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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263
第 16章
选择片键
使用分片时最重要的任务是选择数据的分发方式。为了在这方面做出明智的选择，必须了
解
MongoDB
是如何分发数据的。本章旨在帮助你更好地选择片键，包括：
•
如何在多个可用的片键中做出选择；
•
不同使用场景中的片键选择；
•
哪些键不能作为片键；
•
自定义数据分发方式的可选策略；
•
如何手动对数据分片。
本章假设你已经理解了前两章介绍的关于分片基本组件的知识。
16.1
评估使用情况
在对集合进行分片时，需要选择一两个字段来对数据进行拆分。这个键（或这些键）称为
片键
。一旦对一个集合进行了分片，就不能更改片键了，因此正确选择片键是十分重要的。
要选择一个好的片键，需要了解工作负载以及片键将如何分发应用程序的请求。这可能不
太好描述，可以尝试找出一些例子，或者更进一步，在具有样本流量的备份数据集上做一
些实验。本节有许多图表和解释，但这些都不如在自己的数据上试一试。
对于计划分片的每个集合，首先回答以下问题。
•
计划进行多少个分片？
3
个分片的集群比
1000
个分片的集群具有更大的灵活性
。随着
集群的增长，不应该使用会触发所有分片的查询，因此大部分查询应该包含片键。
•
分片是为了减少读写延迟吗？（延迟指某个操作需要花费的时间。例如，一个写操作要
花费
20
毫秒
，但你需要它
10
毫秒就完成。）降低写延迟通常包括将请求发送到地理位
置更近或功能更强大的机器上。
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