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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