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我的新课**《C2C 电商系统微服务架构120天实战训练营》在公众号儒猿技术窝**上线了，感兴趣的同学，可以点击下方链接了解详情：
一、前情回顾
之前给大家聊了一下，面试时如果遇到消息中间件这个话题，面试官上来可能问的两个问题：
- 你们的系统架构中为什么要引入消息中间件？
- 系统架构中引入消息中间件有什么缺点？
关于这两个问题的回答，可以参见之前的两篇文章：
在问完这两个问题之后，不同风格的面试官可能会展开不同的发问。
针对那种工作年限比较长的资深的同学，可能会开始就候选人所在公司使用的消息中间件，深入里面的技术细节，比如让你聊聊RocketMQ的架构原理和核心源码？
但是另外一种面试风格，会先从你们的项目和业务入手进行考察，比如像下面这样：
- 消息中间件在你们生产项目里具体是哪个业务场景下落地的？
- 这个业务场景有什么技术挑战？
- 为什么必须要在这个业务场景里用消息中间件技术？
- 具体使用消息中间件的时候是怎么来用的？
好！这篇文章，咱们从第二种风格来聊聊。
二、业务场景介绍
我们会落地到某个具体业务系统的某个场景下，看看如何使用消息中间件，然后其效果是什么。
业务场景的话，咱们就用大家都很熟悉的电商业务为例，这里为了便于理解，对其做了一定的抽象和简化。
大家还是来考虑一个下订单的业务流程，比如你下个订单，此时需要干几件事情：
- 更新订单状态为“待发货”（耗时20ms）
- 扣减商品库存（耗时100ms）
- 增加会员积分（耗时80ms）
- 附赠优惠券（耗时50ms）
- 仓储调度发货（耗时几十秒）。
说明一下：上述环节，为了便于大家理解，做了简化。实际真正复杂的电商系统里，整体环节和业务流程会比这个复杂很多倍，而且耗时也绝对不是上面那么简单的。
老规矩！我们还是通过一张手绘图，来看看这整个的业务流程：
如上图，这个下订单的业务流程中：
更新订单状态（20ms） + 扣减商品库存（100ms） + 增加会员积分（80ms） + 附赠优惠券（50ms） = 250ms。
也就是说，仅仅是这4个流程的话，也就200多毫秒的耗时。
200多毫秒的耗时，对用户下单体验来说是非常快速的，几乎就是一瞬间就完成了，不会感到过多的停顿，也就是一下子就可以看到自己下单成功了。
但是，如果加上那个调度仓储发货呢？
那个环节需要读取大量的数据、使用多仓库/多货位的调度算法、还要跟C/S架构的仓储系统进行网络通信，因此我们这里假设这个环节可能会耗时数十秒。
一旦加上那个调度仓储发货的环节到这个下单流程里，就可能导致用户要等页面卡顿几十秒后才会看到下单成功的提示，这个用户体验就相当的差了。
按照之前一篇文章「Java进阶面试系列之一」你们系统架构中为何要引入消息中间件？的说法。对于这种场景，完全适合使用消息中间件来进行异步化调用。
也就是说，订单服务对仓储调度发货，仅仅是发送一个消息到MQ里，然后仓储服务消费消息之后再慢慢的执行调度算法，然后分配商品发货任务给对应的仓库即可。
这样的话，就可以把耗时几十秒的仓储调度发货的环节，从下单流程里摘除出去了。进而保证下单流程就仅仅是耗时200多毫秒而已。
至于那个耗时几十秒的仓储调度发货环节，我们通过异步的方式慢慢执行即可，不会影响用户下单的体验。
以上过程，我们同样来一张图，大家直观的感受一下：
三、初步落地
好！接下来我们就假设大家在实际生产中还没用过消息中间件，咱们从0开始，看看如何落地？
对于已经在生产中使用过消息中间件的小伙伴，不妨也看看，权当复习，温故知新！
我们以RabbitMQ为例，假如你用的消息中间件是RabbitMQ，那么我们对这个消息中间件应该如何安装和部署呢？
很简单，RabbitMQ的官方文档里提供了非常详细的安装部署步骤，你可以在自己的笔记本电脑本地安装，也可以在公司的服务器上部署。
现在假设你已经参考了官方文档并安装完成，那么接下来在代码层面应该怎么来引入RabbitMQ以及在系统里实现收发消息呢？
下面通过一些HelloWorld级别的代码和一些简单的示例图，给大家演示一下RabbitMQ是如何收发消息的。
对于很多在实际生产中使用过MQ的同学，这些代码可能对实际生产中使用过MQ的同学，显得太简单了。
不过考虑到很多初学者可能连用都没有用过MQ，甚至是才听说消息中间件不久，所以笔者认为这些demo代码以及手工绘图，还是很有必要。
好！看完了代码，这个时候，我们可以通过一张图来想象一下两个服务之间的通信。
订单服务你可以启动多个，不同的订单服务都可以往一个RabbitMQ的queue里推送消息。
仓储服务你也可以启动多个，多个仓储服务会采用round-robin的轮询算法，每个服务实例都可以从RabbitMQ queue里消费到一部分的消息。
上面的图里，订单服务在MQ专业术语中叫做**“生产者”**，英文是“Producer”，意思就是这个服务是专门负责生产消息投递到MQ的。
仓储服务在MQ专业术语中叫做**“消费者”**，英文是“Consumer”，意思就是这个服务专门是负责从MQ消费消息然后处理的。
这个时候，这套异步通信的架构就可以跑起来了。
好了，到目前为止，虽然这个代码还存在不少问题，但是没关系，大体上我们已经给一些不太熟悉MQ技术的同学，从一个比较形象易于理解简化后的电商业务场景出发，通过HelloWorld级别的示例代码和手工绘图，将MQ这个技术落地跑起来了。
更进一步，各位同学完全可以参照这个文章里的案例，思考一下：自己负责的项目里，有没有类似的业务场景可以使用MQ的？
然后想办法在自己的项目里落地使用MQ的技术来做一下异步化，提升核心流程的性能。
这样未来在跳槽面试的时候，才可以做到游刃有余，有自己的一套东西可以说。
END
【Java进阶面试系列之四】哥们，如果消息中间件的消费者宕机了，会怎么样？【敬请期待】
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