Sentinel sentinel mechanism

1. Sentinel Sentinel mechanism
   1. Start and initialize Sentinel
      1. 1. Initialize server
      2. 2. Will be ordinary Redis Replace the code used by the server with Sentinel Special code
      3. 3. initialization Sentinel state
      4. 4. According to the given profile , initialization Sentinel The list of monitoring primary servers .
      5. 5. Create a network connection to the primary server
   2. ️ Thank you for your

Sentinel Sentinel mechanism

The sentinel mechanism is Redis Enable highly available solutions ： By one or more Sentinel Examples of Sentinel The system can monitor any number of master and slave servers , If the primary server goes offline ,Sentinel Automatically upgrade the slave server under the offline master server to a new master server , The master server of the alternative principle continues to process command requests , After the offline master server is restored, it is set as the slave server of the new master server

Start and initialize Sentinel

When one Sentinel Startup time , The following steps are required

1. Initialize server

Sentinel The essence is a system that operates in a special mode Redis The server , start-up Sentinel The first step is to initialize a normal Redis The server , Normal servers are initialized by loading RDB Documents or AOF File to restore database state , But because Sentinel Do not use databases , So initialization Sentinel It doesn't load RDB Documents or AOF file

2. Will be ordinary Redis Replace the code used by the server with Sentinel Special code

Ordinary Redis The common port is 6379,Sentinel The common port is 26379

Sentinel Server command table ：sentinel.c Of sentinelcmds

struct redisCommand sentinelcmds[] = {    {"ping",pingCommand,1,"",0,NULL,0,0,0,0,0},    {"sentinel",sentinelCommand,-2,"",0,NULL,0,0,0,0,0},    {"subscribe",subscribeCommand,-2,"",0,NULL,0,0,0,0,0},    {"unsubscribe",unsubscribeCommand,-1,"",0,NULL,0,0,0,0,0},    {"psubscribe",psubscribeCommand,-2,"",0,NULL,0,0,0,0,0},    {"punsubscribe",punsubscribeCommand,-1,"",0,NULL,0,0,0,0,0},    {"info",sentinelInfoCommand,-1,"",0,NULL,0,0,0,0,0}};

Command table only 7 An order , Better than average redis The command list of the server is much less

3. initialization Sentinel state

Server initialization sentinelState structure , This structure holds all and Sentinel Function related states

/* Main state. */struct sentinelState {    dict *masters;      /* Dictionary of master sentinelRedisInstances. Key is the instance name, value is the sentinelRedisInstance structure pointer. */    int tilt;           /* Are we in TILT mode? */    int running_scripts;    /* Number of scripts in execution right now. */    mstime_t tilt_start_time;   /* When TITL started. */    mstime_t previous_time;     /* Time last time we ran the time handler. */    list *scripts_queue;    /* Queue of user scripts to execute. */} sentinel;

4. According to the given profile , initialization Sentinel The list of monitoring primary servers .

Sentinel Of masters The dictionary records all the Sentinel Information about the monitored master server , Where the key of the dictionary is the name of the monitored master server , The value of the dictionary corresponds to the monitored master server sentinelRedisInstance structure . Every sentinelRedisInstance The structure represents a quilt Sentinel Monitored Redis Server instance

Sentinel Initialization of raises a pair of masters Initialization of a dictionary ,masters The beginning of the dictionary is based on what is loaded Sentinel Configuration file to do .

5. Create a network connection to the primary server

Sentinel Two... Connected to the primary server will be created Asynchronous network connection ： One is command connection , Dedicated to sending commands to the primary server , And receive command reply , One is subscription connection , For subscribing to the master server _sentinel_:hello channel

Sentinel You need to create multiple network connections with multiple instances , therefore Sentinel Asynchronous connection is used .

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