May 2020
Beginner to intermediate
319 pages
7h 3m
Chinese
Content preview from 数据库系统内幕
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264
｜
第
14
章
a
）
b
）
c
）
c
）
图
14-11
：
Raft
的提交过程（
P
1
为领导者）
图 14-12 展示了一个共识回合的例子，其中 P
1
是领导者，它拥有最新的事件视图。领导
者将日志条目复制给跟随者，并在收集到确认消息后提交这些命令。提交一个条目时也
会提交它以前的所有条目。只有领导者才能决定是否可以提交该条目。每个日志条目上
都标有任期 ID（日志条目方框右上角的数字）和日志下标，以标识其在日志中的位置。
已提交的条目一定已被复制给 Quorum 的参与者，并且可以安全地按照日志中的顺序应
用到状态机。
已提交的条目
日志下标
（
领导者
）
图
14-12
：
Raft
状态机
14.4.2
故障场景
当多个跟随者决定成为候选者时，可能会没有任何一个候选者收集到多数派的投票，这
共识
｜
265
种情况被称为
平票
（split vote）。 Raft 用随机定时器来降低再次平票的概率。其中一个候
选者可以略早发起下一轮选举，从而收集到足够的票数，而其他人则等待并做出让步。
这种方法可以在不引入其他协调的情况下加快选举速度。
跟随者可能会宕机或反应迟缓，领导者必须尽最大努力保证消息传递。如果未能在预期
时间内收到确认，它可以尝试再次发送消息。作为一种性能优化的方式，它也可以并行
发送多个消息。
由于领导者复制的每个条目都是唯一标识的 ...
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I'm always learning. So when I got on to O'Reilly, I was like a kid in a candy store. There are playlists. There are answers. There's on-demand training. It's worth its weight in gold, in terms of what it allows me to do.