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 RedisSentinel 代码的理解，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 或者 redis-server --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：

程序使用 redis-sentinel 可执行文件执行。
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);

/* Translate the command string flags description into an actual 
         * set of flags. */
if (populateCommandTableParseFlags(cmd,cmd->sflags) == C_ERR)  
            serverPanic("Unsupported command flag");  
    }

/* Initialize various data structures. */
    sentinel.current_epoch = 0;  
    sentinel.masters = dictCreate(&instancesDictType,NULL);  
    sentinel.tilt = 0;  
    sentinel.tilt_start_time = 0;  
    sentinel.previous_time = mstime();  
    .............  
}

使用 Sentinel 自带的命令表去替代 Redis 服务器原生的命令. Sentinel 支持的命令表如下：

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},  
    {"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}  
};

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：

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。
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。
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();  
}

本文转载自中间件小哥公众号。
原文链接：https://mp.weixin.qq.com/s/GWe3eBGrFcOmM5M0QYXHkQ

创作场景

Redis Sentinel 源码分析（一）：Sentinel 的初始化