Redis sentinel mechanism

effect

Solve the problem of master-slave mode , Master slave switchover after master instance failure ( Will be promoted from the instance
Main example ), To ensure that the system is available

Monitor the running status of master-slave instances

While the master-slave instance is running , There is also a special redis The process is running ( sentry )
It is responsible for monitoring the master , From the running state of the instance （ The heartbeat detection : Respond to the sentry within the specified time ）
If the instance does not respond within the specified time , Mark it as offline
If the primary instance does not respond within the specified time , Then start the master-slave switching process
Because a single sentinel may respond overtime due to its own network fluctuations or the pressure of the primary instance at that time , Misjudge the primary instance as offline （ Subjective offline ）. So as to execute unnecessary master-slave switching logic . Therefore, sentinels are generally made into clusters , Judge whether the main instance is offline by obeying the majority ( Objective offline )

Select the main process

When the main instance is judged to be offline , Operation scoring mechanism
1. First exclude the offline from the instance
2. Exclude nodes that have been disconnected from the main instance for a certain number of times during the operation of the instance
3. Determine the priority of the slave instance ( Priority can be achieved by ：slave-priority To configure ), High priority , Score high
4. Compare the ring buffer from the Library slave_repl_offset( This parameter is used to record the replication difference between the primary instance and the primary instance , When the main instance hangs ,master_repl_offset There will be no increase in , So from the example slave_repl_offset The bigger it is , The smaller the difference with the main instance ),slave_repl_offset The bigger it is , The higher the score
5. perform redis The default rule for ： runID The smaller it is , The higher the score
After the above scoring process , After choosing the Lord
The sentry tells the master's information to other slave instances
From the instance through replicaof Synchronize with the new primary instance
The sentinel informs the client to connect to the new primary instance