前言

下面是我摘抄的有关主观下线和客观下线的介绍:

主观下线 SDOWN: 如果监控的服务器节点在 down-after-milliseconds
设置的毫秒时效内没有响应检测，则会被判定为主观下线；这个状态适用所有服务器节点
客观下线 ODOWN: 主服务器节点在发生故障的时候，sentinel 会通过 is-master-down-by-addr 命令向其他 sentinel 节点询问该主服务器节点的状态，如果超过 quorum 个数的哨兵节点都认为主服务器节点不可达，则判定为客观下线；这个状态只会针对主服务器节点(对于从服务器没有客观下线，从下面源码中能找到答案)

那客观下线与主观下线与sentinel 关系其实也是redis和sentinel的交互

每个 sentinel 以每秒一次的频率，向它所知的主服务器节点和从服务器节点以及其他 sentinel节点发送PING 命令，检测其状态
如果一个节点或者实例距离最后一次有效回复 PING 命令的时间超过 down-after-milliseconds设定的值，那么这个节点或者实例会被 sentinel 标记为 SDOWN 主观下线
随后正在监控这个主服务器节点的所有 sentinel 都要以每秒一次的频率确认这个主服务器节点是否真正进入了主观下线状态，一旦有足够数量的 sentinel 在指定时间范围内同意这个下线判断，那么这个服务器就要被标记为 ODOWN 客观下线，然后sentinel 向客观下线主服务器的所有从服务器发送 INFO 命令的频率，会从 10 秒一次改为每秒一次
当然，如果中途当没有足够数量的 sentinel 同意下线判断，那这个 客观下线 状态就会被移除，而且当主服务器重新对 PING进行有效响应的时候，那主观下线状态就会被移除
接上面，如果还是 客观下线状态，就需要进行 主服务器选举逻辑

sentinelCheckSubjectivelyDown(主观下线)

在sentinelHandleRedisInstance方法下有sentinelCheckSubjectivelyDown方法来进行判定，
主要的流程:如果监控的服务器节点在 down-after-milliseconds 设置的毫秒时效内没有响应检测，则会被判定为主观下线；这个状态适用所有服务器节点


/* Is this instance down from our point of view?
从主观的角度看这个实例是否是下线状态*/
void sentinelCheckSubjectivelyDown(sentinelRedisInstance *ri) {
    mstime_t elapsed = 0;
    // 获取命令响应已经过去的时长
    if (ri->link->act_ping_time)
        elapsed = mstime() - ri->link->act_ping_time;
    // 实例连接断开了
    else if (ri->link->disconnected)
        elapsed = mstime() - ri->link->last_avail_time;

    /* Check if we are in need for a reconnection of one of the
     * links, because we are detecting low activity.
     *低活跃度重新连接，即 连接超过了 1.5秒，并且之前发送了 PING 命令但是活跃度很低
     * 1) Check if the command link seems connected, was connected not less
     *    than SENTINEL_MIN_LINK_RECONNECT_PERIOD, but still we have a
     *    pending ping for more than half the timeout.
     检查命令链接是否似乎已连接，连接时间不少于 SENTINEL_MIN_LINK_RECONNECT_PERIOD，但我们仍有超过一半超时的待处理 ping*/
    if (ri->link->cc &&
        (mstime() - ri->link->cc_conn_time) >
        SENTINEL_MIN_LINK_RECONNECT_PERIOD &&
        ri->link->act_ping_time != 0 && /* There is a pending ping... */
        /* The pending ping is delayed, and we did not receive
         * error replies as well. */
        (mstime() - ri->link->act_ping_time) > (ri->down_after_period/2) &&
        (mstime() - ri->link->last_pong_time) > (ri->down_after_period/2))
    {
         // 断开实例的连接
        instanceLinkCloseConnection(ri->link,ri->link->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.
     */
      // 检查 订阅发布的连接是不是也是出于 低活跃度
    if (ri->link->pc &&
        (mstime() - ri->link->pc_conn_time) >
         SENTINEL_MIN_LINK_RECONNECT_PERIOD &&
        (mstime() - ri->link->pc_last_activity) > (SENTINEL_PUBLISH_PERIOD*3))
    {    // 断开实例连接
        instanceLinkCloseConnection(ri->link,ri->link->pc);
    }

    /* Update the SDOWN flag. We believe the instance is SDOWN if:
     *更新 SDOWN 标志。我们认为实例是 SDOWN 如果
     * 1) It is not replying. 它没有回复
     * 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. sentinel 认为是主节点但是 报告是从节点*/
    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 // 设置主观下线标识*/
        if ((ri->flags & SRI_S_DOWN) == 0) {
            sentinelEvent(LL_WARNING,"+sdown",ri,"%@");
            ri->s_down_since_time = mstime();
            ri->flags |= SRI_S_DOWN;
        }
    } else {
        /* Is subjectively up  // 如果已经是主观下线状态，则取消标识*/
        if (ri->flags & SRI_S_DOWN) {
            sentinelEvent(LL_WARNING,"-sdown",ri,"%@");
            ri->flags &= ~(SRI_S_DOWN|SRI_SCRIPT_KILL_SENT);
        }
    }
}

sentinelCheckObjectivelyDown（客观下线）

sentinelHandleRedisInstance方法下的sentinelCheckObjectivelyDown方法，这里需要注意的是，客观下线是针对主服务器节点才能做的状态标识，从服务器节点就不会有这个状态标识

/* Is this instance down according to the configured quorum?
 *
 * Note that ODOWN is a weak quorum, it only means that enough Sentinels
 * reported in a given time range that the instance was not reachable.
 * However messages can be delayed so there are no strong guarantees about
 * N instances agreeing at the same time about the down state.
 请注意，ODOWN 是一个弱仲裁，它仅表示在给定时间范围内报告了足够多的 Sentinel 报告该实例无法访问。但是消息可能会延迟，因此无法保证 N 个实例同时同意关闭状态*/
void sentinelCheckObjectivelyDown(sentinelRedisInstance *master) {
    dictIterator *di;
    dictEntry *de;
    unsigned int quorum = 0, odown = 0;
     // 如果已经被判断为主观下线
    if (master->flags & SRI_S_DOWN) {
        /* Is down for enough sentinels? */
        // 当前 哨兵认为下线，投票1
        quorum = 1; /* the current sentinel. */
        /* Count all the other sentinels. */
        di = dictGetIterator(master->sentinels);
        // 遍历所有监控该主节点的所有 sentinel
        while((de = dictNext(di)) != NULL) {
            sentinelRedisInstance *ri = dictGetVal(de);
             // 如果 sentinel 投票主观下线，投票 +1
            if (ri->flags & SRI_MASTER_DOWN) quorum++;
        }
        dictReleaseIterator(di);
         // 如果投票数够了，则认为 客观下线
        if (quorum >= master->quorum) odown = 1;
    }

    /* Set the flag accordingly to the outcome. */
     // 如果已经判定为 客观下线了
    if (odown) {
     // 如果主服务器节点还没改变状态，则修改其 客观下线状态
        if ((master->flags & SRI_O_DOWN) == 0) {
            sentinelEvent(LL_WARNING,"+odown",master,"%@ #quorum %d/%d",
                quorum, master->quorum);
            master->flags |= SRI_O_DOWN;
            master->o_down_since_time = mstime();
        }
    } else {
     // 如果票数不足，原本 客观下线就要改回来为 主观下线
        if (master->flags & SRI_O_DOWN) {
            sentinelEvent(LL_WARNING,"-odown",master,"%@");
            master->flags &= ~SRI_O_DOWN;
        }
    }
}