MongoDB复制集原理
https://mongoing.com/archives/2155
mongDB replication
https://docs.mongodb.com/manual/replication
跨两个或多个数据中心分布的副本集
Replica Sets Distributed Across Two or More Data Centers
https://docs.mongodb.com/manual/core/replica-set-architecture-geographically-distributed
zk 三个节点的情况下,挂掉节点会发生什么?