class="line">79</span><br><span class="line">80</span><br><span class="line">81</span><br><span class="line">82</span><br><span class="line">83</span><br><span class="line">84</span><br><span class="line">85</span><br><span class="line">86</span><br><span class="line">87</span><br><span class="line">88</span><br><span class="line">89</span><br><span class="line">90</span><br><span class="line">91</span><br><span class="line">92</span><br><span class="line">93</span><br><span class="line">94</span><br><span class="line">95</span><br><span class="line">96</span><br><span class="line">97</span><br><span class="line">98</span><br><span class="line">99</span><br><span class="line">100</span><br><span class="line">101</span><br><span class="line">102</span><br><span class="line">103</span><br><span class="line">104</span><br><span class="line">105</span><br><span class="line">106</span><br><span class="line">107</span><br><span class="line">108</span><br><span class="line">109</span><br><span class="line">110</span><br><span class="line">111</span><br><span class="line">112</span><br><span class="line">113</span><br><span class="line">114</span><br><span class="line">115</span><br><span class="line">116</span><br><span class="line">117</span><br><span class="line">118</span><br><span class="line">119</span><br><span class="line">120</span><br><span class="line">121</span><br><span class="line">122</span><br><span class="line">123</span><br><span class="line">124</span><br><span class="line">125</span><br><span class="line">126</span><br><span class="line">127</span><br><span class="line">128</span><br><span class="line">129</span><br><span class="line">130</span><br><span class="line">131</span><br><span class="line">132</span><br><span class="line">133</span><br><span class="line">134</span><br><span class="line">135</span><br><span class="line">136</span><br><span class="line">137</span><br><span class="line">138</span><br><span class="line">139</span><br><span class="line">140</span><br></pre></td><td class="code"><pre><span class="line">@Configuration</span><br><span class="line">@ConditionalOnClass(RabbitServiceAutoConfiguration.class)</span><br><span class="line">public class StreamAutoConfiguration {</span><br><span class="line"></span><br><span class="line"> @Autowired</span><br><span class="line"> private ConfigurableEnvironment environment;</span><br><span class="line"></span><br><span class="line"> private static final String PROPERTY_SOURCE_NAME = "creams-stream-rabbit-spring-boot-starter:stream-rabbit-config-properties";</span><br><span class="line"></span><br><span class="line"> private static final String RABBIT_PREFIX = "spring.cloud.stream.rabbit.bindings.";</span><br><span class="line"></span><br><span class="line"> @Autowired</span><br><span class="line"> private ApplicationContext context;</span><br><span class="line"></span><br><span class="line"></span><br><span class="line"> @Bean</span><br><span class="line"> @Primary</span><br><span class="line"> BindingServiceProperties customProps(BindingServiceProperties bindingServiceProperties) {</span><br><span class="line"></span><br><span class="line"> Map<String, Object> beanMap = context.getBeansWithAnnotation(EnableBinding.class);</span><br><span class="line"></span><br><span class="line"> Map<String, Object> map = new HashMap<>();</span><br><span class="line"></span><br><span class="line"> beanMap.forEach((key, value) -> {</span><br><span class="line"> EnableBinding annotation = context.findAnnotationOnBean(key, EnableBinding.class);</span><br><span class="line"> if (annotation != null) {</span><br><span class="line"> setPropsForAnnotation(annotation, map, bindingServiceProperties);</span><br><span class="line"> }</span><br><span class="line"> });</span><br><span class="line"></span><br><span class="line"> addOrReplace(environment.getPropertySources(), map);</span><br><span class="line"></span><br><span class="line"> return bindingServiceProperties;</span><br><span class="line"> }</span><br><span class="line"></span><br><span class="line"> private void setPropsForAnnotation(EnableBinding annotation, Map<String, Object> map, BindingServiceProperties bindingServiceProperties) {</span><br><span class="line"> Stream.of(annotation.value()).forEach(type -> ReflectionUtils.doWithMethods(type, method -> {</span><br><span class="line"> Input input = AnnotationUtils.findAnnotation(method, Input.class);</span><br><span class="line"> Output output = AnnotationUtils.findAnnotation(method, Output.class);</span><br><span class="line"> Destination destination = AnnotationUtils.findAnnotation(method, Destination.class);</span><br><span class="line"></span><br><span class="line"> String destinationName = Optional.ofNullable(destination).map(Destination::destination).orElse("");</span><br><span class="line"> if (input != null) {</span><br><span class="line"> // 设置 input 的内容</span><br><span class="line"> if (StringUtils.isEmpty(destinationName)) {</span><br><span class="line"> destinationName = input.value();</span><br><span class="line"> }</span><br><span class="line"> String name = BindingBeanDefinitionRegistryUtils</span><br><span class="line"> .getBindingTargetName(input, method);</span><br><span class="line"> BindingProperties properties = bindingServiceProperties.getBindingProperties(name);</span><br><span class="line"> properties.setGroup(getApplicationName());</span><br><span class="line"> properties.setDestination(destinationName);</span><br><span class="line"> String rabbitPrefix = RABBIT_PREFIX + name + ".consumer";</span><br><span class="line"> map.put(rabbitPrefix + ".autoBindDlq", true);</span><br><span class="line"> map.put(rabbitPrefix + ".republishToDlq", true);</span><br><span class="line"> map.put(rabbitPrefix + ".deadLetterQueueName", getErrorQueueName());</span><br><span class="line"> // 如果注解上声明了 isDelayedExchange 为 true，则加上对应的配置</span><br><span class="line"> if (Optional.ofNullable(destination).map(Destination::isDelayedExchange).orElse(false)) {</span><br><span class="line"> map.put(rabbitPrefix + ".delayedExchange", true);</span><br><span class="line"> }</span><br><span class="line"> } else if (output != null) {</span><br><span class="line"> // 设置 output 的内容</span><br><span class="line"> if (StringUtils.isEmpty(destinationName)) {</span><br><span class="line"> destinationName = output.value();</span><br><span class="line"> }</span><br><span class="line"> String name = BindingBeanDefinitionRegistryUtils</span><br><span class="line"> .getBindingTargetName(output, method);</span><br><span class="line"> BindingProperties properties = bindingServiceProperties.getBindingProperties(name);</span><br><span class="line"> properties.setDestination(destinationName);</span><br><span class="line"></span><br><span class="line"> String rabbitPrefix = RABBIT_PREFIX + name + ".producer";</span><br><span class="line"> // 如果注解上声明了 isDelayedExchange 为 true，则加上对应的配置</span><br><span class="line"> if (Optional.ofNullable(destination).map(Destination::isDelayedExchange).orElse(false)) {</span><br><span class="line"> map.put(rabbitPrefix + ".delayedExchange", true);</span><br><span class="line"> }</span><br><span class="line"> }</span><br><span class="line"> }));</span><br><span class="line"> }</span><br><span class="line"></span><br><span class="line"> private String getApplicationName() {</span><br><span class="line"> return environment.getProperty("spring.application.name");</span><br><span class="line"> }</span><br><span class="line"></span><br><span class="line"> // 定义 DLQ 的名称</span><br><span class="line"> public String getErrorQueueName() {</span><br><span class="line"> return getPureServiceName() + "-error.dlq";</span><br><span class="line"> }</span><br><span class="line"></span><br><span class="line"> // 这里由于我们自己的服务通常起名 creams-xxx-service, 所以会做一次去头去尾的操作，你可以自定义</span><br><span class="line"> private String getPureServiceName() {</span><br><span class="line"> String[] names = getApplicationName().split("-");</span><br><span class="line"> List<String> nameList = new ArrayList<>(Arrays.asList(names));</span><br><span class="line"> if (nameList.size() >= 3) {</span><br><span class="line"> nameList.remove(0);</span><br><span class="line"> nameList.remove(nameList.size() - 1);</span><br><span class="line"> }</span><br><span class="line"> return String.join("-", nameList);</span><br><span class="line"> }</span><br><span class="line"></span><br><span class="line"> // 添加或者替换 KV</span><br><span class="line"> private void addOrReplace(MutablePropertySources propertySources,</span><br><span class="line"> Map<String, Object> map) {</span><br><span class="line"></span><br><span class="line"> MapPropertySource target = null;</span><br><span class="line"> if (propertySources.contains(PROPERTY_SOURCE_NAME)) {</span><br><span class="line"> PropertySource<?> source = propertySources.get(PROPERTY_SOURCE_NAME);</span><br><span class="line"> if (source instanceof MapPropertySource) {</span><br><span class="line"> target = (MapPropertySource) source;</span><br><span class="line"> for (String key : map.keySet()) {</span><br><span class="line"> if (!target.containsProperty(key)) {</span><br><span class="line"> target.getSource().put(key, map.get(key));</span><br><span class="line"> }</span><br><span class="line"> }</span><br><span class="line"> }</span><br><span class="line"> }</span><br><span class="line"> if (target == null) {</span><br><span class="line"> target = new MapPropertySource(PROPERTY_SOURCE_NAME, map);</span><br><span class="line"> }</span><br><span class="line"> if (!propertySources.contains(PROPERTY_SOURCE_NAME)) {</span><br><span class="line"> propertySources.addLast(target);</span><br><span class="line"> }</span><br><span class="line"> }</span><br><span class="line">}</span><br><span class="line"></span><br><span class="line">// 提供一个注解，用于自定义一些内容</span><br><span class="line">@Target({ ElementType.FIELD, ElementType.METHOD, ElementType.ANNOTATION_TYPE,</span><br><span class="line"> ElementType.PARAMETER })</span><br><span class="line">@Retention(RetentionPolicy.RUNTIME)</span><br><span class="line">@Inherited</span><br><span class="line">@Documented</span><br><span class="line">public @interface Destination {</span><br><span class="line"></span><br><span class="line"> @AliasFor("destination")</span><br><span class="line"> String value() default "";</span><br><span class="line"></span><br><span class="line"> @AliasFor("value")</span><br><span class="line"> String destination() default "";</span><br><span class="line"></span><br><span class="line"> boolean isDelayedExchange() default false;</span><br><span class="line">}</span><br></pre></td></tr></table></figure>
<h3 id="延迟消息的实现"><a href="#延迟消息的实现" class="headerlink" title="延迟消息的实现"></a>延迟消息的实现</h3><p>这里我以实现一个小 Demo 来展示所要实现的东西，这个 Demo 实现发送延迟消息，并自我接收后消费消息。为了节省篇幅，这里略去 Demo 工程的建立以及配置过程，文末将附上地址。</p>
<ul>
<li>建立输入输出的 <code>interface</code> ：</li>
</ul>
<figure class="highlight plain"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br></pre></td><td class="code"><pre><span class="line">public interface BindingChannelOutput {</span><br><span class="line"></span><br><span class="line"> String DEMO_EVENTS = "demo-events";</span><br><span class="line"></span><br><span class="line"> @Output(DEMO_EVENTS)</span><br><span class="line"> @Destination(isDelayedExchange = true)</span><br><span class="line"> MessageChannel contract();</span><br><span class="line">}</span><br><span class="line"></span><br><span class="line">public interface BindingChannelInput {</span><br><span class="line"></span><br><span class="line"> String DEMO_INPUTS = "demo-events";</span><br><span class="line"></span><br><span class="line"> @Input(DEMO_INPUTS)</span><br><span class="line"> @Destination(isDelayedExchange = true)</span><br><span class="line"> SubscribableChannel input();</span><br><span class="line">}</span><br></pre></td></tr></table></figure>
<p>注意这里和一般的 <code>Stream</code> 例子 有一些不一样， 用到了我们之前优化的注解，<code>isDelayedExchange</code> 可以指定为 <code>delay exchange</code>。</p>
<ul>
<li>再建立一个 <code>publisher</code> 用于发送消息，以及一个 <code>Listener</code> 用于接收消息</li>
</ul>
<figure class="highlight plain"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br><span class="line">19</span><br><span class="line">20</span><br><span class="line">21</span><br><span class="line">22</span><br><span class="line">23</span><br><span class="line">24</span><br><span class="line">25</span><br><span class="line">26</span><br><span class="line">27</span><br><span class="line">28</span><br></pre></td><td class="code"><pre><span class="line">@Component</span><br><span class="line">@AllArgsConstructor</span><br><span class="line">@Slf4j</span><br><span class="line">public class Publisher {</span><br><span class="line"></span><br><span class="line"> private final BindingChannelOutput outputChannel;</span><br><span class="line"></span><br><span class="line"> public void send(String content, Long delayMillSeconds) {</span><br><span class="line"></span><br><span class="line"> MessageBuilder<String> builder = MessageBuilder.withPayload(content);</span><br><span class="line"> if (delayMillSeconds > 0) {</span><br><span class="line"> builder.setHeader("x-delay", delayMillSeconds);</span><br><span class="line"> }</span><br><span class="line"> log.info(MessageFormat.format("pushing message [{0}] to remote", content));</span><br><span class="line"> outputChannel.output().send(builder.build());</span><br><span class="line"> }</span><br><span class="line">}</span><br><span class="line"></span><br><span class="line">@Component</span><br><span class="line">@Slf4j</span><br><span class="line">public class Listener {</span><br><span class="line"></span><br><span class="line"> @StreamListener(BindingChannelInput.DEMO_INPUTS)</span><br><span class="line"> public void update(@Payload String test) {</span><br><span class="line"> // 发通知/邮件/短信</span><br><span class="line"> log.info(MessageFormat.format("received message [{0}]", test));</span><br><span class="line"> }</span><br><span class="line">}</span><br></pre></td></tr></table></figure>
<p>启动工程后在 <code>RabbitMQ</code> 的控制台应该可以看到两个 <code>queue</code> ，一个是 <code>demo-events.demo-service</code> 另一个是 <code>demo-service-error.dlq</code> ，这说明我们的配置优化生效了。</p>
<ul>
<li>写一个接口来测试延迟消息</li>
</ul>
<figure class="highlight plain"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br></pre></td><td class="code"><pre><span class="line">@RestController</span><br><span class="line">@AllArgsConstructor</span><br><span class="line">public class TestController {</span><br><span class="line"> </span><br><span class="line"> private Publisher publisher;</span><br><span class="line"> </span><br><span class="line"> @PostMapping("/test")</span><br><span class="line"> public void test(@RequestParam("content") String content) {</span><br><span class="line"> </span><br><span class="line"> publisher.send(content, 20000L);</span><br><span class="line"> }</span><br><span class="line">}</span><br></pre></td></tr></table></figure>
<p>在控制台可以看到打印时间相差 20s</p>
<figure class="highlight plain"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br></pre></td><td class="code"><pre><span class="line">2020-03-10 17:17:40.691 INFO 77487 --- [nio-8080-exec-5] com.example.demo.Publisher : pushing message ["测试"] to remote</span><br><span class="line">2020-03-10 17:18:00.483 INFO 77487 --- [.demo-service-1] com.example.demo.Listener : received message ["测试"]</span><br></pre></td></tr></table></figure>
<p>至此延迟消息的实现就完成了，具体到业务的话，都是在消费端进行，比如做一些邮件/短信发送或者通知之类的。</p>
<h2 id="问题-amp-思考"><a href="#问题-amp-思考" class="headerlink" title="问题&思考"></a>问题&思考</h2><h3 id="发出去的消息不可查询"><a href="#发出去的消息不可查询" class="headerlink" title="发出去的消息不可查询"></a>发出去的消息不可查询</h3><p>当我们的业务服务向 <code>RabbitMQ</code> 发送消息后，<code>RabbitMQ</code> 在没有到时间的时候是看不到的，那么要保证可靠，就需要在业务服务中记录是否投递到 <code>RabbitMQ</code>，可以参考可靠消息的投递，在本地做记录。</p>
<h3 id="延迟时间的限制"><a href="#延迟时间的限制" class="headerlink" title="延迟时间的限制"></a>延迟时间的限制</h3><p>文档中提到过延迟的最大毫秒数是 2^32-1 ，大概是 49 天多一点，不过一般也不会往 <code>RabbitMQ</code> 堆这么长时间的消息。一般如果是 1-2 天内需要推送/发通知的业务，我们会直接丢给 <code>RabbitMQ</code>，剩下的比如定了一个一年后的提醒之类的业务，都采用在夜间流量少的时候去查一下第二天需要发送的内容，然后丢到 <code>RabbitMQ</code> 来完成。所以定时之类的操作根据业务的具体情况可能还会存在，但是会大大减少。</p>
<h3 id="禁用插件后所有的未发布的消息会丢失"><a href="#禁用插件后所有的未发布的消息会丢失" class="headerlink" title="禁用插件后所有的未发布的消息会丢失"></a>禁用插件后所有的未发布的消息会丢失</h3><p>这个其实还好，一般不会去做这样的操作。</p>
<h3 id="delay-exchange-的路由类型"><a href="#delay-exchange-的路由类型" class="headerlink" title="delay exchange 的路由类型"></a>delay exchange 的路由类型</h3><p>路由类型还是取决于 exchange 本身的类型，由于 <code>Spring Cloud Stream</code> 默认都是 topic 类型，所以这里保持一致。</p>
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<a href="/2019/06/14/服务优化/" class="post-title-link" itemprop="url">Event Sourcing 和 CQRS落地（八）：服务优化</a>
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<h2 id="服务优化"><a href="#服务优化" class="headerlink" title="服务优化"></a>服务优化</h2><h3 id="失败消息的补偿机制"><a href="#失败消息的补偿机制" class="headerlink" title="失败消息的补偿机制"></a>失败消息的补偿机制</h3><p>由于消息存在发送失败的情况，比如 broker 临时下线或者不可用了，尽管这种情况很少，我们最好做一个机制可以定期或者手动检查，并且尝试自己发送，这里我们就来实现这个机制。</p>
<h4 id="提供未发送的-event-查询"><a href="#提供未发送的-event-查询" class="headerlink" title="提供未发送的 event 查询"></a>提供未发送的 event 查询</h4><ol>
<li>在<code>CustomDomainEventEntryRepository</code>中加入：</li>
</ol>
<figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br></pre></td><td class="code"><pre><span class="line"><span class="comment">/**</span></span><br><span class="line"><span class="comment"> * 查找未发送的事件</span></span><br><span class="line"><span class="comment"> *</span></span><br><span class="line"><span class="comment"> * <span class="doctag">@param</span> pageable</span></span><br><span class="line"><span class="comment"> *</span></span><br><span class="line"><span class="comment"> * <span class="doctag">@return</span></span></span><br><span class="line"><span class="comment"> */</span></span><br><span class="line"><span class="function">Page<CustomDomainEventEntry> <span class="title">findBySentFalse</span><span class="params">(Pageable pageable)</span></span>;</span><br><span class="line"></span><br><span class="line"><span class="comment">/**</span></span><br><span class="line"><span class="comment"> * 查询未发送事件的数量</span></span><br><span class="line"><span class="comment"> * <span class="doctag">@return</span></span></span><br><span class="line"><span class="comment"> */</span></span><br><span class="line"><span class="function">Long <span class="title">countBySentFalse</span><span class="params">()</span></span>;</span><br></pre></td></tr></table></figure>
<ol start="2">
<li>将未发送事件的数量集成到 <code>actuator</code>，让我们可以事实看到失败消息的数量：</li>
</ol>
<figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br></pre></td><td class="code"><pre><span class="line"><span class="meta">@Component</span></span><br><span class="line"><span class="meta">@AllArgsConstructor</span></span><br><span class="line"><span class="keyword">public</span> <span class="class"><span class="keyword">class</span> <span class="title">EventHealthContributor</span> <span class="keyword">implements</span> <span class="title">InfoContributor</span> </span>{</span><br><span class="line"></span><br><span class="line"> <span class="keyword">private</span> <span class="keyword">final</span> CustomDomainEventEntryRepository customDomainEventEntryRepository;</span><br><span class="line"></span><br><span class="line"> <span class="meta">@Override</span></span><br><span class="line"> <span class="function"><span class="keyword">public</span> <span class="keyword">void</span> <span class="title">contribute</span><span class="params">(Info.Builder builder)</span> </span>{</span><br><span class="line"> Long count = customDomainEventEntryRepository.countBySentFalse();</span><br><span class="line"></span><br><span class="line"> builder.withDetail(<span class="string">"failedMessage"</span>, count);</span><br><span class="line"> }</span><br><span class="line">}</span><br></pre></td></tr></table></figure>
<p>打开 <code>http://localhost:8080/actuator/info</code> 应该就可以看到我们的失败消息数量。</p>
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<a href="/2019/06/13/Spring-Cloud-Stream-优化/" class="post-title-link" itemprop="url">Event Sourcing 和 CQRS落地（七）：Spring-Cloud-Stream 优化</a>
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<h2 id="Spring-Cloud-Stream-优化"><a href="#Spring-Cloud-Stream-优化" class="headerlink" title="Spring Cloud Stream 优化"></a>Spring Cloud Stream 优化</h2><h3 id="有哪些问题"><a href="#有哪些问题" class="headerlink" title="有哪些问题"></a>有哪些问题</h3><p><code>Spring Cloud Stream</code>（以下简称 SCS ）是 Spring Cloud 提供的消息中间件的抽象，但是目前也就支持 kafka 和 rabbitmq，这篇文章主要会讨论一下如何让 SCS 更好的服务我们之前搭建的 Event Sourcing、CQRS 模型。以下是我在使用 SCS 的过程中存在的一些问题：</p>
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<li><code>StreamListener</code>用来做事件路由分发并不是很理想，SPEL 可能会写的很长（我尝试过用自定义注解代替原生的注解，从而达到简化的目的，但是会出现一些莫名其妙的事件混乱）。</li>
<li>如果配合之前的模型使用，我们需要保证消息的顺序消费，每个方法都需要去 check 事件的当前 seq，很不方便。</li>
<li>在没有 handler 处理某个 type 的事件时，框架会给出一个 warn，然而这个事件可能在 consumer 这里根本不关心。
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