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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207
第 11章
副本集的组成
本章介绍副本集的各个部分是如何组织在一起的，包括：
•
副本集成员如何复制新数据；
•
如何让新成员开始工作；
•
选举是如何进行的；
•
可能出现的服务器端和网络故障场景。
11.1
同步
复制是指在多台服务器上保持相同的数据副本。
MongoDB
实现此功能的方式是保存操作
日志（
oplog
）
，其中包含了主节点执行的每一次写操作。
oplog
是存在于主节点
local
数据
库中的一个固定集合。从节点通过查询此集合以获取需要复制的操作。
每个从节点都维护着自己的
oplog
，用来记录它从主节点复制的每个操作
。这使得每个成
员都可以被用作其他成员的同步源，如图
1
1-1
所示。从节点从同步源中获取操作，将其应
用到自己的数据集上，然后再写入
oplog
中
。如果应用某个操作失败（只有在基础数据已
损坏或数据与主节点不一致时才会发生这种情况），则从节点会停止从当前数据源复制数
据。
如果一个从节点由于某种原因而停止运行，那么当它重新启动后，就会从
oplog
中的最后
一个操作开始同步。由于这些操作是先应用到数据上然后再写入
oplog
，因此从节点可能
会重复已经应用到其数据上的操作。
MongoDB
在设计时就考虑到了这种情况
：将
oplog
中
的同一个操作执行多次与只执行一次效果是一样的。
oplog
中的每个操作都是幂等的
。也
就是说，无论对目标数据集应用一次还是多次，
oplog
操作都会产生相同的结果。
208
｜
第
11
章
6 7 8 9 10 11 12 13 14
主节点
6 7 8 9 10
从节点 1
查询
{"$gt":10} ...
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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.