1、前情回顾

上篇文章「Java进阶面试系列之一」大家系统架构中为什么要引入消息中间件？，给你们讲了讲消息中间件引入系统架构的做用，主要是解决哪些问题的。性能优化

其比较常见的实践场景是：微信

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

2、正式开始

这篇文章给你们讲讲，若是你在系统架构里引入了消息中间件以后，会有哪些缺点？markdown

1）系统可用性下降网络

首先是你的系统总体可用性绝对会下降，给你举个例子，咱们就拿以前的一幅图来讲明。架构

好比说一个核心链路里面，系统A -> 系统B -> 系统C，而后系统C是经过MQ异步调用系统D的。并发

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。app

可是你有没有考虑另一个问题，就是万一你依赖的那个MQ中间件忽然挂掉了怎么办？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，就致使你的系统的核心业务流程中断了。原本你要是不引入MQ中间件，那其实就是一些系统之间的调用，可是如今你引入了MQ，就致使你多了一个依赖。一旦多了一个依赖，就会致使你的可用性下降。

所以，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，你还要考虑若是MQ一旦挂了之后，你的系统有没有备用兜底的技术方案，能够保证系统继续运行下去。

以前写过两篇文章，都涉及到了MQ挂掉以后的高可用保障方案。

大伙若是感兴趣，能够参考一下：

经过这两篇文章，具体看看咱们在各类场景下遇到MQ故障采起的高可用降级方案。

2）系统稳定性下降

仍是上面那张图，你们再来看一下。

不知道你们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以外，可能还有一些其余的技术问题。

好比说，莫名其妙的，系统C发了一个消息到MQ，结果那个消息由于网络故障等问题，就丢失了。这就致使系统D没有收到那条消息。

这可就惨了，这样会致使系统D没完成本身该作的任务，此时可能整个系统会出现业务错乱，数据丢失，严重的bug，用户体验不好等各类问题。

这还只是其中之一，万一说系统C给MQ发送消息，不当心一抽风重复发了一条如出一辙的，致使消息重复了，这个时候该怎么办？

可能会致使系统D一会儿把一条数据插入了两次，致使数据错误，脏数据的产生，最后同样会致使各类问题。

或者说若是系统D忽然宕机了几个小时，致使没法消费消息，结果大量的消息在MQ中间件里积压了好久，这个时候怎么办？

即便系统D恢复了，也须要慢慢的消费数据来进行处理。

因此这就是引入MQ中间件的第二个大问题，系统稳定性可能会降低，故障会增多，各类各样乱七八糟的问题均可能产生。

并且一旦产生了一个问题，就会致使系统总体出问题。就须要为了解决各类MQ引起的技术问题，采起不少的技术方案。

关于这个，咱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，好比说系统C如今处理本身本地数据库成功了，而后发送了一个消息给MQ，系统D也确实是消费到了。

可是结果不幸的是，系统D操做本身本地数据库失败了，那这个时候咋办？

系统C成功了，系统D失败了，会致使系统总体数据不一致了啊。

因此此时又须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，能够参考以前的一篇文章：

最终一致性分布式事务如何保障实际生产中99.99%高可用？

咱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。

3、总结

最后，咱们来作一个简单的小结。

在面试中要答好这个问题，首先必定要熟悉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，可是他自身又会带来哪些问题。

此外，对于引入MQ之后，是否对他自身可能引起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。这个也有作好相应的准备。

END

若有收获，请帮忙转发，您的鼓励是做者最大的动力，谢谢！

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【Java进阶面试系列之二】：哥们，那你说说系统架构引入消息中间件有什么缺点？

1、前情回顾

2、正式开始

3、总结