May 2024
Intermediate
278 pages
5h 24m
Chinese
Content preview from 构建可扩展分布式系统：方法与实践
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220
｜
第
13
章
会响应客户端。
WAIT
的代价是更长的延迟。此外，它仅保证数据驻留在内存中的副本内。
在下一个
AOF
写入快照之前，若副本崩溃，则仍可能会导致更新丢失。
13.1.3.5
可用性
Redis Cluster
为单个数据库分片实现了一个久经考验的主副本架构。但是，写入可用性
不可避免地受到领导者故障的影响，在将副本提升为领导者之前，给定分片的写入将不
可用。
网络故障可以将
Redis
集群拆分为多数分区和少数分区。这会影响可用性和数据安全性。
只要分区中至少有一个可用副本，客户端写入请求就可以继续写到两个分区中的所有领导
节点。如果有一个领导者与它的副本分离，落到少数分区，而副本落在多数分区，那么客
户端在少数分区的写入仍然可用。经过一个超时周期后，少数分区的领导者将停止接受写
入，因为它无法向副本发送更新。同时，落到多数分区中的副本将进行领导者选举，副本
将被提升为主节点。当分区恢复时，在少数分区中的领导者的写入修改将丢失。
13.2 MongoDB
自
2009
年首次发布以来，
MongoDB
一直走在
NoSQL
数据库的前沿。它在本质上协调
数据库模型与对象模型，直接解决了众所周知的对象关系阻抗不匹配问题。我们可以将
生成的文档数据库视为
JSON
数据库。你可以将业务对象转换为
JSON
，并将数据直接
作为文档存储、查询和操作。不需要复杂的对象关系映射器，直观且更简单地展示业务
逻辑。
1
MongoDB
最初流行起来是因为它易于编程和使用。早期版本中的底层存储引擎，称为
MMAPv1
注
2
，还有一些不足之处
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