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一、前情回顾
上篇文章「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？，给大家讲了讲消息中间件引入系统架构的作用，主要是解决哪些问题的。
其比较常见的实践场景是：
- 复杂系统的解耦
- 复杂链路的异步调用
- 瞬时高峰的削峰处理
二、正式开始
这篇文章给大家讲讲，如果你在系统架构里引入了消息中间件之后，会有哪些缺点？
1）系统可用性降低
首先是你的系统整体可用性绝对会降低，给你举个例子，我们就拿之前的一幅图来说明。
比如说一个核心链路里面，系统A -> 系统B -> 系统C，然后系统C是通过MQ异步调用系统D的。
看起来很好，你用这个MQ异步化的手段解决了一个核心链路执行性能过差的问题。
但是你有没有考虑另外一个问题，就是万一你依赖的那个MQ中间件突然挂掉了怎么办？这个还真的不是异想天开，MQ、Redis、MySQL这些组件都有可能会挂掉。
一旦你的MQ挂了，就导致你的系统的核心业务流程中断了。本来你要是不引入MQ中间件，那其实就是一些系统之间的调用，但是现在你引入了MQ，就导致你多了一个依赖。一旦多了一个依赖，就会导致你的可用性降低。
因此，一旦引入了MQ中间件，你就必须去考虑这个MQ是如何部署的，如何保证高可用性。
甚至在复杂的高可用的场景下，你还要考虑如果MQ一旦挂了以后，你的系统有没有备用兜底的技术方案，可以保证系统继续运行下去。
之前写过两篇文章，都涉及到了MQ挂掉之后的高可用保障方案。
大伙如果感兴趣，可以参考一下：
通过这两篇文章，具体看看我们在各种场景下遇到MQ故障采取的高可用降级方案。
2）系统稳定性降低
还是上面那张图，大家再来看一下。
不知道大家有没有发现一个问题，这个链路除了MQ中间件挂掉这个可能存在的隐患之外，可能还有一些其他的技术问题。
比如说，莫名其妙的，系统C发了一个消息到MQ，结果那个消息因为网络故障等问题，就丢失了。这就导致系统D没有收到那条消息。
这可就惨了，这样会导致系统D没完成自己该做的任务，此时可能整个系统会出现业务错乱，数据丢失，严重的bug，用户体验很差等各种问题。
这还只是其中之一，万一说系统C给MQ发送消息，不小心一抽风重复发了一条一模一样的，导致消息重复了，这个时候该怎么办？
可能会导致系统D一下子把一条数据插入了两次，导致数据错误，脏数据的产生，最后一样会导致各种问题。
或者说如果系统D突然宕机了几个小时，导致无法消费消息，结果大量的消息在MQ中间件里积压了很久，这个时候怎么办？
即使系统D恢复了，也需要慢慢的消费数据来进行处理。
所以这就是引入MQ中间件的第二个大问题，系统稳定性可能会下降，故障会增多，各种各样乱七八糟的问题都可能产生。
而且一旦产生了一个问题，就会导致系统整体出问题。就需要为了解决各种MQ引发的技术问题，采取很多的技术方案。
关于这个，我们后面会用专门的文章聊聊MQ中间件的这些问题的解决方案，包括：
- 消息高可靠传递（0丢失）
- 消息幂等性传递（绝对不重复）
- 百万消息积压的线上故障处理
3）分布式一致性问题
引入消息中间件，还有分布式一致性的问题。
举个例子，比如说系统C现在处理自己本地数据库成功了，然后发送了一个消息给MQ，系统D也确实是消费到了。
但是结果不幸的是，系统D操作自己本地数据库失败了，那这个时候咋办？
系统C成功了，系统D失败了，会导致系统整体数据不一致了啊。
所以此时又需要使用可靠消息最终一致性的分布式事务方案来保障。
关于这个，可以参考之前的一篇文章：
我们在里面详细阐述了系统之间异步调用场景下，如何采用分布式事务方案保证其数据一致性。
三、总结
最后，我们来做一个简单的小结。
在面试中要答好这个问题，首先一定要熟悉MQ这个技术的优缺点。了解清楚把他引入系统是为了解决哪些问题的，但是他自身又会带来哪些问题。
此外，对于引入MQ以后，是否对他自身可能引发的问题有一些方案的设计，来保证你的系统高可用、高可靠的运行，保证数据的一致性。这个也有做好相应的准备。
END
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