Redis Sentinel源码分析（一）：Sentinel的初始化-InfoQ

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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);   ..........    }

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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;
}

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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();      }      ............

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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. */}

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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();      .............  }

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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}  };

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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;

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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();  }