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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18
｜
第
2
章
•
在这
4
字节中也隐含了每个文档的创建时间。大多数驱动程序提供了从
ObjectId
中提
取此信息的方法。
由于
ObjectId
使用的是当前时间，因此很多人会担心需要对服务器进行时钟同步。尽管
出于其他原因进行时钟同步是一个好主意（参见
24.5.1
节）
，但在这里实际的时间戳对
ObjectId
并不重要，只要它总是不停增加就可以了（每秒
1
次 ）。
ObjectId
中接下来的
5
字节是一个随机值。最后
3
字节是一个计数器，以一个随机数作为
起始值，用来避免在不同机器上生成相互冲突的
ObjectId
。
因此，前
9
字节保证了
ObjectId
在
1
秒内跨机器和进程的唯一性。最后
3
字节只是一个
递增计数器，负责确保单个进程中
1
秒内的唯一性
。这允许在
1
秒内为
每个进程
生成多达
256
3
（
16 777 216
）个唯一的
ObjectId
。
2.
自动生成
_id
如前所述，如果插入文档时不存在
"_id"
键，则会自动创建一个并添加到插入的文档中。
这个工作可以由
MongoDB
的服务器端处理，但通常由客户端的驱动程序来完成。
2.7
使用
MongoDB shell
本节介绍如何将
shell
作为命令行工具包的一部分来使用，如何对其进行自定义，以及
shell
的一些高级功能。
尽管在上面的例子中只是连接到了一个本地的
mongod
实例
，但实际上可以将
shell
连接到
机器可以访问的任何
MongoDB
实例
。要想连接到其他机器或端口上的
mongod
，需要在启
动
shell
时指定主机名、端口和数据库：
$ mongo some-host:30000/myDB ...
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