Redis源码分析（十九）--- replication主从数据复制的实现

              replication的英文单词的原意是“复制”的意思,replication文件作为我在Data目录下的分析的最后一个文件，足以说明他的重要性，代码量1800+，的确非常难啃。只能说个我看代码下来的大致印象吧，要我画个结构图好好理理这里面各个API的关系图，这个我目前还真做不到。说到主从复制，这个是实现读写分离的最好手段了，也很常见，当用户数达到一定量，当一个服务器承受不了达到上千万的pv时，采取主从数据库的形式也是一般架构师能够想到的一种手段。Redis的主从数据库在我这里就称为主客户端，从客户端，因为客户端中有所属于的db，因为数据库基于客户单本身进行复制操作的。也就是说，一个Redis，存在一个master主客户端，多个slave从客户端，到时实现的就是slave向主客户端进行复制操作。因为API比较多，进行了稍稍的归类:

/* ---------------------------------- MASTER -------------------------------- */void createReplicationBacklog(void) /* 创建backlog的buffer */void resizeReplicationBacklog(long long newsize) /* 调整复制备份日志的大小，当replication backlog被修改的时候 */void freeReplicationBacklog(void) /* 释放备份日志 */void feedReplicationBacklog(void *ptr, size_t len) /* 往备份日志中添加添加数据操作，会引起master_repl_offset偏移量的增加 */void feedReplicationBacklogWithObject(robj *o) /* 往backlog添加数据，以Redis 字符串对象作为参数 */void replicationFeedSlaves(list *slaves, int dictid, robj **argv, int argc) /* 将主数据库复制到从数据库 */void replicationFeedMonitors(redisClient *c, list *monitors, int dictid, robj **argv, int argc) /* 发送数据给monitor监听者客户端 */long long addReplyReplicationBacklog(redisClient *c, long long offset) /* slave从客户单添加备份日志 */int masterTryPartialResynchronization(redisClient *c) /* 主数据库尝试分区同步 */void syncCommand(redisClient *c) /* 同步命令函数 */void replconfCommand(redisClient *c) /* 此函数用于从客户端进行配置复制进程中的执行参数设置 */void sendBulkToSlave(aeEventLoop *el, int fd, void *privdata, int mask) /* 给slave客户端发送BULK数据 */void updateSlavesWaitingBgsave(int bgsaveerr) /* 此方法将用于后台保存进程快结束时调用，更新slave从客户端 *//* ----------------------------------- SLAVE -------------------------------- */void replicationAbortSyncTransfer(void) /* 中止与master主数据的同步操作 */void replicationSendNewlineToMaster(void) /* 从客户端发送空行给主客户端，破坏了原本的协议格式，避免让主客户端检测出从客户端超时的情况 */void replicationEmptyDbCallback(void *privdata) /* 清空数据库后的回调方法，当老数据被刷新出去之后等待加载新数据的时候调用 */void readSyncBulkPayload(aeEventLoop *el, int fd, void *privdata, int mask) /* 从客户端读取同步的Sync的BULK数据 */char *sendSynchronousCommand(int fd, ...) /* 从客户端发送给主客户端同步数据的命令，附上验证信息，和一些参数配置信息 */int slaveTryPartialResynchronization(int fd) /* 从客户端尝试分区同步操作 */void syncWithMaster(aeEventLoop *el, int fd, void *privdata, int mask) /* 与主客户端保持同步，期间包括端口号等的确认，socket连接 */int connectWithMaster(void) /* 连接主客户端 */void undoConnectWithMaster(void) /* 撤销连接主客户端 */int cancelReplicationHandshake(void) /* 当已经存在一个复制进程时，中止一个非阻塞的replication复制的尝试 */void replicationSetMaster(char *ip, int port) /* 设定主客户端的ip地址和端口号 */void replicationUnsetMaster(void)void slaveofCommand(redisClient *c)void roleCommand(redisClient *c)void replicationSendAck(void) /* 发送ACK包给主客户端 ，告知当前的进程偏移量 *//* ---------------------- MASTER CACHING FOR PSYNC -------------------------- */void replicationCacheMaster(redisClient *c) /* 缓存客户端信息 */void replicationDiscardCachedMaster(void) /* 当某个客户端将不会再回复的时候，可以释放掉缓存的主客户端 */void replicationResurrectCachedMaster(int newfd) /* 将缓存客户端复活 *//* ------------------------- MIN-SLAVES-TO-WRITE  --------------------------- */void refreshGoodSlavesCount(void) /* 更新slave从客户端数量 */void replicationScriptCacheInit(void)void replicationScriptCacheFlush(void)void replicationScriptCacheAdd(sds sha1)int replicationScriptCacheExists(sds sha1)void replicationCron(void)

找一个标准的slave从客户端向主客户端实现同步的操作:

/* 与主客户端保持同步，期间包括端口号等的确认，socket连接 */void syncWithMaster(aeEventLoop *el, int fd, void *privdata, int mask) {    char tmpfile[256], *err;    int dfd, maxtries = 5;    int sockerr = 0, psync_result;    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. */    /* socket连接是否正常 */    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). */    /* 连接测试，将由主客户端发送PING命令给从客户端，在给定的延迟时间内观察是否有回复 */    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. */        //发送PING命令        syncWrite(fd,"PING\r\n",6,100);        return;    }    /* Receive the PONG command. */    //收到回复了    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 accept only two replies as valid, a positive +PONG reply         * (we just check for "+") or an authentication error.         * Note that older versions of Redis replied with "operation not         * permitted" instead of using a proper error code, so we test         * both. */        if (buf[0] != '+' &&            strncmp(buf,"-NOAUTH",7) != 0 &&            strncmp(buf,"-ERR operation not permitted",28) != 0)        {            redisLog(REDIS_WARNING,"Error reply to PING from master: '%s'",buf);            goto error;        } else {            redisLog(REDIS_NOTICE,                "Master replied to PING, replication can continue...");        }    }    /* AUTH with the master if required. */    //auth身份验证    if(server.masterauth) {        err = sendSynchronousCommand(fd,"AUTH",server.masterauth,NULL);        if (err[0] == '-') {            redisLog(REDIS_WARNING,"Unable to AUTH to MASTER: %s",err);            sdsfree(err);            goto error;        }        sdsfree(err);    }    /* Set the slave port, so that Master's INFO command can list the     * slave listening port correctly. */    /* 设置从客户端监听端口 */    {        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[0] == '-') {            redisLog(REDIS_NOTICE,"(Non critical) Master does not understand REPLCONF listening-port: %s", err);        }        sdsfree(err);    }    /* Try a partial resynchonization. If we don't have a cached master     * slaveTryPartialResynchronization() will at least try to use PSYNC     * to start a full resynchronization so that we get the master run id     * and the global offset, to try a partial resync at the next     * reconnection attempt. */    psync_result = slaveTryPartialResynchronization(fd);    if (psync_result == PSYNC_CONTINUE) {        redisLog(REDIS_NOTICE, "MASTER <-> SLAVE sync: Master accepted a Partial Resynchronization.");        return;    }    /* Fall back to SYNC if needed. Otherwise psync_result == PSYNC_FULLRESYNC     * and the server.repl_master_runid and repl_master_initial_offset are     * already populated. */    if (psync_result == PSYNC_NOT_SUPPORTED) {        redisLog(REDIS_NOTICE,"Retrying with SYNC...");        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: %s (fd=%d)",            strerror(errno),fd);        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;}

         在replication中，要一个cacheMaster的概念，就是可以临时缓存主客户端的信息，一般用于突然master和slave断开连接的时候，可以下次进行主从同步的时候快速恢复:

/* 缓存客户端信息 */void replicationCacheMaster(redisClient *c) {    listNode *ln;    redisAssert(server.master != NULL && server.cached_master == NULL);    redisLog(REDIS_NOTICE,"Caching the disconnected master state.");    /* Remove from the list of clients, we don't want this client to be     * listed by CLIENT LIST or processed in any way by batch operations. */    //首先移除此客户端    ln = listSearchKey(server.clients,c);    redisAssert(ln != NULL);    listDelNode(server.clients,ln);    /* Save the master. Server.master will be set to null later by     * replicationHandleMasterDisconnection(). */    //保存为缓存客户端    server.cached_master = server.master;    /* Remove the event handlers and close the socket. We'll later reuse     * the socket of the new connection with the master during PSYNC. */    //删除在这个客户端上的读写事件    aeDeleteFileEvent(server.el,c->fd,AE_READABLE);    aeDeleteFileEvent(server.el,c->fd,AE_WRITABLE);    close(c->fd);    /* Set fd to -1 so that we can safely call freeClient(c) later. */    c->fd = -1;    /* Invalidate the Peer ID cache. */    if (c->peerid) {        sdsfree(c->peerid);        c->peerid = NULL;    }    /* Caching the master happens instead of the actual freeClient() call,     * so make sure to adjust the replication state. This function will     * also set server.master to NULL. */    replicationHandleMasterDisconnection();}

当想让这个master的复活的时候，调用下面的方法:

/* Turn the cached master into the current master, using the file descriptor * passed as argument as the socket for the new master. * * This funciton is called when successfully setup a partial resynchronization * so the stream of data that we'll receive will start from were this * master left. *//* 将缓存客户端复活 */void replicationResurrectCachedMaster(int newfd) {//将cached_master赋值为主客户端    server.master = server.cached_master;    server.cached_master = NULL;    server.master->fd = newfd;    server.master->flags &= ~(REDIS_CLOSE_AFTER_REPLY|REDIS_CLOSE_ASAP);    server.master->authenticated = 1;    server.master->lastinteraction = server.unixtime;    server.repl_state = REDIS_REPL_CONNECTED;    /* Re-add to the list of clients. */    //重新添加入客户端列表中    listAddNodeTail(server.clients,server.master);    if (aeCreateFileEvent(server.el, newfd, AE_READABLE,                          readQueryFromClient, server.master)) {        redisLog(REDIS_WARNING,"Error resurrecting the cached master, impossible to add the readable handler: %s", strerror(errno));        freeClientAsync(server.master); /* Close ASAP. */    }    /* We may also need to install the write handler as well if there is     * pending data in the write buffers. */    if (server.master->bufpos || listLength(server.master->reply)) {        if (aeCreateFileEvent(server.el, newfd, AE_WRITABLE,                          sendReplyToClient, server.master)) {            redisLog(REDIS_WARNING,"Error resurrecting the cached master, impossible to add the writable handler: %s", strerror(errno));            freeClientAsync(server.master); /* Close ASAP. */        }    }}

当然如果确定在未来不糊在使用缓存的master的时，可以彻底摧毁:

/* Free a cached master, called when there are no longer the conditions for * a partial resync on reconnection. *//* 当某个客户端将不会再回复的时候，可以释放掉缓存的主客户端 */void replicationDiscardCachedMaster(void) {    if (server.cached_master == NULL) return;    redisLog(REDIS_NOTICE,"Discarding previously cached master state.");    server.cached_master->flags &= ~REDIS_MASTER;    //直接释放客户端    freeClient(server.cached_master);    //server的缓存客户端赋值为NULL    server.cached_master = NULL;}

在这里面靠的就是server.cached_master属性。slave在和master连接的时候，要进行master的ip地址和Port端口的确认：

/* Set replication to the specified master address and port. *//* 设定主客户端的ip地址和端口号 */void replicationSetMaster(char *ip, int port) {    sdsfree(server.masterhost);    server.masterhost = sdsdup(ip);    server.masterport = port;    //设置完毕之后，断开所有的连接，中止replication进程    if (server.master) freeClient(server.master);    disconnectSlaves(); /* Force our slaves to resync with us as well. */    replicationDiscardCachedMaster(); /* Don't try a PSYNC. */    freeReplicationBacklog(); /* Don't allow our chained slaves to PSYNC. */    cancelReplicationHandshake();    server.repl_state = REDIS_REPL_CONNECT;    server.master_repl_offset = 0;}

主从复制的实现其实还有很多细节和步骤的。稍稍分析了一下，以后有机会研究的更深入一点