摘要：本系列通过作者对Redis Sentinel源码的了解，具体阐明Sentinel的代码实现形式。

Redis Sentinel 是Redis提供的高可用模型解决方案。Sentinel能够主动监测一个或多个Redis主备实例，并在主实例宕机的状况下主动履行主备倒换。本系列通过作者对Redis Sentinel源码的了解，具体阐明Sentinel的代码实现形式。

Sentinel应用Redis内核雷同的事件驱动代码框架， 但Sentinel有本人独特的初始化步骤。在这篇文章里，作者会介绍Sentinel与Redis服务器不同的初始化局部。

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

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. 程序应用redis-sentinel可执行文件执行。
  2. 程序参数列表中有--sentinel 标记。

以上任何一种条件成立则Redis会应用Sentinel的形式运行。

/* 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的形式运行当前，咱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，会应用Sentinel特定的端口（默认为26379）来代替Redis的默认端口（6379）。另外，在Sentinel模式下，须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主状态构造，Sentinel主状态的定义及正文如下。

/* 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. 查看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状态扭转的话，会一直将本人以后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 会应用这个ID作为运行ID，相同地，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 源码剖析（1）Sentinel的初始化》，原文作者：中间件小哥 。

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