Redis主从复制

Redis 是如何基于状态机的设计思路来实现主从复制的?

(1) Redis主从复制

从原理上来说,Redis 的主从复制主要包括了全量复制、增量复制和长连接同步三种情况。

  1. 全量复制传输 RDB 文件;
  2. 增量复制传输主从断连期间的命令;
  3. 长连接同步则是把主节点正常收到的请求传输给从节点。

(2) 为什么需要主从复制

为了提高服务的分区容错性,一般都会通过冗余来提高分区容错性。
主从复制技术类似于冗余,是提高分区容错性的一种办法。

在使用 Redis 或 MySQL 数据库时,经常会使用主从复制来实现主从节点间的数据同步,以此提升服务的高可用性。

(3) Redis主从复制原理

  1. 从库设置主库
  2. 主从建立连接
  3. 主从握手并判断复制类型
  4. 复制类型判断与执行

(3.1) 从库设置主库

主要是获得了主库的 IP 和端口号。
可以用三种方式来设置。
方式一:在实例 A 上执行 replicaof masterip masterport 的主从复制命令,指明实例 B 的 IP(masterip)和端口号(masterport)。
方式二:在实例 A 的配置文件中设置 replicaof masterip masterport,实例 A 可以通过解析文件获得主库 IP 和端口号。
方式三:在实例 A 启动时,设置启动参数–replicaof [masterip] [masterport]。实例 A 解析启动参数,就能获得主库的 IP 和端口号。

(3.2) 主从建立连接

从库获得了主库的IP和端口号,就会尝试和主库建立TCP网络连接,并且会在建立好的网络连接上,监听是否有主库发送的命令。

(3.3) 主从握手

当从库和主库建立好连接之后,从库就开始和主库进行握手。
简单来说,握手过程就是主从库间相互发送PING-PONG消息,同时从库根据配置信息向主库进行验证。
从库把自己的 IP、端口号,以及对无盘复制和 PSYNC 2 协议的支持情况发给主库。

(3.4) 复制类型判断与执行

等到主从库之间的握手完成后,从库就会给主库发送 PSYNC 命令。主库会根据从库发送的命令参数作出相应的三种回复,分别是执行全量复制、执行增量复制、发生错误。
最后,从库在收到上述回复后,就会根据回复的复制类型,开始执行具体的复制操作。


(4) 主从复制源码解读

主从复制中的状态机具体对应的是什么呢?

// file: src/server.h

/*
 */
struct redisServer {

   // ...

   /* 复制相关 */
    char *masterauth;         // 用于和主库进行验证的密码
    char *masterhost;         // 主库主机名
    int masterport;           // 主库端口号
    int repl_timeout;         //
    client *master;           // 从库上用来和主库连接的客户端
    client *cached_master;    // 从库上缓存的主库信息
    int repl_state;           // 从库的复制状态
    off_t repl_transfer_size;  
    off_t repl_transfer_read;  
    off_t repl_transfer_last_fsync_off;  
    connection *repl_transfer_s; 
    int repl_transfer_fd;     
    char *repl_transfer_tmpfile;  
    time_t repl_transfer_lastio;  
    int repl_serve_stale_data;  
    int repl_slave_ro;           
    int repl_slave_ignore_maxmemory;   
    time_t repl_down_since;  
    int repl_disable_tcp_nodelay;    
    int slave_priority;              
    int slave_announce_port;         
    char *slave_announce_ip;         
    // ...

}

(4.1) 从库设置主库

// file: src/server.c

void initServerConfig(void) {

   // 初始化复制状态  默认为没有
   server.repl_state = REPL_STATE_NONE;
   
}

实例执行了 replicaof masterip masterport 命令

// file: src/replication.c 

/*
 */
void replicaofCommand(client *c) {
    /* SLAVEOF is not allowed in cluster mode as replication is automatically
     * configured using the current address of the master node. */
    if (server.cluster_enabled) {
        addReplyError(c,"REPLICAOF not allowed in cluster mode.");
        return;
    }

    /* The special host/port combination "NO" "ONE" turns the instance
     * into a master. Otherwise the new master address is set. */
    if (!strcasecmp(c->argv[1]->ptr,"no") &&
        !strcasecmp(c->argv[2]->ptr,"one")) {
        if (server.masterhost) {
            replicationUnsetMaster();
            sds client = catClientInfoString(sdsempty(),c);
            serverLog(LL_NOTICE,"MASTER MODE enabled (user request from '%s')",
                client);
            sdsfree(client);
        }
    } else {
        long port;

        if (c->flags & CLIENT_SLAVE)
        {
            /* If a client is already a replica they cannot run this command,
             * because it involves flushing all replicas (including this
             * client) */
            addReplyError(c, "Command is not valid when client is a replica.");
            return;
        }

        if ((getLongFromObjectOrReply(c, c->argv[2], &port, NULL) != C_OK))
            return;

        /* Check if we are already attached to the specified slave */
        if (server.masterhost && !strcasecmp(server.masterhost,c->argv[1]->ptr)
            && server.masterport == port) {
            serverLog(LL_NOTICE,"REPLICAOF would result into synchronization "
                                "with the master we are already connected "
                                "with. No operation performed.");
            addReplySds(c,sdsnew("+OK Already connected to specified "
                                 "master\r\n"));
            return;
        }
        /* There was no previous master or the user specified a different one,
         * we can continue. */
        replicationSetMaster(c->argv[1]->ptr, port);
        sds client = catClientInfoString(sdsempty(),c);
        serverLog(LL_NOTICE,"REPLICAOF %s:%d enabled (user request from '%s')",
            server.masterhost, server.masterport, client);
        sdsfree(client);
    }
    addReply(c,shared.ok);
}
/* Set replication to the specified master address and port. */
void replicationSetMaster(char *ip, int port) {
    int was_master = server.masterhost == NULL;

    sdsfree(server.masterhost);
    server.masterhost = sdsnew(ip);
    server.masterport = port;
    if (server.master) {
        freeClient(server.master);
    }
    disconnectAllBlockedClients(); /* Clients blocked in master, now slave. */

    /* Update oom_score_adj */
    setOOMScoreAdj(-1);

    /* Force our slaves to resync with us as well. They may hopefully be able
     * to partially resync with us, but we can notify the replid change. */
    disconnectSlaves();
    cancelReplicationHandshake();
    /* Before destroying our master state, create a cached master using
     * our own parameters, to later PSYNC with the new master. */
    if (was_master) {
        replicationDiscardCachedMaster();
        replicationCacheMasterUsingMyself();
    }

    /* Fire the role change modules event. */
    moduleFireServerEvent(REDISMODULE_EVENT_REPLICATION_ROLE_CHANGED,
                          REDISMODULE_EVENT_REPLROLECHANGED_NOW_REPLICA,
                          NULL);

    /* Fire the master link modules event. */
    if (server.repl_state == REPL_STATE_CONNECTED)
        moduleFireServerEvent(REDISMODULE_EVENT_MASTER_LINK_CHANGE,
                              REDISMODULE_SUBEVENT_MASTER_LINK_DOWN,
                              NULL);

    server.repl_state = REPL_STATE_CONNECT;
}

replicationSetMaster 函数除了会记录主库的 IP、端口号之外,还会把从库实例的状态机设置为 REPL_STATE_CONNECT。
此时,主从复制的状态机会从 REPL_STATE_NONE 变迁为 REPL_STATE_CONNECT

(4.2) 主从建立连接

从库是何时开始和主库建立网络连接的呢?

replicationCron() 任务。这个任务的执行频率是每 1000ms 执行一次

replicationCron() 任务的函数实现逻辑是在 server.c 中,在该任务中,一个重要的判断就是,检查从库的复制状态机状态。如果状态机状态是 REPL_STATE_CONNECT,那么从库就开始和主库建立连接。连接的建立是通过调用 connectWithMaster() 函数来完成的。

/*
 *
 * @param *eventLoop
 * @param id
 * @param *clientData
 */
int serverCron(struct aeEventLoop *eventLoop, long long id, void *clientData) {

    /* Replication cron function -- used to reconnect to master,
     * detect transfer failures, start background RDB transfers and so forth. */
    run_with_period(1000) replicationCron();

}
// file: src/replication.c


/* --------------------------- REPLICATION CRON  ---------------------------- */

/* Replication cron function, called 1 time per second. */
void replicationCron(void) {
    static long long replication_cron_loops = 0;

    /* Non blocking connection timeout? */
    if (server.masterhost &&
        (server.repl_state == REPL_STATE_CONNECTING ||
         slaveIsInHandshakeState()) &&
         (time(NULL)-server.repl_transfer_lastio) > server.repl_timeout)
    {
        serverLog(LL_WARNING,"Timeout connecting to the MASTER...");
        cancelReplicationHandshake();
    }

    /* Bulk transfer I/O timeout? */
    if (server.masterhost && server.repl_state == REPL_STATE_TRANSFER &&
        (time(NULL)-server.repl_transfer_lastio) > server.repl_timeout)
    {
        serverLog(LL_WARNING,"Timeout receiving bulk data from MASTER... If the problem persists try to set the 'repl-timeout' parameter in redis.conf to a larger value.");
        cancelReplicationHandshake();
    }

    /* Timed out master when we are an already connected slave? */
    if (server.masterhost && server.repl_state == REPL_STATE_CONNECTED &&
        (time(NULL)-server.master->lastinteraction) > server.repl_timeout)
    {
        serverLog(LL_WARNING,"MASTER timeout: no data nor PING received...");
        freeClient(server.master);
    }

    /* Check if we should connect to a MASTER */
    if (server.repl_state == REPL_STATE_CONNECT) {
        serverLog(LL_NOTICE,"Connecting to MASTER %s:%d",
            server.masterhost, server.masterport);
        if (connectWithMaster() == C_OK) {
            serverLog(LL_NOTICE,"MASTER <-> REPLICA sync started");
        }
    }

    /* Send ACK to master from time to time.
     * Note that we do not send periodic acks to masters that don't
     * support PSYNC and replication offsets. */
    if (server.masterhost && server.master &&
        !(server.master->flags & CLIENT_PRE_PSYNC))
        replicationSendAck();

    /* If we have attached slaves, PING them from time to time.
     * So slaves can implement an explicit timeout to masters, and will
     * be able to detect a link disconnection even if the TCP connection
     * will not actually go down. */
    listIter li;
    listNode *ln;
    robj *ping_argv[1];

    /* First, send PING according to ping_slave_period. */
    if ((replication_cron_loops % server.repl_ping_slave_period) == 0 &&
        listLength(server.slaves))
    {
        /* Note that we don't send the PING if the clients are paused during
         * a Redis Cluster manual failover: the PING we send will otherwise
         * alter the replication offsets of master and slave, and will no longer
         * match the one stored into 'mf_master_offset' state. */
        int manual_failover_in_progress =
            server.cluster_enabled &&
            server.cluster->mf_end &&
            clientsArePaused();

        if (!manual_failover_in_progress) {
            ping_argv[0] = createStringObject("PING",4);
            replicationFeedSlaves(server.slaves, server.slaveseldb,
                ping_argv, 1);
            decrRefCount(ping_argv[0]);
        }
    }

    /* Second, send a newline to all the slaves in pre-synchronization
     * stage, that is, slaves waiting for the master to create the RDB file.
     *
     * Also send the a newline to all the chained slaves we have, if we lost
     * connection from our master, to keep the slaves aware that their
     * master is online. This is needed since sub-slaves only receive proxied
     * data from top-level masters, so there is no explicit pinging in order
     * to avoid altering the replication offsets. This special out of band
     * pings (newlines) can be sent, they will have no effect in the offset.
     *
     * The newline will be ignored by the slave but will refresh the
     * last interaction timer preventing a timeout. In this case we ignore the
     * ping period and refresh the connection once per second since certain
     * timeouts are set at a few seconds (example: PSYNC response). */
    listRewind(server.slaves,&li);
    while((ln = listNext(&li))) {
        client *slave = ln->value;

        int is_presync =
            (slave->replstate == SLAVE_STATE_WAIT_BGSAVE_START ||
            (slave->replstate == SLAVE_STATE_WAIT_BGSAVE_END &&
             server.rdb_child_type != RDB_CHILD_TYPE_SOCKET));

        if (is_presync) {
            connWrite(slave->conn, "\n", 1);
        }
    }

    /* Disconnect timedout slaves. */
    if (listLength(server.slaves)) {
        listIter li;
        listNode *ln;

        listRewind(server.slaves,&li);
        while((ln = listNext(&li))) {
            client *slave = ln->value;

            if (slave->replstate == SLAVE_STATE_ONLINE) {
                if (slave->flags & CLIENT_PRE_PSYNC)
                    continue;
                if ((server.unixtime - slave->repl_ack_time) > server.repl_timeout) {
                    serverLog(LL_WARNING, "Disconnecting timedout replica (streaming sync): %s",
                          replicationGetSlaveName(slave));
                    freeClient(slave);
                    continue;
                }
            }
            /* We consider disconnecting only diskless replicas because disk-based replicas aren't fed
             * by the fork child so if a disk-based replica is stuck it doesn't prevent the fork child
             * from terminating. */
            if (slave->replstate == SLAVE_STATE_WAIT_BGSAVE_END && server.rdb_child_type == RDB_CHILD_TYPE_SOCKET) {
                if (slave->repl_last_partial_write != 0 &&
                    (server.unixtime - slave->repl_last_partial_write) > server.repl_timeout)
                {
                    serverLog(LL_WARNING, "Disconnecting timedout replica (full sync): %s",
                          replicationGetSlaveName(slave));
                    freeClient(slave);
                    continue;
                }
            }
        }
    }

    /* If this is a master without attached slaves and there is a replication
     * backlog active, in order to reclaim memory we can free it after some
     * (configured) time. Note that this cannot be done for slaves: slaves
     * without sub-slaves attached should still accumulate data into the
     * backlog, in order to reply to PSYNC queries if they are turned into
     * masters after a failover. */
    if (listLength(server.slaves) == 0 && server.repl_backlog_time_limit &&
        server.repl_backlog && server.masterhost == NULL)
    {
        time_t idle = server.unixtime - server.repl_no_slaves_since;

        if (idle > server.repl_backlog_time_limit) {
            /* When we free the backlog, we always use a new
             * replication ID and clear the ID2. This is needed
             * because when there is no backlog, the master_repl_offset
             * is not updated, but we would still retain our replication
             * ID, leading to the following problem:
             *
             * 1. We are a master instance.
             * 2. Our slave is promoted to master. It's repl-id-2 will
             *    be the same as our repl-id.
             * 3. We, yet as master, receive some updates, that will not
             *    increment the master_repl_offset.
             * 4. Later we are turned into a slave, connect to the new
             *    master that will accept our PSYNC request by second
             *    replication ID, but there will be data inconsistency
             *    because we received writes. */
            changeReplicationId();
            clearReplicationId2();
            freeReplicationBacklog();
            serverLog(LL_NOTICE,
                "Replication backlog freed after %d seconds "
                "without connected replicas.",
                (int) server.repl_backlog_time_limit);
        }
    }

    /* If AOF is disabled and we no longer have attached slaves, we can
     * free our Replication Script Cache as there is no need to propagate
     * EVALSHA at all. */
    if (listLength(server.slaves) == 0 &&
        server.aof_state == AOF_OFF &&
        listLength(server.repl_scriptcache_fifo) != 0)
    {
        replicationScriptCacheFlush();
    }

    /* Start a BGSAVE good for replication if we have slaves in
     * WAIT_BGSAVE_START state.
     *
     * In case of diskless replication, we make sure to wait the specified
     * number of seconds (according to configuration) so that other slaves
     * have the time to arrive before we start streaming. */
    if (!hasActiveChildProcess()) {
        time_t idle, max_idle = 0;
        int slaves_waiting = 0;
        int mincapa = -1;
        listNode *ln;
        listIter li;

        listRewind(server.slaves,&li);
        while((ln = listNext(&li))) {
            client *slave = ln->value;
            if (slave->replstate == SLAVE_STATE_WAIT_BGSAVE_START) {
                idle = server.unixtime - slave->lastinteraction;
                if (idle > max_idle) max_idle = idle;
                slaves_waiting++;
                mincapa = (mincapa == -1) ? slave->slave_capa :
                                            (mincapa & slave->slave_capa);
            }
        }

        if (slaves_waiting &&
            (!server.repl_diskless_sync ||
             max_idle > server.repl_diskless_sync_delay))
        {
            /* Start the BGSAVE. The called function may start a
             * BGSAVE with socket target or disk target depending on the
             * configuration and slaves capabilities. */
            startBgsaveForReplication(mincapa);
        }
    }

    /* Remove the RDB file used for replication if Redis is not running
     * with any persistence. */
    removeRDBUsedToSyncReplicas();

    /* Refresh the number of slaves with lag <= min-slaves-max-lag. */
    refreshGoodSlavesCount();
    replication_cron_loops++; /* Incremented with frequency 1 HZ. */
}
/*
 * 
 */
int connectWithMaster(void) {

    // 
    server.repl_transfer_s = server.tls_replication ? connCreateTLS() : connCreateSocket();

    // 
    if (connConnect(server.repl_transfer_s, server.masterhost, server.masterport,
                NET_FIRST_BIND_ADDR, syncWithMaster) == C_ERR) {
        serverLog(LL_WARNING,"Unable to connect to MASTER: %s",
                connGetLastError(server.repl_transfer_s));
        connClose(server.repl_transfer_s);
        server.repl_transfer_s = NULL;
        return C_ERR;
    }


    //  
    server.repl_transfer_lastio = server.unixtime;
    // 连接建立后,将状态机设置为REPL_STATE_CONNECTING
    server.repl_state = REPL_STATE_CONNECTING;
    return C_OK;
}

(4.3) 主从握手并判断复制类型

从库建立TCP连接后,从库实例其实并没有立即开始进行数据同步,而是会先和主库之间进行握手通信。

握手通信的目的,主要包括从库和主库进行验证,以及从库将自身的 IP 和端口号发给主库。

(4.4) 复制类型判断与执行

/* Try a partial resynchronization with the master if we are about to reconnect.
 * If there is no cached master structure, at least try to issue a
 * "PSYNC ? -1" command in order to trigger a full resync using the PSYNC
 * command in order to obtain the master replid and the master replication
 * global offset.
 *
 * This function is designed to be called from syncWithMaster(), so the
 * following assumptions are made:
 *
 * 1) We pass the function an already connected socket "fd".
 * 2) This function does not close the file descriptor "fd". However in case
 *    of successful partial resynchronization, the function will reuse
 *    'fd' as file descriptor of the server.master client structure.
 *
 * The function is split in two halves: if read_reply is 0, the function
 * writes the PSYNC command on the socket, and a new function call is
 * needed, with read_reply set to 1, in order to read the reply of the
 * command. This is useful in order to support non blocking operations, so
 * that we write, return into the event loop, and read when there are data.
 *
 * When read_reply is 0 the function returns PSYNC_WRITE_ERR if there
 * was a write error, or PSYNC_WAIT_REPLY to signal we need another call
 * with read_reply set to 1. However even when read_reply is set to 1
 * the function may return PSYNC_WAIT_REPLY again to signal there were
 * insufficient data to read to complete its work. We should re-enter
 * into the event loop and wait in such a case.
 *
 * The function returns:
 *
 * PSYNC_CONTINUE: If the PSYNC command succeeded and we can continue.
 * PSYNC_FULLRESYNC: If PSYNC is supported but a full resync is needed.
 *                   In this case the master replid and global replication
 *                   offset is saved.
 * PSYNC_NOT_SUPPORTED: If the server does not understand PSYNC at all and
 *                      the caller should fall back to SYNC.
 * PSYNC_WRITE_ERROR: There was an error writing the command to the socket.
 * PSYNC_WAIT_REPLY: Call again the function with read_reply set to 1.
 * PSYNC_TRY_LATER: Master is currently in a transient error condition.
 *
 * Notable side effects:
 *
 * 1) As a side effect of the function call the function removes the readable
 *    event handler from "fd", unless the return value is PSYNC_WAIT_REPLY.
 * 2) server.master_initial_offset is set to the right value according
 *    to the master reply. This will be used to populate the 'server.master'
 *    structure replication offset.
 */

#define PSYNC_WRITE_ERROR 0
#define PSYNC_WAIT_REPLY 1
#define PSYNC_CONTINUE 2
#define PSYNC_FULLRESYNC 3
#define PSYNC_NOT_SUPPORTED 4
#define PSYNC_TRY_LATER 5

/*
 */
int slaveTryPartialResynchronization(connection *conn, int read_reply) {
    char *psync_replid;
    char psync_offset[32];
    sds reply;

    /* Writing half */
    if (!read_reply) {
        /* Initially set master_initial_offset to -1 to mark the current
         * master replid and offset as not valid. Later if we'll be able to do
         * a FULL resync using the PSYNC command we'll set the offset at the
         * right value, so that this information will be propagated to the
         * client structure representing the master into server.master. */
        server.master_initial_offset = -1;

        if (server.cached_master) {
            psync_replid = server.cached_master->replid;
            snprintf(psync_offset,sizeof(psync_offset),"%lld", server.cached_master->reploff+1);
            serverLog(LL_NOTICE,"Trying a partial resynchronization (request %s:%s).", psync_replid, psync_offset);
        } else {
            serverLog(LL_NOTICE,"Partial resynchronization not possible (no cached master)");
            psync_replid = "?";
            memcpy(psync_offset,"-1",3);
        }

        /* Issue the PSYNC command */
        reply = sendSynchronousCommand(SYNC_CMD_WRITE,conn,"PSYNC",psync_replid,psync_offset,NULL);
        if (reply != NULL) {
            serverLog(LL_WARNING,"Unable to send PSYNC to master: %s",reply);
            sdsfree(reply);
            connSetReadHandler(conn, NULL);
            return PSYNC_WRITE_ERROR;
        }
        return PSYNC_WAIT_REPLY;
    }

    /* Reading half */
    reply = sendSynchronousCommand(SYNC_CMD_READ,conn,NULL);
    if (sdslen(reply) == 0) {
        /* The master may send empty newlines after it receives PSYNC
         * and before to reply, just to keep the connection alive. */
        sdsfree(reply);
        return PSYNC_WAIT_REPLY;
    }

    connSetReadHandler(conn, NULL);

    if (!strncmp(reply,"+FULLRESYNC",11)) {
        char *replid = NULL, *offset = NULL;

        /* FULL RESYNC, parse the reply in order to extract the replid
         * and the replication offset. */
        replid = strchr(reply,' ');
        if (replid) {
            replid++;
            offset = strchr(replid,' ');
            if (offset) offset++;
        }
        if (!replid || !offset || (offset-replid-1) != CONFIG_RUN_ID_SIZE) {
            serverLog(LL_WARNING,
                "Master replied with wrong +FULLRESYNC syntax.");
            /* This is an unexpected condition, actually the +FULLRESYNC
             * reply means that the master supports PSYNC, but the reply
             * format seems wrong. To stay safe we blank the master
             * replid to make sure next PSYNCs will fail. */
            memset(server.master_replid,0,CONFIG_RUN_ID_SIZE+1);
        } else {
            memcpy(server.master_replid, replid, offset-replid-1);
            server.master_replid[CONFIG_RUN_ID_SIZE] = '\0';
            server.master_initial_offset = strtoll(offset,NULL,10);
            serverLog(LL_NOTICE,"Full resync from master: %s:%lld",
                server.master_replid,
                server.master_initial_offset);
        }
        /* We are going to full resync, discard the cached master structure. */
        replicationDiscardCachedMaster();
        sdsfree(reply);
        return PSYNC_FULLRESYNC;
    }

    if (!strncmp(reply,"+CONTINUE",9)) {
        /* Partial resync was accepted. */
        serverLog(LL_NOTICE,
            "Successful partial resynchronization with master.");

        /* Check the new replication ID advertised by the master. If it
         * changed, we need to set the new ID as primary ID, and set or
         * secondary ID as the old master ID up to the current offset, so
         * that our sub-slaves will be able to PSYNC with us after a
         * disconnection. */
        char *start = reply+10;
        char *end = reply+9;
        while(end[0] != '\r' && end[0] != '\n' && end[0] != '\0') end++;
        if (end-start == CONFIG_RUN_ID_SIZE) {
            char new[CONFIG_RUN_ID_SIZE+1];
            memcpy(new,start,CONFIG_RUN_ID_SIZE);
            new[CONFIG_RUN_ID_SIZE] = '\0';

            if (strcmp(new,server.cached_master->replid)) {
                /* Master ID changed. */
                serverLog(LL_WARNING,"Master replication ID changed to %s",new);

                /* Set the old ID as our ID2, up to the current offset+1. */
                memcpy(server.replid2,server.cached_master->replid,
                    sizeof(server.replid2));
                server.second_replid_offset = server.master_repl_offset+1;

                /* Update the cached master ID and our own primary ID to the
                 * new one. */
                memcpy(server.replid,new,sizeof(server.replid));
                memcpy(server.cached_master->replid,new,sizeof(server.replid));

                /* Disconnect all the sub-slaves: they need to be notified. */
                disconnectSlaves();
            }
        }

        /* Setup the replication to continue. */
        sdsfree(reply);
        replicationResurrectCachedMaster(conn);

        /* If this instance was restarted and we read the metadata to
         * PSYNC from the persistence file, our replication backlog could
         * be still not initialized. Create it. */
        if (server.repl_backlog == NULL) createReplicationBacklog();
        return PSYNC_CONTINUE;
    }

    /* If we reach this point we received either an error (since the master does
     * not understand PSYNC or because it is in a special state and cannot
     * serve our request), or an unexpected reply from the master.
     *
     * Return PSYNC_NOT_SUPPORTED on errors we don't understand, otherwise
     * return PSYNC_TRY_LATER if we believe this is a transient error. */

    if (!strncmp(reply,"-NOMASTERLINK",13) ||
        !strncmp(reply,"-LOADING",8))
    {
        serverLog(LL_NOTICE,
            "Master is currently unable to PSYNC "
            "but should be in the future: %s", reply);
        sdsfree(reply);
        return PSYNC_TRY_LATER;
    }

    if (strncmp(reply,"-ERR",4)) {
        /* If it's not an error, log the unexpected event. */
        serverLog(LL_WARNING,
            "Unexpected reply to PSYNC from master: %s", reply);
    } else {
        serverLog(LL_NOTICE,
            "Master does not support PSYNC or is in "
            "error state (reply: %s)", reply);
    }
    sdsfree(reply);
    replicationDiscardCachedMaster();
    return PSYNC_NOT_SUPPORTED;
}

(5) 参考资料

https://weikeqin.com/tags/redis/

Redis源码剖析与实战 学习笔记 Day21 主从复制:基于状态机的设计与实现 https://time.geekbang.org/col...

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