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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持久性
｜
335
操作之前等待日志记录确实会造成性能损失。
在解决持久性问题时，必须仔细评估应用程序的需求，并权衡所选择的持久性设置对性能
的影响。
20.3
使用读关注的集群级别持久性
在
MongoDB
中，读关注（
read concern
）允许对何时读取结果进行配置。这可以让客户端
在写操作被持久化之前就看到写入的结果。读关注可以与写关注一起使用，以控制对应用
程序的一致性和可用性的保证级别。不要将读关注与读偏好（
read
preference
）相混淆，后
者处理从何处读取数据的问题。具体来说，读偏好决定了副本集中承载数据的成员。默认
的读偏好是从主节点中读取。
读关注决定了正在读取的数据的一致性和隔离性。默认的
readConcern
是
local
，它所返
回的数据不保证已经被写入了大多数承载数据的副本集成员。这可能会导致数据在将来的
某个时刻被回滚。
majority
读关注只返回被大多数副本集成员确认的持久数据（不会被回
滚 ）。
MongoDB 3.4
中增加了
linearizable
读关注，它确保返回的数据反映了在读操作开
始之前已成功完成的经过大多数确认的写操作。在返回结果之前，它可能会等待那些正在
并发执行的写操作完成。
和写关注一样，在为应用程序选择合适的关注选项之前，需要权衡读关注对性能的影响，
以及它们提供的持久性和隔离性保证。
20.4
使用写关注的事务持久性
在
MongoDB
中，对单个文档的操作是原子的。可以在单个文档中使用内嵌文档和数组来
表示实体之间的关系，而不是使用范式化的数据模型将实体和关系拆分到多个集合中。因 ...
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I’ve been on the O’Reilly platform for more than eight years. I use a couple of learning platforms, but I'm on O'Reilly more than anybody else. When you're there, you start learning. I'm never disappointed.Amir M.
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.