redis源碼分析:主從模式中從服務器同步策略

從服務器在與主服務器建立連接後向主服務器發送同步命令,要求進行同步。發送同步命令後,將相應的讀操作設爲readSyncBulkPayload。

void syncWithMaster(aeEventLoop *el, int fd, void *privdata, int mask) {
    char tmpfile[256], *err;
    int dfd, maxtries = 5;
    int sockerr = 0;
    socklen_t errlen = sizeof(sockerr);
    REDIS_NOTUSED(el);
    REDIS_NOTUSED(privdata);
    REDIS_NOTUSED(mask);

    /* If this event fired after the user turned the instance into a master
     * with SLAVEOF NO ONE we must just return ASAP. */
    if (server.repl_state == REDIS_REPL_NONE) {
        close(fd);
        return;
    }

    /* Check for errors in the socket. */
    if (getsockopt(fd, SOL_SOCKET, SO_ERROR, &sockerr, &errlen) == -1)
        sockerr = errno;
    if (sockerr) {
        aeDeleteFileEvent(server.el,fd,AE_READABLE|AE_WRITABLE);
        redisLog(REDIS_WARNING,"Error condition on socket for SYNC: %s",
            strerror(sockerr));
        goto error;
    }

    /* If we were connecting, it's time to send a non blocking PING, we want to
     * make sure the master is able to reply before going into the actual
     * replication process where we have long timeouts in the order of
     * seconds (in the meantime the slave would block). */
    if (server.repl_state == REDIS_REPL_CONNECTING) {
        redisLog(REDIS_NOTICE,"Non blocking connect for SYNC fired the event.");
        /* Delete the writable event so that the readable event remains
         * registered and we can wait for the PONG reply. */
        aeDeleteFileEvent(server.el,fd,AE_WRITABLE);
        server.repl_state = REDIS_REPL_RECEIVE_PONG;
        /* Send the PING, don't check for errors at all, we have the timeout
         * that will take care about this. */
        syncWrite(fd,"PING\r\n",6,100);
        return;
    }

    /* Receive the PONG command. */
    if (server.repl_state == REDIS_REPL_RECEIVE_PONG) {
        char buf[1024];

        /* Delete the readable event, we no longer need it now that there is
         * the PING reply to read. */
        aeDeleteFileEvent(server.el,fd,AE_READABLE);

        /* Read the reply with explicit timeout. */
        buf[0] = '\0';
        if (syncReadLine(fd,buf,sizeof(buf),
            server.repl_syncio_timeout*1000) == -1)
        {
            redisLog(REDIS_WARNING,
                "I/O error reading PING reply from master: %s",
                strerror(errno));
            goto error;
        }

        /* We don't care about the reply, it can be +PONG or an error since
         * the server requires AUTH. As long as it replies correctly, it's
         * fine from our point of view. */
        if (buf[0] != '-' && buf[0] != '+') {
            redisLog(REDIS_WARNING,"Unexpected reply to PING from master.");
            goto error;
        } else {
            redisLog(REDIS_NOTICE,
                "Master replied to PING, replication can continue...");
        }
    }

    /* AUTH with the master if required. */
    if(server.masterauth) {
        err = sendSynchronousCommand(fd,"AUTH",server.masterauth,NULL);
        if (err) {
            redisLog(REDIS_WARNING,"Unable to AUTH to MASTER: %s",err);
            sdsfree(err);
            goto error;
        }
    }

    /* Set the slave port, so that Master's INFO command can list the
     * slave listening port correctly. */
    {
        sds port = sdsfromlonglong(server.port);
        err = sendSynchronousCommand(fd,"REPLCONF","listening-port",port,
                                         NULL);
        sdsfree(port);
        /* Ignore the error if any, not all the Redis versions support
         * REPLCONF listening-port. */
        if (err) {
            redisLog(REDIS_NOTICE,"(non critical): Master does not understand REPLCONF listening-port: %s", err);
            sdsfree(err);
        }
    }

    /* Issue the SYNC command */
    if (syncWrite(fd,"SYNC\r\n",6,server.repl_syncio_timeout*1000) == -1) {
        redisLog(REDIS_WARNING,"I/O error writing to MASTER: %s",
            strerror(errno));
        goto error;
    }

    /* Prepare a suitable temp file for bulk transfer */
    while(maxtries--) {
        snprintf(tmpfile,256,
            "temp-%d.%ld.rdb",(int)server.unixtime,(long int)getpid());
        dfd = open(tmpfile,O_CREAT|O_WRONLY|O_EXCL,0644);
        if (dfd != -1) break;
        sleep(1);
    }
    if (dfd == -1) {
        redisLog(REDIS_WARNING,"Opening the temp file needed for MASTER <-> SLAVE synchronization: %s",strerror(errno));
        goto error;
    }

    /* Setup the non blocking download of the bulk file. */
    if (aeCreateFileEvent(server.el,fd, AE_READABLE,readSyncBulkPayload,NULL)
            == AE_ERR)
    {
        redisLog(REDIS_WARNING,"Can't create readable event for SYNC");
        goto error;
    }

    server.repl_state = REDIS_REPL_TRANSFER;
    server.repl_transfer_size = -1;
    server.repl_transfer_read = 0;
    server.repl_transfer_last_fsync_off = 0;
    server.repl_transfer_fd = dfd;
    server.repl_transfer_lastio = server.unixtime;
    server.repl_transfer_tmpfile = zstrdup(tmpfile);
    return;

error:
    close(fd);
    server.repl_transfer_s = -1;
    server.repl_state = REDIS_REPL_CONNECT;
    return;
}

readSyncBulkPayload函數負責從連接中讀取主服務器建立的同步文件,可分多次讀完所有同步數據。在數據超過8MB後,每次讀取操作都會進行寫磁盤操作,如果在最後才進行寫磁盤操作可能會造成極大的延遲。

/* Asynchronously read the SYNC payload we receive from a master */
#define REPL_MAX_WRITTEN_BEFORE_FSYNC (1024*1024*8) /* 8 MB */
void readSyncBulkPayload(aeEventLoop *el, int fd, void *privdata, int mask) {
    char buf[4096];
    ssize_t nread, readlen;
    off_t left;
    REDIS_NOTUSED(el);
    REDIS_NOTUSED(privdata);
    REDIS_NOTUSED(mask);

    /* If repl_transfer_size == -1 we still have to read the bulk length
     * from the master reply. */
    if (server.repl_transfer_size == -1) {
        if (syncReadLine(fd,buf,1024,server.repl_syncio_timeout*1000) == -1) {
            redisLog(REDIS_WARNING,
                "I/O error reading bulk count from MASTER: %s",
                strerror(errno));
            goto error;
        }

        if (buf[0] == '-') {
            redisLog(REDIS_WARNING,
                "MASTER aborted replication with an error: %s",
                buf+1);
            goto error;
        } else if (buf[0] == '\0') {
            /* At this stage just a newline works as a PING in order to take
             * the connection live. So we refresh our last interaction
             * timestamp. */
            server.repl_transfer_lastio = server.unixtime;
            return;
        } else if (buf[0] != '$') {
            redisLog(REDIS_WARNING,"Bad protocol from MASTER, the first byte is not '$', are you sure the host and port are right?");
            goto error;
        }
        server.repl_transfer_size = strtol(buf+1,NULL,10);
        redisLog(REDIS_NOTICE,
            "MASTER <-> SLAVE sync: receiving %ld bytes from master",
            server.repl_transfer_size);
        return;
    }

    /* Read bulk data */
    left = server.repl_transfer_size - server.repl_transfer_read;
    readlen = (left < (signed)sizeof(buf)) ? left : (signed)sizeof(buf);
    nread = read(fd,buf,readlen);
    if (nread <= 0) {
        redisLog(REDIS_WARNING,"I/O error trying to sync with MASTER: %s",
            (nread == -1) ? strerror(errno) : "connection lost");
        replicationAbortSyncTransfer();
        return;
    }
    server.repl_transfer_lastio = server.unixtime;
    if (write(server.repl_transfer_fd,buf,nread) != nread) {
        redisLog(REDIS_WARNING,"Write error or short write writing to the DB dump file needed for MASTER <-> SLAVE synchronization: %s", strerror(errno));
        goto error;
    }
    server.repl_transfer_read += nread;

    /* Sync data on disk from time to time, otherwise at the end of the transfer
     * we may suffer a big delay as the memory buffers are copied into the
     * actual disk. */
    if (server.repl_transfer_read >=
        server.repl_transfer_last_fsync_off + REPL_MAX_WRITTEN_BEFORE_FSYNC)
    {
        off_t sync_size = server.repl_transfer_read -
                          server.repl_transfer_last_fsync_off;
        rdb_fsync_range(server.repl_transfer_fd,
            server.repl_transfer_last_fsync_off, sync_size);
        server.repl_transfer_last_fsync_off += sync_size;
    }

    /* Check if the transfer is now complete */
    if (server.repl_transfer_read == server.repl_transfer_size) {
        if (rename(server.repl_transfer_tmpfile,server.rdb_filename) == -1) {
            redisLog(REDIS_WARNING,"Failed trying to rename the temp DB into dump.rdb in MASTER <-> SLAVE synchronization: %s", strerror(errno));
            replicationAbortSyncTransfer();
            return;
        }
        redisLog(REDIS_NOTICE, "MASTER <-> SLAVE sync: Loading DB in memory");
        emptyDb();
        /* Before loading the DB into memory we need to delete the readable
         * handler, otherwise it will get called recursively since
         * rdbLoad() will call the event loop to process events from time to
         * time for non blocking loading. */
        aeDeleteFileEvent(server.el,server.repl_transfer_s,AE_READABLE);
        if (rdbLoad(server.rdb_filename) != REDIS_OK) {
            redisLog(REDIS_WARNING,"Failed trying to load the MASTER synchronization DB from disk");
            replicationAbortSyncTransfer();
            return;
        }
        /* Final setup of the connected slave <- master link */
        zfree(server.repl_transfer_tmpfile);
        close(server.repl_transfer_fd);
        server.master = createClient(server.repl_transfer_s);
        server.master->flags |= REDIS_MASTER;
        server.master->authenticated = 1;
        server.repl_state = REDIS_REPL_CONNECTED;
        redisLog(REDIS_NOTICE, "MASTER <-> SLAVE sync: Finished with success");
        /* Restart the AOF subsystem now that we finished the sync. This
         * will trigger an AOF rewrite, and when done will start appending
         * to the new file. */
        if (server.aof_state != REDIS_AOF_OFF) {
            int retry = 10;

            stopAppendOnly();
            while (retry-- && startAppendOnly() == REDIS_ERR) {
                redisLog(REDIS_WARNING,"Failed enabling the AOF after successful master synchrnization! Trying it again in one second.");
                sleep(1);
            }
            if (!retry) {
                redisLog(REDIS_WARNING,"FATAL: this slave instance finished the synchronization with its master, but the AOF can't be turned on. Exiting now.");
                exit(1);
            }
        }
    }

    return;

error:
    replicationAbortSyncTransfer();
    return;
}

在從服務器數據未同步完的情況下,如果從服務器沒有設置只讀,是可以進行數據寫入的。如果設置了只讀,在同步中,只有主節點可以執行寫操作,其他節點都不可以執行寫操作。從服務器收到完整同步文件後清空內存再加載rdb文件到內存。

 

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