November 2021
Intermediate to advanced
410 pages
12h 1m
Chinese
Content preview from MongoDB权威指南（第3版）
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创建副本集
｜
203
更复杂的需求可能需要不同的配置，但应该考虑副本集在不利条件下将如何满足“大多
数”的要求。
如果
MongoDB
的一个副本集可以拥有多个主节点
，上面这些复杂问题就迎刃而解了。不
过，多个主节点会带来其他的复杂性。在拥有两个主节点的情况下，就需要处理写入冲
突。（例如，有人在第一个主节点上更新一个文档，而另一个人在另一个主节点上删除了
这个文档。）在支持多线程写入的系统中有两种常见的冲突处理方式：手动解决冲突或者
让系统任选一个作为“胜利者”。但是这两种方式对于开发者来说都不容易实现，因为无
法确保写入的数据不会被其他节点修改。因此，
MongoDB
选择只支持单一主节点
。这样
可以使开发更容易，但是可能导致副本集变为只读状态。
如何进行选举
当一个从节点无法与主节点连通时，它就会联系并请求其他的副本集成员将自己选举为主
节点。其他成员会做几项健全性检查：它们能否连接到一个主节点，而这个主节点是发起
选举的节点无法连接到的？这个发起选举的从节点是否有最新数据？有没有其他更高优先
级的成员可以被选举为主节点？
MongoDB
在其
3.2
版本中引入了第
1
版复制协议
。第
1
版协议基于斯坦福大学的
Diego
Ongaro
和
John
Ousterhout
开发的
RAFT
共识协议。这是一个类
RAFT
的协议，并且包含
了一些特定于
MongoDB
的副本集概念
，比如仲裁节点、优先级、非选举成员、写入关注
点（
write concern
）等
。第
1
版协议提供了很多新特性的基础，比如更短的故障转移时间，
以及大大减少了检测主节点失效的时间。它还通过使用 ...
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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.