一文带你探究Sentinel的独特初始化

摘要：本系列通过作者对Redis Sentinel源码的理解，详细说明Sentinel的代码实现方式。

Redis Sentinel 是Redis提供的高可用模型解决方案。Sentinel可以自动监测一个或多个Redis主备实例，并在主实例宕机的情况下自动实行主备倒换。本系列通过作者对Redis Sentinel源码的理解，详细说明Sentinel的代码实现方式。

Sentinel使用Redis内核相同的事件驱动代码框架， 但Sentinel有自己独特的初始化步骤。在这篇文章里，作者会介绍Sentinel与Redis服务器不同的初始化部分。

我们可以通过redis-sentinel <path-to-configfile> 或者 redis-server <path-to-configfile> --sentinel 这两种方式启动并运行Sentinel实例，这两种方式是等价的。在Redis server.c 的main函数中，我们会看到Redis如何判断用户指定以Sentinel方式运行的逻辑：

int main(int argc, char **argv) {
 ..........
 server.sentinel_mode = checkForSentinelMode(argc,argv);
 ..........
} 

其中checkForSentinelMode函数会监测以下两种条件：

1. 程序使用redis-sentinel可执行文件执行。

2. 程序参数列表中有--sentinel 标志。

以上任何一种条件成立则Redis会使用Sentinel的方式运行。

/* Returns 1 if there is --sentinel among the arguments or if 

 * argv[0] contains "redis-sentinel". */ 

int checkForSentinelMode(int argc, char **argv) { 

      int j; 

       if (strstr(argv[0],"redis-sentinel") != NULL) return 1; 

       for (j = 1; j < argc; j++)
       if (!strcmp(argv[j],"--sentinel")) return 1; 

   return 0; 

}

在Redis 判断是否以Sentinel的方式运行以后，我们会看到如下代码段：

int main(int argc, char **argv) {
    struct timeval tv;
    int j; 

    ............
/* We need to init sentinel right now as parsing the configuration file
    * in sentinel mode will have the effect of populating the sentinel
    * data structures with master nodes to monitor. */
   if (server.sentinel_mode) {
       initSentinelConfig();
       initSentinel();
 }
 ............

在initSentinelConfig函数中，会使用Sentinel特定的端口（默认为26379）来替代Redis的默认端口（6379）。另外，在Sentinel模式下，需要禁用服务器运行保护模式。

/* This function overwrites a few normal Redis config default with Sentinel
 * specific defaults. */
void initSentinelConfig(void) {
 server.port = REDIS_SENTINEL_PORT;
 server.protected_mode = 0; /* Sentinel must be exposed. */
}

与此同时，initSentinel函数会做如下操作：

/* Perform the Sentinel mode initialization. */
void initSentinel(void) {
 unsigned int j; 

 /* Remove usual Redis commands from the command table, then just add
 * the SENTINEL command. */
 dictEmpty(server.commands,NULL);
 for (j = 0; j < sizeof(sentinelcmds)/sizeof(sentinelcmds[0]); j++) {
 int retval;
 struct redisCommand *cmd = sentinelcmds+j; 

 retval = dictAdd(server.commands, sdsnew(cmd->name), cmd);
 serverAssert(retval == DICT_OK); 

 {"punsubscribe",punsubscribeCommand,-1,"",0,NULL,0,0,0,0,0},
 {"publish",sentinelPublishCommand,3,"",0,NULL,0,0,0,0,0},
 {"info",sentinelInfoCommand,-1,"",0,NULL,0,0,0,0,0},
 {"role",sentinelRoleCommand,1,"ok-loading",0,NULL,0,0,0,0,0},
 {"client",clientCommand,-2,"read-only no-script",0,NULL,0,0,0,0,0},
 {"shutdown",shutdownCommand,-1,"",0,NULL,0,0,0,0,0},
 {"auth",authCommand,2,"no-auth no-script ok-loading ok-stale fast",0,NULL,0,0,0,0,0},
 {"hello",helloCommand,-2,"no-auth no-script fast",0,NULL,0,0,0,0,0}
};

2.初始化Sentinel主状态结构，Sentinel主状态的定义及注释如下。

/* Main state. */ 

struct sentinelState { 

 char myid[CONFIG_RUN_ID_SIZE+1]; /* This sentinel ID. */ 

 uint64_t current_epoch; /* Current epoch. */ 

 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; /* Last time we ran the time handler. */ 

 list *scripts_queue; /* Queue of user scripts to execute. */ 

 char *announce_ip; /* IP addr that is gossiped to other sentinels if 

 not NULL. */ 

 int announce_port; /* Port that is gossiped to other sentinels if 

 non zero. */ 

  unsigned long simfailure_flags; /* Failures simulation. */ 

 int deny_scripts_reconfig; /* Allow SENTINEL SET ... to change script 

 paths at runtime? */ 

} sentinel; 

其中masters字典指针中的每个值都对应着一个Sentinel检测的主实例。

在读取配置信息后，Redis服务器主函数会调用sentinelIsRunning函数， 做以下几个工作：

1. 检查配置文件是否被设置，并且检查程序对配置文件是否有写权限，因为如果Sentinel状态改变的话，会不断将自己当前状态记录在配置文件中。

2. 如果在配置文件中指定运行ID，Sentinel 会使用这个ID作为运行ID，相反地，如果没有指定运行ID，Sentinel会生成一个ID用来作为Sentinel的运行ID。

3. 对所有的Sentinel监测实例产生初始监测事件。

/* This function gets called when the server is in Sentinel mode, started,
 * loaded the configuration, and is ready for normal operations. */
void sentinelIsRunning(void) {
 int j; 

 if (server.configfile == NULL) {
 serverLog(LL_WARNING,
 "Sentinel started without a config file. Exiting...");
 exit(1);
 } else if (access(server.configfile,W_OK) == -1) {
 serverLog(LL_WARNING,
 "Sentinel config file %s is not writable: %s. Exiting...",
 server.configfile,strerror(errno));
 exit(1);
 } 

 /* If this Sentinel has yet no ID set in the configuration file, we
 * pick a random one and persist the config on disk. From now on this
 * will be this Sentinel ID across restarts. */
 for (j = 0; j < CONFIG_RUN_ID_SIZE; j++)
 if (sentinel.myid[j] != 0) break; 

 if (j == CONFIG_RUN_ID_SIZE) {
 /* Pick ID and persist the config. */
 getRandomHexChars(sentinel.myid,CONFIG_RUN_ID_SIZE);
 sentinelFlushConfig();
 } 

/* Log its ID to make debugging of issues simpler. */
 serverLog(LL_WARNING,"Sentinel ID is %s", sentinel.myid); 

 /* We want to generate a +monitor event for every configured master
 * at startup. */
 sentinelGenerateInitialMonitorEvents();
}

参考资料：

https://github.com/antirez/redis

https://redis.io/topics/sentinel

Redis设计与实现第二版 黄健宏著

本文分享自华为云社区《Redis Sentinel 源码分析（1）Sentinel的初始化》，原文作者：中间件小哥 。

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