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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184
｜
第
9
章
要理解如何实现这些不同级别的一致性，就必须了解
MongoDB
的内部机制。服务器端为
各个数据库连接维护了一个请求队列。客户端每次发来的新请求都会被添加到队列的末
尾。连接中的任何后续请求都将依次得到处理。因此，单个连接具有一致的数据库视图，
并且始终可以读取到自己的写操作。
注意，每个列队只对应一个连接：如果打开两个
shell
，连接到相同的数据库
，则会有两个
不同的连接。如果在一个
shell
中执行插入操作
，那么另一个
shell
中的后续查询可能不会
返回插入的文档。然而，在同一个
shell
中，
如果在插入一个文档之后查询，则一定能够查
询到刚插入的文档。想手动重现这种问题可能很困难，但是在繁忙的服务器上，很可能会
出现交错的插入和查询操作。当开发人员使用一个线程插入数据，然后在另一个线程中检
查数据是否成功插入时经常会遇到这种情况。片刻之后，数据看起来好像没有插入成功，
然后又突然出现了。
在使用
Ruby
、
Python
和
Java
驱动程序时尤其需要注意这个问题
，因为这
3
种语言的驱动
程序都使用了连接池。为了提高效率，这些驱动程序会建立多个与服务器端的连接（也就
是一个
连接池
），并在它们之间分发请求。不过，它们都拥有各自的机制来保证由单个连
接处理一系列相关的请求。
MongoDB
驱
动程序连接监控和连接池规范中有关于各种语言
连接池的详细文档。
当向副本集的从节点（参见第
12
章）发送读请求时
，这将成为一个更大的问题。从节点
数据可能落后于主节点，导致读取到的数据是几秒、几分钟甚至几小时之前的。有几种方 ...
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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.