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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264
｜
第
16
章
•
分片是为了提高读写吞吐量吗？（吞吐量指集群在同一时间可以处理的请求数量。例如，
集群可以在
20
毫秒内完成
1000
次写操作，
但你需要它在
20
毫秒内完成
5000
次写操作。）
提高吞吐量通常需要增加更多的并行化，并确保请求在集群中均匀分发。
•
分片是为了增加系统资源吗？（例如，每
GB
数据提供给
MongoDB
更多的
RAM
。）
如
果是这样，你可能会希望保持工作集尽可能小。
根据这些问题的答案来评估以下对片键的描述，以决定所选择的片键是否合适。它是否提
供了所需的目标查询？它是否按所需要的方式改变了系统的吞吐量或延迟？如果需要保持
一个紧凑的工作集，它可以提供吗？
16.2
描绘分发情况
最常见的数据拆分方式是升序片键、随机分发的片键和基于位置的片键。也可以使用其他
类型的键，但大多数场景属于这三类之一。下面几节会讨论不同类型的分发方式。
16.2.1
升序片键
升序片键通常类似于
"date"
字段或
ObjectId
——随着时间稳步增长的字段。自增主键是升
序字段的另一个例子，尽管它在
MongoDB
中并不常见（除非从另一个数据库导入）
。
假设按照升序字段进行分片，比如在集合中使用了
ObjectId
类型的
"_id"
键。如果在
"_id"
上进行分片，那么数据会根据
"_id"
的范围被拆分成多个块，如图
16-1
所示。这些
块会分发在整个分片集群中，假设有
3
个分片，如图
16-2
所示。
$minKey -> ObjectId("5112fa61b4a4b396960262")
ObjectId("5112fa61b4a4b39696 ...
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