May 2020
Beginner to intermediate
319 pages
7h 3m
Chinese
Content preview from 数据库系统内幕
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共识
｜
253
提议者
接受者
图
14-6
：
Paxos
故障场景：提议者故障之后接受者故障
Paxos 算法能容忍接受者故障，但剩余的接受者必须足以构成多数派。
14.3.4 Multi-Paxos
到目前为止，我们讨论了经典的 Paxos 算法。在该算法中，我们选择任意一个提议者来
尝试发起 Paxos 回合。这个方法存在的一个问题是，系统中的每轮复制都需要进行一
轮提议。只有当选出提议者之后（即多数派的接受者向某一提议者回应 Promise 消息
时），才会进入复制阶段。为了避免重复进行提议阶段，使提议者可以重用已被认可的地
位，我们可以使用 Multi-Paxos。Multi-Paxos 引入了
领导者
的概念
—
提议者的负责人
（distinguished proposer）[LAMPORT01]。这一补充十分关键，能显著提升算法的效率。
有了一个已确立的领导者，我们就可以跳过提议阶段，直接进行复制：分发值并收集接
受者的确认。
经典 Paxos 算法中，读取可以通过执行一轮 Paxos 回合实现，它从不完整的回合（如果
存在的话）中收集值。之所以必须这么做，是因为最后一个已知的提议者无法保证一定
包含最新的数据，可能另一个提议者已经修改了状态，而当前提议者并不知道。
Multi-Paxos 中也可能发生类似的情况：我们试图从已知的领导者进行读取，但此时
另一位领导者已经当选，那么请求将返回过期的数据，这就违背了共识的可线性化
（linearizability）保证。为了避免这种情况并确保其他进程都无法成功地提交值 ...
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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.