Redis - sentinel mode

brief introduction

From the original master-slave replication , If the host goes down , You need to manually set a slave to a new master , But this manual configuration requires manual intervention , It takes time and manpower , And it will also cause service unavailability during the time of manual intervention , This is not a good solution for developers .Redis from 2.8 Sentinel mode began to appear .
Sentinel mode can monitor whether the host fails in the background , In case of failure, one of the slave machines will be selected as the host by voting .

Sentinel mode works

1、 By sending commands , Give Way Redis The server returns to monitor its running state , Including the master and slave servers .
2、 When the Sentinels detect master Downtime , Will automatically slave Switch to master, Then the other slave servers are informed through publish subscribe mode , Modify the configuration file , Let them switch hosts .

Sentinel mode principle

Redis The order of the sentry , And it is a special pattern , It will create a completely independent Redis Process of server , One sentry can monitor multiple servers , And there can be multiple sentinels , Each sentinel will send regularly PING Command to the server , And also get the response from the server within a certain period of time , After receiving the response, sentinel mode will determine that your current state is normal . If the host fails to respond to the request it sends within the specified time , Then the Sentry will make a preliminary judgment , The current host is offline , The rest of the Sentinels found that the host did not respond to the data within the specified time , Then the host will be judged once per second , It's really subjective , Then the host will be marked as offline , After the host computer is down , The sentinel will choose one of the suspended hosts as the new host by voting .

Configure sentinel mode

stay Linux Using the editor to write a file , The name of the document is sentinel.conf, Here we need to pay attention to , The name of the configuration file is fixed , Don't Scribble .

Establish different server configurations , Implement one master and two slaves mode , How to configure it can be viewed in my previous articles ：https://blog.csdn.net/aaa123_456aaa/article/details/122932495?spm=1001.2014.3001.5502
We can check these services ：

The core configuration of the sentry ：sentinel monitor myredis 127.0.0.1 6379 1
And the numbers 1 Express ： When a sentinel subjectively thinks the host is disconnected , It can be objectively considered that the host is faulty , Then start electing a new host .

#  Sentinel service default port 
port 26379
#  Sentry mode default working directory 
dir /tmp
#  Property format ：sentinel monitor  Host name  IP port 1  Back number 1 On behalf of the host is hung up ,Slave Vote who will be the new host 
sentinel monitor myredis 127.0.0.1 6379 1

edit sentinel.conf

Activate the sentry ：

Test sentinel mode

Let's set a value on the host first , Then turn off the host , See if a new host will be created ：

Once the host is shut down , We will find that the sentinel side will also change , That is, monitor the host changes ：

We see the 6381, It is found that it is still a slave ：

So there's no doubt about it , According to the principle of sentinel mode ,6380 Will become a host , So let's see 6380 You can see that it becomes a host ：

Be careful ：
If the host comes back , Then it will still become 6380 Slave of , This is the time 6380 Is already the boss .

The advantages and disadvantages of sentinel mode

advantage ：
1、 The sentry cluster , Based on master-slave replication mode , All the advantages of master-slave replication , It all has .
2、 Master slave can switch , Faults can be transferred , The availability of the system is better .
3、 Sentry mode is an upgrade of master-slave mode , Manual to automatic , More robust
shortcoming ：
1、Redis Not good for online expansion , Once the cluster capacity reaches the upper limit , Online expansion is very troublesome
2、 The configuration of sentinel mode is very troublesome , There are many configuration items in it

Full configuration of sentinel mode

Complete sentinel mode configuration file sentinel.conf

# Example sentinel.conf
 
#  sentry sentinel The port on which the instance runs   Default 26379
port 26379
 
#  sentry sentinel Working directory of 
dir /tmp
 
#  sentry sentinel Monitored redis The master node  ip port 
# master-name   You can name the master node by yourself   Only letters A-z、 Numbers 0-9 、 These three characters ".-_" form .
# quorum  When these quorum Number sentinel The sentry thought master Primary node lost connection   Then at this time   It is objectively believed that the primary node is disconnected 
# sentinel monitor <master-name> <ip> <redis-port> <quorum>
sentinel monitor mymaster 127.0.0.1 6379 1
 
#  When in Redis In the example, the requirepass foobared  Authorization code   So all connections Redis The client of the instance must provide the password 
#  Set up a sentry sentinel  Password to connect master and slave   Note that the same authentication password must be set for master slave 
# sentinel auth-pass <master-name> <password>
sentinel auth-pass mymaster MySUPER--secret-0123passw0rd
 
 
#  Specify how many milliseconds later   The master node didn't answer the sentry sentinel  here   Sentinels subjectively think that the primary node is offline   Default 30 second 
# sentinel down-after-milliseconds <master-name> <milliseconds>
sentinel down-after-milliseconds mymaster 30000
 
#  This configuration item specifies what is happening failover How many can there be at most during the primary / standby handover slave Simultaneously on the new master Conduct   Sync ,
 The smaller the number , complete failover The longer it takes ,
 But if the number is bigger , That means the more   Much of the slave because replication And is not available .
 You can do this by setting this value to zero  1  To make sure there's only one at a time slave  In a state where command requests cannot be processed .
# sentinel parallel-syncs <master-name> <numslaves>
sentinel parallel-syncs mymaster 1
 
 
 
#  Timeout for failover  failover-timeout  It can be used in the following ways ： 
#1.  The same sentinel To the same master two failover The time between .
#2.  When one slave From a wrong one master Where the synchronized data starts to calculate the time . until slave Corrected to be correct master Where the data is synchronized .
#3. When you want to cancel an ongoing failover The time required .  
#4. When doing failover when , Configure all slaves Point to the new master Maximum time required . however , Even after this timeout ,slaves It will still be correctly configured to point master, But it won't parallel-syncs Here comes the configured rule 
#  Default three minutes 
# sentinel failover-timeout <master-name> <milliseconds>
sentinel failover-timeout mymaster 180000
 
# SCRIPTS EXECUTION
 
# Configure the scripts that need to be executed when an event occurs , The administrator can be notified by script , For example, when the system is not running normally, send an email to inform the relevant personnel .
# There are the following rules for the result of the script ：
# If the script returns 1, Then the script will be executed again later , The number of repetitions currently defaults to 10
# If the script returns 2, Or 2 A higher return value , The script will not repeat .
# If the script is terminated during execution due to receiving a system interrupt , Then the same return value is 1 The same behavior when .
# The maximum execution time of a script is 60s, If I exceed that time , The script will be a SIGKILL Signal termination , And then re execute .
 
# Notification scripts : When sentinel When any warning level event occurs （ for instance redis Subjective failure and objective failure of examples, etc ）, Will call this script ,
# At this point the script should be sent by email ,SMS Wait for the way to inform the system administrator about the abnormal operation of the system . When the script is called , Two parameters will be passed to the script ,
# One is the type of event ,
# One is the description of the event .
# If sentinel.conf The script path is configured in the configuration file , Then you have to make sure that the script exists in this path , And it's executable , otherwise sentinel Failed to start normally successfully .
# Notification script 
# sentinel notification-script <master-name> <script-path>
  sentinel notification-script mymaster /var/redis/notify.sh
 
#  The client reconfigures the master node parameter script 
#  When one master because failover And when it changes , This script will be called , Notify related clients about master Information that the address has changed .
#  The following parameters will be passed to the script when the script is called :
# <master-name> <role> <state> <from-ip> <from-port> <to-ip> <to-port>
#  at present <state> Always “failover”,
# <role> yes “leader” perhaps “observer” One of them . 
#  Parameters  from-ip, from-port, to-ip, to-port It's for the old master And the new master( I.e. old slave) communication 
#  This script should be generic , Can be called many times , It's not targeted .
# sentinel client-reconfig-script <master-name> <script-path>
sentinel client-reconfig-script mymaster /var/redis/reconfig.sh