【解构系统设计面试】什么是系统设计?以及如何设计一个新鲜事系统?

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的过程可能要持续一段时间,有一些粉丝可能已经收到这个明星发布的新鲜事了,但是有的粉丝可能半个小时之后才收到,这是影响用户体验的,一个吃瓜群众肯定是希望第一时间吃到瓜的。"}]}]}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"heading","attrs":{"align":null,"level":3},"content":[{"type":"text","text":"哪种方案更好?"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"比较两个方案的好坏,首先我们要分别列举出两个方案的优缺点:"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"image","attrs":{"src":"https://static001.geekbang.org/infoq/9a/9a1fd8299ed4a8c8f21058535895c7e6.png","alt":null,"title":null,"style":null,"href":null,"fromPaste":true,"pastePass":true}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"我们从表格中可以看到,这两个方案其实是各有千秋的。但是在实际系统中,往往更多系统设计会选择使用【拉模型】,比如:"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"bulletedlist","content":[{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"Facebook 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模型就好了”。这是错误的回答方式。"}]}]},{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"strong"}],"text":"要展现出 tradeoff 的能力:"},{"type":"text","text":"要针对当前系统设计方案的缺陷去做权衡。"}]}]}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"heading","attrs":{"align":null,"level":3},"content":[{"type":"text","text":"如何优化?"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"这一部分我们主要讲的是如何针对上述两种方案的缺陷进行优化。这一部分对应的就是 4S 分析法中的【扩展 - Scale】部分,专注于解决设计中的缺陷。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"heading","attrs":{"align":null,"level":4},"content":[{"type":"text","text":" 解决 Pull 模型的缺陷"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"我们来分析一下 Pull 模型可能存在的系统瓶颈(bottleneck),我们会发现这一模型最慢的部分发生在用户请求新鲜事列表时,并且这一过程需要消耗用户的等待时间。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"针对这一情况我们可以选择在系统中加入缓存(Cache),在访问 DB 之前加入一个缓存层,如果在缓存中命中了,则直接将数据返回给用户,可以大大缩减用户的等待时间。我们知道,一般缓存是存放在系统的内存中的,对缓存的操作是远远快于操作数据库的。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"但是缓存我们该缓存什么呢?是缓存每个用户的时间线呢?还是缓存每个用户的新鲜事列表呢?"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"bulletedlist","content":[{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"strong"}],"text":"缓存每个用户的时间线(Timeline):"},{"type":"text","text":"如果我们缓存每个用户的时间线,就相当于把之前的 N 次(N 为关注的好友的个数) DB 请求替换成了 N 次缓存访问,明显是可以提速的。但是,缓存的空间一般是内存,是很宝贵的,是不能存储海量数据的,因此我们就要做出相应的 tradeoff,比如说,每个用户只缓存最新 1000 条或者最新 100 条的新鲜事;比如说,我们还可以将明星、热点用户(用友大量关注者的用户)的缓存长期保存在缓存系统中,不轻易让缓存失效;"}]}]},{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"strong"}],"text":"缓存每个用户的新鲜事列表(News Feed)"},{"type":"text","text":":如果我们来将每个用户的新鲜事列表缓存起来,那么拉取新鲜事列表是就仅仅需要一次 Cache 访问了。针对没有缓存新鲜事列表的用户,归并 N 个关注好友的最新 100 条新鲜事,取出前 100 条放入缓存中;针对已经做了缓存的用户,只需要归并 N 个用户在某个时间之后的所有新鲜事,加入缓存即可。在这一方案中,我们也可以针对不同的用户来做优化,比如说针对经常使用系统、经常频繁刷新新鲜事的用户做优化,将他们的缓存长期存放,对于一些长期不使用的僵尸用户,将他们的缓存清理掉。"}]}]}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"提到了缓存,不得不提的就是,由于系统的突然爆发式的访问,造成了缓存的失效该如何处理呢?这就是我们经常会听到的"},{"type":"text","marks":[{"type":"strong"}],"text":"系统雪崩"},{"type":"text","text":"和*"},{"type":"text","marks":[{"type":"italic"}],"text":"缓存重建"},{"type":"text","text":"*的问题。针对这一问题,我们会在接下来的问题中探讨一些解决的思路和方案。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"heading","attrs":{"align":null,"level":4},"content":[{"type":"text","text":" 解决 Push 模型的缺陷"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"首先,Push 模型有一个缺陷是,浪费数据库的存储空间,但是其实这个并不算问题,因为毕竟当前“Disk is cheap.”。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"第二,由于 Fanout 的时间可能会比较长,导致用户有可能在自己所关注的用户发布了新鲜事的一段时间后才能够在自己的新鲜事列表里刷到该条消息。对此,我们可以采用一定的方案来提升用户体验,比如说在 Fanout 的时候先对粉丝按照某种规则进行排序,例如,按照用户的活跃度进行排序(可以按最新登录系统的时间进行排序),针对活跃度越高的用户优先进行推送。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"另外,针对某些明星用户或者说热点用户(关注他们的用户数量远远大于他们自己所关注用户的数目),整个 Fanout 的时间可能会很长。解决这一个问题有一个很粗暴的方案,就是增加机器的数量,采用分布式 Fanout 的方案。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"heading","attrs":{"align":null,"level":4},"content":[{"type":"text","text":"Push 模型 + Pull 模型的方案"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"其实,还有一种方案,是将 Push 方案 和 Pull 方案结合起来进行优化。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"比如说,针对普通用户我们采用 Push 的方案;针对明星这类热点用户,我们在系统里进行标记。对于明星用户发布新鲜事,不进行 Push,也就是不将新鲜事 Push 到关注了这一明星的列表里。当用户需要获取自己的新鲜事列表时,到自己所关注的明星用户的时间线上取,并且合并到自己的 News Feed 列表里。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"heading","attrs":{"align":null,"level":2},"content":[{"type":"text","text":"Q2:热点用户(明星)的迅速涨粉/掉粉(摇摆问题)"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"基于上述针对“明星用户”的解决方案,会引发一个“摇摆问题”。这一问题的描述大致如下:"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"在系统中我们需要标记为热点明星用户采用不同的策略,例如,在上一问题的解决方案中,对于明星用户的处理方式是,将明星用户进行标记,对于非明星用户使用“Push”的方案,对于明星用户采用“Pull”的方案。这会带来什么问题呢?如果系统定义被关注数量有 1M 以上的用户为明星用户,此时有一个用户的被关注数量为 1M,同时存在大量的用户在点击对该用户的关注/取关,这一用户在系统中就会被频繁的被标记为“明星”/“非明星”用户。于是针对这一类“明星用户”,会在“Pull”和“Push”模型之中来回切换,于是会导致有用户收不到该“明星用户”发布的新鲜事。这一问题被称为“摇摆问题”。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"image","attrs":{"src":"https://static001.geekbang.org/infoq/10/10dd5453c9c5f78c2abb9c5135b7d26d.png","alt":null,"title":"","style":[{"key":"width","value":"75%"},{"key":"bordertype","value":"boxShadow"}],"href":"http://blog.luoyuanhang.com/2020/05/24/system-design-0/","fromPaste":false,"pastePass":false}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"strong"}],"text":"该如何解决这一问题呢?"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"其实解决这一问题最简单的方法就是,不会动态的对明星用户进行标记。也就是说,用户的关注、取关行为不会立刻影响到该用户会不会被标记为“明星用户”,这一标记过程可以异步使用一个线程,定期来做。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"还有另外一种解决思路,是使用一个缓冲地带,比如给用户加入一个“伪明星”的状态,针对这一状态的用户单独处理。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"heading","attrs":{"align":null,"level":2},"content":[{"type":"text","text":"Q3:僵尸粉会带来哪些问题?"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"我们可以来定义一下“僵尸粉”,该类用户主要表示的是在系统中长期不活跃的用户,但是他们也有许多关注的用户,也会有少量的粉丝。而这种用户有时会给系统带来一些不必要的负载,从而影响到整个系统的性能。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"其实,我们在系统设计过程中,可以单独针对这一类用户单独处理。比如,我们在系统中对这些长期不活跃的用户进行标记,系统如果使用的是 Push 模型的话,在 Fanout 的过程中,将该类用户设置优先级,放到该过程的最后来执行。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"heading","attrs":{"align":null,"level":2},"content":[{"type":"text","text":"Q3:如何实现关注(Follow)与取关(Unfollow)?"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"针对 Follow 与 UnFollow 的实现方案,如果采用的是 Push 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Facebook 提供的 Comment 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"},{"type":"link","attrs":{"href":"http://blog.luoyuanhang.com","title":""},"content":[{"type":"text","text":"罗远航"}]},{"type":"text","text":" 所有,采用 [Attribution-NonCommercial 3.0 License](http://creativecommons.org/licenses/by-nc/3.0/)。任何人可以进行转载、分享,但不可在未经允许的情况下用于商业用途;转载请注明出处。感谢配合!"}]}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}}]}
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