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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：
- 程序应用 redis-sentinel 可执行文件执行。
- 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：
- 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。
- 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。
- 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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