Acting half：

一、進入故障轉移狀態之前：

 void sentinelHandleRedisInstance(sentinelRedisInstance *ri) {
    // ...
    // ...
    /* ============== ACTING HALF ============= */
    /* We don't proceed with the acting half if we are in TILT mode.
     * TILT happens when we find something odd with the time, like a
     * sudden change in the clock. */
    if (sentinel.tilt) {
        if (mstime()-sentinel.tilt_start_time < SENTINEL_TILT_PERIOD) return;
        /* 如果30秒內一切正常，退出Tilt模式 */
        sentinel.tilt = 0;
        sentinelEvent(REDIS_WARNING,"-tilt",NULL,"#tilt mode exited");
    }

    /* Every kind of instance */
    sentinelCheckSubjectivelyDown(ri);

    /* Masters and slaves */
    if (ri->flags & (SRI_MASTER|SRI_SLAVE)) {
        /* Nothing so far. */
    }

   /* Only masters */
    if (ri->flags & SRI_MASTER) {
        /* 由於訊息的收發都是非同步的，所以這裡不一定可以直接判斷出結果，
         * 所以本sentinel會在一次次定時器呼叫中判斷是否需要客觀下線 */
        sentinelCheckObjectivelyDown(ri);
        /* 判斷是否進行故障轉移，如果進行故障轉移，master->failover_state的變化將觸發一次命令的傳送
         * 即本機sentinel要求其他sentinel選舉本機為leader*/
        if (sentinelStartFailoverIfNeeded(ri))
             /* 這裡的呼叫是本機要求選舉自己做領頭Sentinel，這裡只會觸發一次 */
            sentinelAskMasterStateToOtherSentinels(ri,SENTINEL_ASK_FORCED);
        sentinelFailoverStateMachine(ri);
        /* 這裡傳送的訊息應該不固定，可能是詢問客觀下線狀態，
         * 也可能是要求選舉（如果初始選舉沒有產生leader，
         * 這一行會不斷觸發，直至產生leader） */
        sentinelAskMasterStateToOtherSentinels(ri,SENTINEL_NO_FLAGS);
    }
}
 

• L16：檢測主觀下線狀態；
• L27：檢測客觀下線狀態，注意 L24： Only masters；
• L32、L37： 第二節分析 sentinelAskMasterStateToOtherSentinels；
• L30：第三節分析進入故障轉移階段的條件；
• L33：進入故障轉移階段，下一篇文章分析故障轉移狀態機；

二、傳送命令 sentinelAskMasterStateToOtherSentinels：

​ 這個函式很特殊，所以專門用一節講它。首先先說明這個函式會發送命令：

​ SENTINEL IS-MASTER-DOWN-BY-ADDR <ip> <port> <current_epoch> <runid>

而根據runid引數不同，會執行兩個不同功能：

• 第一個功能：runid為'*'。本機sentinel認為當前master已主觀下線，需要詢問其他sentinels是否認為該master下線（可能主觀，也可能客觀，因為不同Sentinel判定標準不同 //todo分析）;
• 第二個功能：runid為本機sentinel的runid。本機sentinel已經認定當前master客觀下線，要求其他sentinels選舉自己為領頭Sentinel；

void sentinelAskMasterStateToOtherSentinels(sentinelRedisInstance *master, int flags) {
    dictIterator *di;
    dictEntry *de;

    di = dictGetIterator(master->sentinels);
    while((de = dictNext(di)) != NULL) {
        sentinelRedisInstance *ri = dictGetVal(de);
        mstime_t elapsed = mstime() - ri->last_master_down_reply_time;
        char port[32];
        int retval;

        /* If the master state from other sentinel is too old, we clear it. */
        if (elapsed > SENTINEL_ASK_PERIOD*5) {
            ri->flags &= ~SRI_MASTER_DOWN;
            sdsfree(ri->leader);
            ri->leader = NULL;
        }

        /* Only ask if master is down to other sentinels if:
         *
         * 1) We believe it is down, or there is a failover in progress.
         * 2) Sentinel is connected.
         * 3) We did not received the info within SENTINEL_ASK_PERIOD ms. */
        if ((master->flags & SRI_S_DOWN) == 0) continue;
        if (ri->flags & SRI_DISCONNECTED) continue;
        if (!(flags & SENTINEL_ASK_FORCED) &&
            mstime() - ri->last_master_down_reply_time < SENTINEL_ASK_PERIOD)
            continue;

        /* Ask */
        ll2string(port,sizeof(port),master->addr->port);
        retval = redisAsyncCommand(ri->cc,
                    /**/
                    sentinelReceiveIsMasterDownReply, NULL,
                    "SENTINEL is-master-down-by-addr %s %s %llu %s",
                    master->addr->ip, port,
                    sentinel.current_epoch,
                    /* 根據master當前failover_state判斷這是sentinel在進行判斷是否客觀下線還是要選舉該sentinel為leader */
                    (master->failover_state > SENTINEL_FAILOVER_STATE_NONE) ?
                    server.runid : "*");
        if (retval == REDIS_OK) ri->pending_commands++;
    }
    dictReleaseIterator(di);
}

 

• 看一下官方註釋

  If we think the master is down, we start sending SENTINEL IS-MASTER-DOWN-BY-ADDR requests(1) to other sentinels in order to get the replies(2) that allow to reach the quorum needed to mark(3) the master in ODOWN state and trigger(4) a failover

• (1)：該函式第一個功能；

  • (2)：L34： 回撥函式sentinelReceiveIsMasterDownReply接收回復，根據回覆會設定ri->flags & SRI_MASTER_DOWN；
  • (3)：ri->flags & SRI_MASTER_DOWN會在sentinelCheckObjectivelyDown函式中使quorum++；
  • (4)：如果客觀下線要求達到，那麼第一節的sentinelStartFailoverIfNeeded會進入故障轉移階段，即trigger a failover;

• L34：第三節分析傳送SENTINEL is-master-down-by-addr命令的回撥函式sentinelReceiveIsMasterDownReply；

三、進入故障轉移的條件：

int sentinelStartFailoverIfNeeded(sentinelRedisInstance *master) {
    /* We can't failover if the master is not in O_DOWN state. */
    if (!(master->flags & SRI_O_DOWN)) return 0;

    /* Failover already in progress? */
    if (master->flags & SRI_FAILOVER_IN_PROGRESS) return 0;

    /* Last failover attempt started too little time ago? */
    if (mstime() - master->failover_start_time <
        master->failover_timeout*2) return 0;
    /* 進行故障轉移 */
    sentinelStartFailover(master);
    return 1;
}

• L3：主伺服器必須已經被判定客觀下線才會進入故障轉移；
• L6：主伺服器不能已經在故障轉移狀態中；
• L12： 進行故障轉移，下面繼續分析原始碼：

void sentinelStartFailover(sentinelRedisInstance *master) {
    redisAssert(master->flags & SRI_MASTER);
    /* 設定FAILOVER_STATE_WAIT_START狀態，該狀態會在 
     * sentinelAskMasterStateToOtherSentinels 方法中
     * 允許本sentinel要求其他Sentinel選舉自己為leader */
    master->failover_state = SENTINEL_FAILOVER_STATE_WAIT_START;
    master->flags |= SRI_FAILOVER_IN_PROGRESS;
    /* 回合數++ */
    master->failover_epoch = ++sentinel.current_epoch;
    sentinelEvent(REDIS_WARNING,"+new-epoch",master,"%llu",
        (unsigned long long) sentinel.current_epoch);
    sentinelEvent(REDIS_WARNING,"+try-failover",master,"%@");
    /* 維護time資訊 */
    master->failover_start_time = mstime();
    master->failover_state_change_time = mstime();
}

• L6、L7：master的failover_state和flags都被更新，HandleInstance程式下一次進入故障轉移狀態機函式時，邏輯才進入故障轉移；

• L9：回合數++，可能不是第一個回合；