第八章 Read Concern

第八章 Read Concern

Read Concern选项一般用来保证数据的隔离性、一致性。

1.readPreference和readConcern的区别

readPreference 决定客户端Driver到哪个节点读取数据。

• primary：只从primary节点读取数据。
• primaryPreferred：优先从primary节点读数据，primary节点不可用时，从secondary节点读取数据。
• secondary：只从secondary节点读取数据。
• secondaryPreferred：优先从secondary节点读取数据，没有secondary节点时，从primary节点读取。
• nearest：就近读取数据。

readConcern：决定从节点数据读取到什么样的数据。有可能是脏数据。

• local：读取任意数据。（默认从primary节点读取，不能保证数据被复制到大多数节点）
• avaiable ：读取任意数据。（默认从secondary节点读取）
• majority：读取已经被复制到大多数节点的数据。这个读取级别要求使用 WiredTiger 存储引擎。

2.写入数据过程

先来看一下，数据被写入副本集群的过程，writeConcern为majority。

还没有执行写操作前的数据状态：Write_pre
执行完写操作后的数据状态：Write0

时间点	事件	最新数据状态	满足majority的数据状态
T0	Primary执行写操作	Primary: Write0 Secondary1: Write_prev Secondary2: Write_prev	Primary: Write_prev Secondary1: Write_prev Secondary2: Write_prev
T1	Secondary1执行写操作	Primary: Write0 Secondary1: Write0 Secondary2: Write_prev	Primary: Write_prev Secondary1: Write_prev Secondary2: Write_prev
T2	Secondary2执行写操作	Primary: Write0 Secondary1: Write0 Secondary2: Write0	Primary: Write_prev Secondary1: Write_prev Secondary2: Write_prev
T3	Primary知道Secondary1成功的复制，并返回ack给客户端	Primary: Write0 Secondary1: Write0 Secondary2: Write0	Primary: Write0 Secondary1: Write_prev Secondary2: Write_prev
T4	Primary知道Secondary2成功的复制	Primary: Write0 Secondary1: Write0 Secondary2: Write0	Primary: Write0 Secondary1: Write_prev Secondary2: Write_prev
T5	Secondary1通过定期复制机制更新它的快照	Primary: Write0 Secondary1: Write0 Secondary2: Write0	Primary: Write0 Secondary1: Write0 Secondary2: Write_prev
T6	Secondary2通过定期复制机制跟新它的快照	Primary: Write0 Secondary1: Write0 Secondary2: Write0	Primary: Write0 Secondary1: Write0 Secondary2: Write0

2.读级别

local级别会读取节点的最新数据，可能会发生回滚，读到脏数据。

读取节点	时间点	数据状态
Primary	在T0之后	Write0
Secondary1	在T1之前	Write_prev
Secondary1	在T1之后	Write0
Secondary2	在T2之前	Write_prev
Secondary2	在T2之后	Write0

majority：可以保证数据的持久性，不会发生回滚，避免脏读。majority级别需要由存储引擎WiredTiger支持。

读取节点	时间点	数据状态
Primary	在T3之前	Write_prev
Primary	在T3之后	Write0
Secondary1	在T5之前	Write_prev
Secondary1	在T5之后	Write0
Secondary2	在T6之前	Write_prev
Secondary2	在T6之后	Write0