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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356
｜
第
22
章
无论选择哪种技术，都需要提前计划以尽量减少对应用程序的影响。进行备份，依次修改
集合中的每个成员，将数据从一个地方复制到另一个地方，这些操作都需要时间。
22.5
监控复制情况
复制延迟和
oplog
长度是需要跟踪的重要指标。延迟是指从节点无法跟上主节点的速度。
它的计算方法是，将应用在从节点上最后一个操作的时间减去主节点上最后一个操作的时
间。如果从节点刚刚应用了时间戳为
3:26:00 p.m.
的操作
，而主节点刚刚应用了时间戳为
3:29:45 p.m.
的操作
，那么从节点的延迟就是
3
分
45
秒。延迟应该尽可能接近于
0
，并且
通常是毫秒级别的。如果从节点能够与主节点保持同步，那么复制延迟应该如图
22-9
所
示，即基本上始终为
0
。
复制延迟
图 22-9：没有延迟的副本集，这是我们希望看到的
如果从节点复制写操作的速度比主节点的写入速度慢，就会出现非零的延迟。最极端的情
况是当复制过程卡住时：由于某种原因，从节点不能应用更多的操作。此时，延迟将以
“每秒一秒”
1
的速度增长，形成如图
22-10
所示的陡坡。这可能是由于网络问题或缺少
"_id"
索引造成的，要使复制正常工作，每个集合都需要这个索引。
复制延迟
图 22-10：复制发生了阻塞，在 2 月 10 日之前才开始恢复，垂直线表示服务器的重启
注
1
：
由于同步被完全阻塞，因此每过一秒都会使延迟增加一秒。
监控
MongoDB
｜
357
如果一个集合缺少
"_id"
索引，则将服务器从副本集中脱离并作为单机服务
器启动，然后创建
"_id"
索引。确保将
"_id" ...
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I wanted to learn C and C++, but it didn't click for me until I picked up an O'Reilly book. When I went on the O’Reilly platform, I was astonished to find all the books there, plus live events and sandboxes so you could play around with the technology.Addison B.
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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.