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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?

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《C2C 电商系统微服务架构120天实战训练营》

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:

最终一致性分布式事务如何保障实际生产中99.99%高可用?

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。

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