1、检测主观下线状态：

void sentinelCheckSubjectivelyDown(sentinelRedisInstance *ri) {
    mstime_t elapsed = mstime() - ri->last_avail_time;

    /* Check if we are in need for a reconnection of one of the 
     * links, because we are detecting low activity.
     *
     * 1) Check if the command link seems connected, was connected not less
     *    than SENTINEL_MIN_LINK_RECONNECT_PERIOD, but still we have an
     *    idle time that is greater than down_after_period / 2 seconds. */
    if (ri->cc &&
        (mstime() - ri->cc_conn_time) > SENTINEL_MIN_LINK_RECONNECT_PERIOD &&
        (mstime() - ri->last_pong_time) > (ri->down_after_period/2))
    {
        sentinelKillLink(ri,ri->cc);
    }

    /* 2) Check if the pubsub link seems connected, was connected not less
     *    than SENTINEL_MIN_LINK_RECONNECT_PERIOD, but still we have no
     *    activity in the Pub/Sub channel for more than
     *    SENTINEL_PUBLISH_PERIOD * 3.
     */
    if (ri->pc &&
        (mstime() - ri->pc_conn_time) > SENTINEL_MIN_LINK_RECONNECT_PERIOD &&
        (mstime() - ri->pc_last_activity) > (SENTINEL_PUBLISH_PERIOD*3))
    {
        sentinelKillLink(ri,ri->pc);
    }

    /* Update the SDOWN flag. We believe the instance is SDOWN if:
     *
     * 1) It is not replying.
     * 2) We believe it is a master, it reports to be a slave for enough time
     *    to meet the down_after_period, plus enough time to get two times
     *    INFO report from the instance. */
    if (elapsed > ri->down_after_period ||
        (ri->flags & SRI_MASTER &&
         ri->role_reported == SRI_SLAVE &&
         mstime() - ri->role_reported_time >
          (ri->down_after_period+SENTINEL_INFO_PERIOD*2)))
    {
        /* Is subjectively down */
        if ((ri->flags & SRI_S_DOWN) == 0) {
            sentinelEvent(REDIS_WARNING,"+sdown",ri,"%@");
            ri->s_down_since_time = mstime();
            /* 打开主观下线标志 */
            ri->flags |= SRI_S_DOWN;
        }
    } else {
        /* Is subjectively up */
        if (ri->flags & SRI_S_DOWN) {
            sentinelEvent(REDIS_WARNING,"-sdown",ri,"%@");
            ri->flags &= ~(SRI_S_DOWN|SRI_SCRIPT_KILL_SENT);
        }
    }
}

• 主要是通过判断时间来判断主观下线的；
• L46：打开主观下线标志；

2、判断客观下线状态：

void sentinelCheckObjectivelyDown(sentinelRedisInstance *master) {
    dictIterator *di;
    dictEntry *de;
    int quorum = 0, odown = 0;

    if (master->flags & SRI_S_DOWN) {
        /* Is down for enough sentinels? */
        quorum = 1; /* the current sentinel. */
        /* Count all the other sentinels. */
        /* 遍历master的sentinels字典，判断他们是否认为该master主观下线 */
        di = dictGetIterator(master->sentinels);
        while((de = dictNext(di)) != NULL) {
            sentinelRedisInstance *ri = dictGetVal(de);

            if (ri->flags & SRI_MASTER_DOWN) quorum++;
        }
        dictReleaseIterator(di);
        /* 可以判断为客观下线 */
        if (quorum >= master->quorum) odown = 1;
    }

    /* Set the flag accordingly to the outcome. */
    if (odown) {
        if ((master->flags & SRI_O_DOWN) == 0) {
            sentinelEvent(REDIS_WARNING,"+odown",master,"%@ #quorum %d/%d",
                quorum, master->quorum);
            master->flags |= SRI_O_DOWN;
            master->o_down_since_time = mstime();
        }
    } else {
        if (master->flags & SRI_O_DOWN) {
            sentinelEvent(REDIS_WARNING,"-odown",master,"%@");
            master->flags &= ~SRI_O_DOWN;
        }
    }
}

• L1：参数是master，记住只有主服务器才需要被检查客观下线状态

• L10~L17：通过监视该master的所有Sentinel投票来决定 本机sentinel认定该master 客观下线；

  ​