一、Monitor half：

void sentinelHandleRedisInstance(sentinelRedisInstance *ri) {
    /* ========== MONITORING HALF ============ */
    /* Every kind of instance */
    /* 建立命令连接(cc)，和订阅连接(pc) */
    sentinelReconnectInstance(ri);
    /* 对实例进行定期操作：INFO（10s | 1s）, PUBLISH (2s), PING(1s) */
    sentinelSendPeriodicCommands(ri);

    /* ============== ACTING HALF ============= */
    // ...
    // ...
}

• L5：建立连接过程中使用了hiredis的异步通信API[1]，以后补充。
• 建立订阅连接会执行SUBSCRIBE命令，设置回调sentinelReceiveHelloMessages，这个函数会通过频道接收其他同样监视该服务器（主从？）的Sentinel更新服务器状态，如果是新的Sentinel，会被新建并添加到该服务器实例结构的sentinels字典中。
• L7： 第二节分析一下周期操作sentinelSendPeriodicCommands；

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二、周期操作sentinelSendPeriodicCommands：

void sentinelSendPeriodicCommands(sentinelRedisInstance *ri) {
    mstime_t now = mstime();
    mstime_t info_period, ping_period;
    int retval;

    /* Return ASAP if we have already a PING or INFO already pending, or
     * in the case the instance is not properly connected. */
    if (ri->flags & SRI_DISCONNECTED) return;

    /* For INFO, PING, PUBLISH that are not critical commands to send we
     * also have a limit of SENTINEL_MAX_PENDING_COMMANDS. We don't
     * want to use a lot of memory just because a link is not working
     * properly (note that anyway there is a redundant protection about this,
     * that is, the link will be disconnected and reconnected if a long
     * timeout condition is detected. */
    if (ri->pending_commands >= SENTINEL_MAX_PENDING_COMMANDS) return;

    /* If this is a slave of a master in O_DOWN condition we start sending
     * it INFO every second, instead of the usual SENTINEL_INFO_PERIOD
     * period. In this state we want to closely monitor slaves in case they
     * are turned into masters by another Sentinel, or by the sysadmin. */
    if ((ri->flags & SRI_SLAVE) &&
        (ri->master->flags & (SRI_O_DOWN|SRI_FAILOVER_IN_PROGRESS))) {
        /* 当前实例是slave，而且它的master已客观下线并且故障转移正在进行中，那么本机sentinel 1s发一次INFO */
        info_period = 1000;
    } else {
        /* 正常情况下是10s发一次INFO */
        info_period = SENTINEL_INFO_PERIOD;
    }

    /* We ping instances every time the last received pong is older than
     * the configured 'down-after-milliseconds' time, but every second
     * anyway if 'down-after-milliseconds' is greater than 1 second. */
    ping_period = ri->down_after_period;
    if (ping_period > SENTINEL_PING_PERIOD) ping_period = SENTINEL_PING_PERIOD;

    if ((ri->flags & SRI_SENTINEL) == 0 &&
        (ri->info_refresh == 0 ||
        (now - ri->info_refresh) > info_period))
    {
        /* Send INFO to masters and slaves, not sentinels. */
        /* 实例不是sentinel，并且超过INFO周期，发送INFO */
        retval = redisAsyncCommand(ri->cc,
            sentinelInfoReplyCallback, NULL, "INFO");
        if (retval == REDIS_OK) ri->pending_commands++;
    } else if ((now - ri->last_pong_time) > ping_period) {
        /* Send PING to all the three kinds of instances. */
        sentinelSendPing(ri);
    } else if ((now - ri->last_pub_time) > SENTINEL_PUBLISH_PERIOD) {
        /* PUBLISH hello messages to all the three kinds of instances. */
        sentinelSendHello(ri);
    }
}

• L22~L25：注意发INFO周期的改变
• L43：发送INFO命令，回调为sentinelInfoReplyCallback；本机sentinel通过这个回调获取主服务器和从服务器的信息，故障转移的role变化也在这个函数执行，以后详解。
• L48：发送PING命令，回调函数只是更新time信息，不细说。
• L51：发送PUBLISH到_sentinel_:hello频道，回调函数只是更新time信息，不细说。

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参考：

1. Redis源码解析：19Hiredis异步API代码解析_程序员的自我修养-CSDN博客