深入剖析Redis主从复制

【http://sofar.blog.51cto.com/353572/1413024/】

 

【Redis 主从复制的内部协议和机制】

 

一、主从概述

Redis 支持 Master-Slave（主从）模式，Redis Server 可以设置为另一个 Redis Server 的主机（从机），从机定期从主机拿数据。特殊的，一个从机同样可以设置为一个 Redis Server 的主机，这样一来 Master-Slave 的分布看起来就是一个有向无环图 DAG，如此形成 Redis Server 集群，无论是主机还是从机都是 Redis Server，都可以提供服务。

在配置后，主机可负责读写服务，从机只负责读。Redis 提高这种配置方式，为的是让其支持数据的弱一致性，即最终一致性。在业务中，选择强一致性还是弱一致性，应该取决于具体的业务需求，像微博，完全可以使用弱一致性模型；像淘宝，可以选用强一致性模型。

Redis 主从复制的实现主要在 replication.c 中。

这篇文章涉及较多的代码，但我已经尽量删繁就简，达到能说明问题本质。为了保留代码的原生性并让读者能够阅读原生代码的注释，剖析 Redis 的几篇文章都没有删除代码中的英文注释，并已加注释。

 

二、积压空间

在《深入剖析 Redis AOF 持久化策略》中，介绍了更新缓存的概念，举一个例子：客户端发来命令：set name Jhon，这一数据更新被记录为：*3\r\n$3\r\nSET\r\n$4\r\nname\r\n$3\r\nJhon\r\n，并存储在更新缓存中。

同样，在主从连接中，也有更新缓存的概念。只是两者的用途不一样，前者被写入本地，后者被写入从机，这里我们把它称为积压空间。

更新缓存存储在 server.repl_backlog，Redis 将其作为一个环形空间来处理，这样做节省了空间，避免内存再分配的情况。

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struct redisServer {     /* Replication (master) */     // 最近一次使用（访问）的数据集     int slaveseldb;                 /* Last SELECTed DB in replication output */                                                                         // 全局的数据同步偏移量     long long master_repl_offset;   /* Global replication offset */                                                                          // 主从连接心跳频率     int repl_ping_slave_period;     /* Master pings the slave every N seconds */                                                                          // 积压空间指针     char *repl_backlog;             /* Replication backlog for partial syncs */                                                                        // 积压空间大小     long long repl_backlog_size;    /* Backlog circular buffer size */                                                                        // 积压空间中写入的新数据的大小     long long repl_backlog_histlen; /* Backlog actual data length */                                                                        // 下一次向积压空间写入数据的起始位置     long long repl_backlog_idx;     /* Backlog circular buffer current offset */                                                                        // 积压数据的起始位置，是一个宏观值     long long repl_backlog_off;     /* Replication offset of first byte                                        in the backlog buffer. */                                                                          // 积压空间有效时间     time_t repl_backlog_time_limit; /* Time without slaves after the backlog gets released. */ }

积压空间中的数据变更记录是什么时候被写入的？在执行一个 Redis 命令的时候，如果存在数据的修改（写），那么就会把变更记录传播。Redis 源码中是这么实现的：call()->propagate()->replicationFeedSlaves()

注释：命令真正执行的地方在 call() 中，call() 如果发现数据被修改（dirty），则传播 propagrate()，replicationFeedSlaves() 将修改记录写入积压空间和所有已连接的从机。

这里可能会有疑问：为什么把数据添加入积压空间，又把数据分发给所有的从机？为什么不仅仅将数据分发给所有从机呢？

因为有一些从机会因特殊情况（？？？）与主机断开连接，注意从机断开前有暂存主机的状态信息，因此这些断开的从机就没有及时收到更新的数据。Redis 为了让断开的从机在下次连接后能够获取更新数据，将更新数据加入了积压空间。从 replicationFeedSlaves() 实现来看，在线的 Slave 能马上收到数据更新记录；因某些原因暂时断开连接的 Slave，需要从积压空间中找回断开期间的数据更新记录。如果断开的时间足够长，Master 会拒绝 Slave 的部分同步请求，从而 Slave 只能进行全同步。

下面是源码注释：

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// call() 函数是执行命令的核心函数，真正执行命令的地方 /* Call() is the core of Redis execution of a command */ void call(redisClient *c, int flags) {     ......     /* Call the command. */     c->flags &= ~(REDIS_FORCE_AOF | REDIS_FORCE_REPL);     redisOpArrayInit(&server.also_propagate);                                                              // 脏数据标记，数据是否被修改     dirty = server.dirty;                                                              // 执行命令对应的函数     c->cmd->proc(c);                                                              dirty = server.dirty - dirty;     duration = ustime() - start;                                                              ......                                                              // 将客户端请求的数据修改记录传播给 AOF 和从机     /* Propagate the command into the AOF and replication link */     if (flags & REDIS_CALL_PROPAGATE) {         int flags = REDIS_PROPAGATE_NONE;                                                                  // 强制主从复制         if (c->flags & REDIS_FORCE_REPL) flags |= REDIS_PROPAGATE_REPL;                                                                  // 强制 AOF 持久化         if (c->flags & REDIS_FORCE_AOF) flags |= REDIS_PROPAGATE_AOF;                                                                  // 数据被修改         if (dirty)             flags |= (REDIS_PROPAGATE_REPL | REDIS_PROPAGATE_AOF);                                                                  // 传播数据修改记录         if (flags != REDIS_PROPAGATE_NONE)             propagate(c->cmd, c->db->id, c->argv, c->argc, flags);     }     ...... }                                                          // 向 AOF 和从机发布数据更新 /* Propagate the specified command (in the context of the specified database id)  * to AOF and Slaves.  *  * flags are an xor between:  * + REDIS_PROPAGATE_NONE (no propagation of command at all)  * + REDIS_PROPAGATE_AOF (propagate into the AOF file if is enabled)  * + REDIS_PROPAGATE_REPL (propagate into the replication link)  */ void propagate(struct redisCommand *cmd, int dbid, robj **argv, int argc,                int flags) {     // AOF 策略需要打开，且设置 AOF 传播标记，将更新发布给本地文件     if (server.aof_state != REDIS_AOF_OFF && flags & REDIS_PROPAGATE_AOF)         feedAppendOnlyFile(cmd, dbid, argv, argc);                                                              // 设置了从机传播标记，将更新发布给从机     if (flags & REDIS_PROPAGATE_REPL)         replicationFeedSlaves(server.slaves,dbid,argv,argc); }                                                          // 向积压空间和从机发送数据 void replicationFeedSlaves(list *slaves, int dictid, robj **argv, int argc) {     listNode *ln;     listIter li;     int j, len;     char llstr[REDIS_LONGSTR_SIZE];                                                              // 没有积压数据且没有从机，直接退出     /* If there aren't slaves, and there is no backlog buffer to populate,      * we can return ASAP. */     if (server.repl_backlog == NULL && listLength(slaves) == 0) return;                                                              /* We can't have slaves attached and no backlog. */     redisAssert(!(listLength(slaves) != 0 && server.repl_backlog == NULL));                                                              /* Send SELECT command to every slave if needed. */     if (server.slaveseldb != dictid) {         robj *selectcmd;                                                                  // 小于等于 10 的可以用共享对象         /* For a few DBs we have pre-computed SELECT command. */         if (dictid >= 0 && dictid < REDIS_SHARED_SELECT_CMDS) {             selectcmd = shared.select[dictid];         } else {         // 不能使用共享对象，生成 SELECT 命令对应的 redis 对象             int dictid_len;                                                                      dictid_len = ll2string(llstr, sizeof(llstr), dictid);             selectcmd = createObject(REDIS_STRING,                 sdscatprintf(sdsempty(),                 "*2\r\n$6\r\nSELECT\r\n$%d\r\n%s\r\n",                 dictid_len, llstr));         }                                                                  // 这里可能会有疑问：为什么把数据添加入积压空间，又把数据分发给所有的从机？         // 为什么不仅仅将数据分发给所有从机呢？         // 因为有一些从机会因特殊情况（？？？）与主机断开连接，注意从机断开前有暂存         // 主机的状态信息，因此这些断开的从机就没有及时收到更新的数据。redis 为了让         // 断开的从机在下次连接后能够获取更新数据，将更新数据加入了积压空间。                                                                  // 将 SELECT 命令对应的 redis 对象数据添加到积压空间         /* Add the SELECT command into the backlog. */         if (server.repl_backlog) feedReplicationBacklogWithObject(selectcmd);                                                                  // 将数据分发所有的从机         /* Send it to slaves. */         listRewind(slaves, &li);         while((ln = listNext(&li))) {             redisClient *slave = ln->value;             addReply(slave, selectcmd);         }                                                                  // 销毁对象         if (dictid < 0 || dictid >= REDIS_SHARED_SELECT_CMDS)             decrRefCount(selectcmd);     }                                                              // 更新最近一次使用（访问）的数据集     server.slaveseldb = dictid;                                                              // 将命令写入积压空间     /* Write the command to the replication backlog if any. */     if (server.repl_backlog) {         char aux[REDIS_LONGSTR_SIZE+3];                                                                  // 命令个数         /* Add the multi bulk reply length. */         aux[0] = '*';         len = ll2string(aux + 1, sizeof(aux) - 1, argc);         aux[len+1] = '\r';         aux[len+2] = '\n';         feedReplicationBacklog(aux, len + 3);                                                                  // 逐个命令写入         for (j = 0; j < argc; j++) {             long objlen = stringObjectLen(argv[j]);                                                                      /* We need to feed the buffer with the object as a bulk reply              * not just as a plain string, so create the $..CRLF payload len              * ad add the final CRLF */             aux[0] = '$';             len = ll2string(aux + 1, sizeof(aux) - 1, objlen);             aux[len+1] = '\r';             aux[len+2] = '\n';                                                                      /* 每个命令格式如下：             $3             *3             SET             *4             NAME             *4             Jhon*/                                                                      // 命令长度             feedReplicationBacklog(aux, len + 3);             // 命令             feedReplicationBacklogWithObject(argv[j]);             // 换行             feedReplicationBacklog(aux + len + 1, 2);         }     }                                                              // 立即给每一个从机发送命令     /* Write the command to every slave. */     listRewind(slaves, &li);     while((ln = listNext(&li))) {         redisClient *slave = ln->value;                                                                  // 如果从机要求全同步，则不对此从机发送数据         /* Don't feed slaves that are still waiting for BGSAVE to start */         if (slave->replstate == REDIS_REPL_WAIT_BGSAVE_START) continue;                                                                  /* Feed slaves that are waiting for the initial SYNC (so these commands          * are queued in the output buffer until the initial SYNC completes),          * or are already in sync with the master. */                                                                  // 向从机命令的长度         /* Add the multi bulk length. */         addReplyMultiBulkLen(slave, argc);                                                                  // 向从机发送命令         /* Finally any additional argument that was not stored inside the          * static buffer if any (from j to argc). */         for (j = 0; j < argc; j++)             addReplyBulk(slave, argv[j]);     } }

 

三、主从数据同步机制概述

Redis 主从同步有两种方式（或者所两个阶段）：全同步和部分同步。

主从刚刚连接的时候，进行全同步；全同步结束后，进行部分同步。当然，如果有需要，Slave 在任何时候都可以发起全同步。Redis 策略是，无论如何，首先会尝试进行部分同步，如不成功，要求从机进行全同步，并启动 BGSAVE……BGSAVE 结束后，传输 RDB 文件；如果成功，允许从机进行部分同步，并传输积压空间中的数据。

下面这幅图，总结了主从同步的机制：

如需设置 Slave，Master 需要向 Slave 发送 SLAVEOF hostname port，从机接收到后会自动连接主机，注册相应读写事件（syncWithMaster())。

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// 修改主机 void slaveofCommand(redisClient *c) {     if (!strcasecmp(c->argv[1]->ptr, "no") &&         !strcasecmp(c->argv[2]->ptr, "one")) {         // slaveof no one 断开主机连接         if (server.masterhost) {             replicationUnsetMaster();             redisLog(REDIS_NOTICE, "MASTER MODE enabled (user request)");         }     } else {         long port;                                                                                                                     if ((getLongFromObjectOrReply(c, c->argv[2], &port, NULL) != REDIS_OK))             return;                                                                                                                     // 可能已经连接需要连接的主机         /* Check if we are already attached to the specified slave */         if (server.masterhost && !strcasecmp(server.masterhost,c->argv[1]->ptr)             && server.masterport == port) {             redisLog(REDIS_NOTICE,                 "SLAVE OF 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;         }                                                                                                                     // 断开之前连接主机的连接，连接新的。 replicationSetMaster() 并不会真正连接主机，         // 只是修改 struct server 中关于主机的设置。真正的主机连接在 replicationCron() 中完成         /* There was no previous master or the user specified a different one,          * we can continue. */         replicationSetMaster(c->argv[1]->ptr, port);         redisLog(REDIS_NOTICE, "SLAVE OF %s:%d enabled (user request)",             server.masterhost, server.masterport);     }     addReply(c,shared.ok); }                                                                                                             // 设置新主机 /* Set replication to the specified master address and port. */ void replicationSetMaster(char *ip, int port) {     sdsfree(server.masterhost);     server.masterhost = sdsdup(ip);     server.masterport = port;                                                                                                                 // 断开之前主机的连接     if (server.master) freeClient(server.master);     disconnectSlaves(); /* Force our slaves to resync with us as well. */                                                                                                                 // 取消缓存主机     replicationDiscardCachedMaster(); /* Don't try a PSYNC. */                                                                                                                 // 释放积压空间     freeReplicationBacklog(); /* Don't allow our chained slaves to PSYNC. */                                                                                                                 // cancelReplicationHandshake() 尝试断开数据传输和主机连接     cancelReplicationHandshake();     server.repl_state = REDIS_REPL_CONNECT;     server.master_repl_offset = 0; }                                                                                                             // 管理主从连接的定时程序定时程序，每秒执行一次 // 在 serverCorn() 中调用 /* --------------------------- REPLICATION CRON  ----------------------------- */                                                                                                             /* Replication cron funciton, called 1 time per second. */ void replicationCron(void) {     ......     // 如果需要（ REDIS_REPL_CONNECT），尝试连接主机，真正连接主机的操作在这里     /* Check if we should connect to a MASTER */     if (server.repl_state == REDIS_REPL_CONNECT) {         redisLog(REDIS_NOTICE, "Connecting to MASTER %s:%d",             server.masterhost, server.masterport);         if (connectWithMaster() == REDIS_OK) {             redisLog(REDIS_NOTICE, "MASTER <-> SLAVE sync started");         }     }     ...... }

四、全同步

接着自动发起 PSYNC 请求 Master 进行全同步。无论如何，Redis 首先会尝试部分同步，如果失败才尝试全同步。而刚刚建立连接的 Master-Slave 需要全同步。

从机连接主机后，会主动发起 PSYNC 命令，从机会提供 master_runid 和 offset，主机验证 master_runid 和 offset 是否有效？master_runid 相当于主机身份验证码，用来验证从机上一次连接的主机，offset 是全局积压空间数据的偏移量。

验证未通过则，则进行全同步：主机返回 +FULLRESYNC master_runid offset（从机接收并记录 master_runid 和 offset，并准备接收 RDB 文件）接着启动 BGSAVE 生成 RDB 文件，BGSAVE 结束后，向从机传输，从而完成全同步。

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// 连接主机 connectWithMaster() 的时候，会被注册为回调函数 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);                                                                                                    ......                                                                                                    // 这里尝试向主机请求部分同步，主机会回复以拒绝或接受请求。如果拒绝部分同步，     // 会返回 +FULLRESYNC master_runid offset     // 从机接收后准备进行全同步    psync_result = slaveTryPartialResynchronization(fd);     if (psync_result == PSYNC_CONTINUE) {         redisLog(REDIS_NOTICE,             "MASTER <-> SLAVE sync: Master accepted a Partial Resynchronization.");         return;     }                                                                                                    // 执行全同步     /* 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. */                                                                                                    // 未知结果，进行出错处理     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;         }     }                                                                                                    // 为什么要尝试 5次？？？     /* 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;     }                                                                                                    // 注册读事件，回调函数 readSyncBulkPayload()， 准备读 RDB 文件     /* 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;     }                                                                                                    // 设置传输 RDB 文件数据的选项     // 状态     server.repl_state = REDIS_REPL_TRANSFER;     // RDB 文件大小     server.repl_transfer_size = -1;     // 已经传输的大小     server.repl_transfer_read = 0;     // 上一次同步的偏移，为的是定时写入磁盘     server.repl_transfer_last_fsync_off = 0;     // 本地 RDB 文件套接字     server.repl_transfer_fd = dfd;     // 上一次同步 IO 时间     server.repl_transfer_lastio = server.unixtime;     // 临时文件名     server.repl_transfer_tmpfile = zstrdup(tmpfile);     return;                                                                                                error:     close(fd);     server.repl_transfer_s = -1;     server.repl_state = REDIS_REPL_CONNECT;     return; }

全同步请求的数据是 RDB 数据文件和积压空间中的数据。关于 RDB 数据文件，请参看《深入剖析 Redis RDB 持久化策略》。如果没有后台持久化 BGSAVE 进程，那么 BGSVAE 会被触发，否则所有请求全同步的 Slave 都会被标记为等待 BGSAVE 结束。BGSAVE 结束后，Master 会马上向所有的从机发送 RDB 文件。

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// 主机 SYNC 和 PSYNC 命令处理函数，会尝试进行部分同步和全同步 /* SYNC ad PSYNC command implemenation. */ void syncCommand(redisClient *c) {     ......     // 主机尝试部分同步，失败的话向从机发送 +FULLRESYNC master_runid offset，接着启动 BGSAVE                                                                          // 执行全同步：     /* Full resynchronization. */     server.stat_sync_full++;                                                                          /* Here we need to check if there is a background saving operation      * in progress, or if it is required to start one */     if (server.rdb_child_pid != -1) {         /*  存在 BGSAVE 后台进程。           1.如果 master 现有所连接的所有从机 slaves 当中有存在 REDIS_REPL_WAIT_BGSAVE_END 的从机，             那么将从机 c 设置为 REDIS_REPL_WAIT_BGSAVE_END；           2.否则，设置为 REDIS_REPL_WAIT_BGSAVE_START*/                                                                              /* Ok a background save is in progress. Let's check if it is a good          * one for replication, i.e. if there is another slave that is          * registering differences since the server forked to save */         redisClient *slave;         listNode *ln;         listIter li;                                                                              // 检测是否已经有从机申请全同步         listRewind(server.slaves, &li);         while((ln = listNext(&li))) {             slave = ln->value;             if (slave->replstate == REDIS_REPL_WAIT_BGSAVE_END) break;         }                                                                              if (ln) {             // 存在状态为 REDIS_REPL_WAIT_BGSAVE_END 的从机 slave，             // 就将此从机 c 状态设置为 REDIS_REPL_WAIT_BGSAVE_END，             // 从而在 BGSAVE 进程结束后，可以发送 RDB 文件，             // 同时将从机 slave 中的更新复制到此从机 c。                                                                                  /* Perfect, the server is already registering differences for              * another slave. Set the right state, and copy the buffer. */                                                                                  // 将其他从机上的待回复的缓存复制到从机 c             copyClientOutputBuffer(c, slave);                                                                                  // 修改从机 c 状态为「等待 BGSAVE 进程结束」             c->replstate = REDIS_REPL_WAIT_BGSAVE_END;             redisLog(REDIS_NOTICE, "Waiting for end of BGSAVE for SYNC");         } else {             // 不存在状态为 REDIS_REPL_WAIT_BGSAVE_END 的从机，就将此从机 c 状态设置为             // REDIS_REPL_WAIT_BGSAVE_START，即等待新的 BGSAVE 进程的开启。             // 修改状态为「等待 BGSAVE 进程开始」             /* No way, we need to wait for the next BGSAVE in order to              * register differences */             c->replstate = REDIS_REPL_WAIT_BGSAVE_START;             redisLog(REDIS_NOTICE, "Waiting for next BGSAVE for SYNC");         }     } else {         // 不存在 BGSAVE 后台进程，启动一个新的 BGSAVE 进程         /* Ok we don't have a BGSAVE in progress, let's start one */         redisLog(REDIS_NOTICE, "Starting BGSAVE for SYNC");         if (rdbSaveBackground(server.rdb_filename) != REDIS_OK) {             redisLog(REDIS_NOTICE, "Replication failed, can't BGSAVE");             addReplyError(c, "Unable to perform background save");             return;         }                                                                              // 将此从机 c 状态设置为 REDIS_REPL_WAIT_BGSAVE_END，从而在 BGSAVE 进程结束后，         // 可以发送 RDB 文件，同时将从机 slave 中的更新复制到此从机 c。         c->replstate = REDIS_REPL_WAIT_BGSAVE_END;                                                                              // 清理脚本缓存？？？         /* Flush the script cache for the new slave. */         replicationScriptCacheFlush();     }                                                                          if (server.repl_disable_tcp_nodelay)         anetDisableTcpNoDelay(NULL, c->fd); /* Non critical if it fails. */     c->repldbfd = -1;     c->flags |= REDIS_SLAVE;     server.slaveseldb = -1; /* Force to re-emit the SELECT command. */     listAddNodeTail(server.slaves,c);     if (listLength(server.slaves) == 1 && server.repl_backlog == NULL)         createReplicationBacklog();     return; }                                                                      // BGSAVE 结束后，会调用 /* A background saving child (BGSAVE) terminated its work. Handle this. */ void backgroundSaveDoneHandler(int exitcode, int bysignal) {     // 其他操作     ......     // 可能从机正在等待 BGSAVE 进程的终止     /* Possibly there are slaves waiting for a BGSAVE in order to be served      * (the first stage of SYNC is a bulk transfer of dump.rdb) */     updateSlavesWaitingBgsave(exitcode == 0 ? REDIS_OK : REDIS_ERR); }                                                                      // 当 RDB 持久化(backgroundSaveDoneHandler())结束后，会调用此函数 // RDB 文件就绪，给所有的从机发送 RDB 文件 /* This function is called at the end of every background saving. * The argument bgsaveerr is REDIS_OK if the background saving succeeded * otherwise REDIS_ERR is passed to the function. * * The goal of this function is to handle slaves waiting for a successful * background saving in order to perform non-blocking synchronization. */ void updateSlavesWaitingBgsave(int bgsaveerr) {     listNode *ln;     int startbgsave = 0;     listIter li;                                                                          listRewind(server.slaves,&li);     while((ln = listNext(&li))) {         redisClient *slave = ln->value;                                                                              // 等待 BGSAVE 开始。调整状态为等待下一次 BGSAVE 进程的结束         if (slave->replstate == REDIS_REPL_WAIT_BGSAVE_START) {             startbgsave = 1;                                                                                  slave->replstate = REDIS_REPL_WAIT_BGSAVE_END;                                                                              // 等待 BGSAVE 结束。准备向 slave 发送 RDB 文件         } else if (slave->replstate == REDIS_REPL_WAIT_BGSAVE_END) {             struct redis_stat buf;                                                                                  // 如果 RDB 持久化失败， bgsaveerr 会被设置为 REDIS_ERR             if (bgsaveerr != REDIS_OK) {                 freeClient(slave);                 redisLog(REDIS_WARNING, "SYNC failed. BGSAVE child returned an error");                 continue;             }                                                                                  // 打开 RDB 文件             if ((slave->repldbfd = open(server.rdb_filename, O_RDONLY)) == -1 ||                 redis_fstat(slave->repldbfd, &buf) == -1) {                 freeClient(slave);                 redisLog(REDIS_WARNING,                     "SYNC failed. Can't open/stat DB after BGSAVE: %s", strerror(errno));                 continue;             }                                                                                  slave->repldboff = 0;             slave->repldbsize = buf.st_size;             slave->replstate = REDIS_REPL_SEND_BULK;                                                                                  // 如果之前有注册写事件，取消             aeDeleteFileEvent(server.el,slave->fd,AE_WRITABLE);                                                                                  // 注册新的写事件,sendBulkToSlave() 传输 RDB 文件             if (aeCreateFileEvent(server.el, slave->fd, AE_WRITABLE, sendBulkToSlave,                 slave) == AE_ERR) {                 freeClient(slave);                 continue;             }         }     }                                                                          // startbgsave == REDIS_ERR 表示 BGSAVE 失败，再一次进行 BGSAVE 尝试     if (startbgsave) {         /* Since we are starting a new background save for one or more slaves,          * we flush the Replication Script Cache to use EVAL to propagate every          * new EVALSHA for the first time, since all the new slaves don't know          * about previous scripts. */         replicationScriptCacheFlush();                                                                              if (rdbSaveBackground(server.rdb_filename) != REDIS_OK) {             /* BGSAVE 可能 fork 失败，所有等待 BGSAVE 的从机都将结束连接。              * 这是 redis 自我保护的措施，fork 失败很可能是内存紧张              */             listIter li;                                                                                  listRewind(server.slaves,&li);             redisLog(REDIS_WARNING, "SYNC failed. BGSAVE failed");             while((ln = listNext(&li))) {                 redisClient *slave = ln->value;                                                                                      if (slave->replstate == REDIS_REPL_WAIT_BGSAVE_START)                     freeClient(slave);             }         }     } }

五、部分同步

如上所说，无论如何，Redis 首先会尝试部分同步。部分同步即把积压空间缓存的数据，即更新记录发送给从机。

从机连接主机后，会主动发起 PSYNC 命令，从机会提供 master_runid 和 offset，主机验证 master_runid 和 offset 是否有效？

验证通过则，进行部分同步：主机返回 +CONTINUE（从机接收后会注册积压数据接收事件），接着发送积压空间数据。

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// 连接主机 connectWithMaster() 的时候，会被注册为回调函数 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);                                                      ......                                                      // 尝试部分同步，主机允许进行部分同步会返回 +CONTINUE，从机接收后注册相应的事件                                                      /* 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. */                                                      // 函数返回三种状态：     // PSYNC_CONTINUE：表示会进行部分同步，在 slaveTryPartialResynchronization()     // 中已经设置回调函数 readQueryFromClient()     // PSYNC_FULLRESYNC：全同步，会下载 RDB 文件     // PSYNC_NOT_SUPPORTED：未知     psync_result = slaveTryPartialResynchronization(fd);     if (psync_result == PSYNC_CONTINUE) {         redisLog(REDIS_NOTICE,             "MASTER <-> SLAVE sync: Master accepted a Partial Resynchronization.");         return;     }                                                      // 执行全同步     ...... }                                                  // 函数返回三种状态： // PSYNC_CONTINUE：表示会进行部分同步，已经设置回调函数 // PSYNC_FULLRESYNC：全同步，会下载 RDB 文件 // PSYNC_NOT_SUPPORTED：未知 #define PSYNC_CONTINUE 0 #define PSYNC_FULLRESYNC 1 #define PSYNC_NOT_SUPPORTED 2 int slaveTryPartialResynchronization(int fd) {     char *psync_runid;     char psync_offset[32];     sds reply;                                                      /* Initially set repl_master_initial_offset to -1 to mark the current      * master run_id 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.repl_master_initial_offset = -1;                                                      if (server.cached_master) {         // 缓存了上一次与主机连接的信息，可以尝试进行部分同步，减少数据传输         psync_runid = server.cached_master->replrunid;         snprintf(psync_offset, sizeof(psync_offset), "%lld",             server.cached_master->reploff + 1);                                                  redisLog(REDIS_NOTICE,             "Trying a partial resynchronization (request %s:%s).",             psync_runid, psync_offset);     } else {         // 未缓存上一次与主机连接的信息，进行全同步         // psync ? -1 可以获取主机的 master_runid         redisLog(REDIS_NOTICE, "Partial resynchronization not possible (no cached master)");         psync_runid = "?";         memcpy(psync_offset, "-1", 3);     }                                                      // 向主机发送命令，并接收回复     /* Issue the PSYNC command */     reply = sendSynchronousCommand(fd, "PSYNC", psync_runid, psync_offset, NULL);                                                      // 全同步     if (!strncmp(reply, "+FULLRESYNC", 11)) {         char *runid = NULL, *offset = NULL;                                                          /* FULL RESYNC, parse the reply in order to extract the run id          * and the replication offset. */         runid = strchr(reply, ' ');         if (runid) {             runid++;             offset = strchr(runid, ' ');             if (offset) offset++;         }         if (!runid || !offset || (offset-runid-1) != REDIS_RUN_ID_SIZE) {             redisLog(REDIS_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              * runid to make sure next PSYNCs will fail. */             memset(server.repl_master_runid, 0, REDIS_RUN_ID_SIZE + 1);         } else {             // 拷贝 runid             memcpy(server.repl_master_runid, runid, offset-runid-1);             server.repl_master_runid[REDIS_RUN_ID_SIZE] = '\0';             server.repl_master_initial_offset = strtoll(offset,NULL,10);             redisLog(REDIS_NOTICE, "Full resync from master: %s:%lld",                 server.repl_master_runid,                 server.repl_master_initial_offset);         }         /* We are going to full resync, discard the cached master structure. */         replicationDiscardCachedMaster();         sdsfree(reply);         return PSYNC_FULLRESYNC;     }                                                      // 部分同步     if (!strncmp(reply, "+CONTINUE", 9)) {         /* Partial resync was accepted, set the replication state accordingly */         redisLog(REDIS_NOTICE, "Successful partial resynchronization with master.");         sdsfree(reply);                                                          // 缓存主机替代现有主机，且为 PSYNC（部分同步） 做好准备c         replicationResurrectCachedMaster(fd);                                                          return PSYNC_CONTINUE;     }                                                      /* If we reach this point we receied either an error since the master does      * not understand PSYNC, or an unexpected reply from the master.      * Reply with PSYNC_NOT_SUPPORTED in both cases. */                                                      // 接收到主机发出的错误信息     if (strncmp(reply, "-ERR", 4)) {         /* If it's not an error, log the unexpected event. */         redisLog(REDIS_WARNING, "Unexpected reply to PSYNC from master: %s", reply);     } else {         redisLog(REDIS_NOTICE,             "Master does not support PSYNC or is in "             "error state (reply: %s)", reply);     }     sdsfree(reply);     replicationDiscardCachedMaster();     return PSYNC_NOT_SUPPORTED; }                                                  // 主机 SYNC 和 PSYNC 命令处理函数，会尝试进行部分同步和全同步 /* SYNC ad PSYNC command implemenation. */ void syncCommand(redisClient *c) {     ......                                                      // 主机尝试部分同步，允许则进行部分同步，会返回 +CONTINUE，接着发送积压空间                                                      /* Try a partial resynchronization if this is a PSYNC command.      * If it fails, we continue with usual full resynchronization, however      * when this happens masterTryPartialResynchronization() already      * replied with:      *      * +FULLRESYNC <runid> <offset>      *      * So the slave knows the new runid and offset to try a PSYNC later      * if the connection with the master is lost. */     if (!strcasecmp(c->argv[0]->ptr, "psync")) {         // 部分同步         if (masterTryPartialResynchronization(c) == REDIS_OK) {             server.stat_sync_partial_ok++;             return; /* No full resync needed, return. */         } else {         // 部分同步失败，会进行全同步，这时会收到来自客户端的 runid             char *master_runid = c->argv[1]->ptr;                                                              /* Increment stats for failed PSYNCs, but only if the              * runid is not "?", as this is used by slaves to force a full              * resync on purpose when they are not albe to partially              * resync. */             if (master_runid[0] != '?')                 server.stat_sync_partial_err++;         }     } else {         /* If a slave uses SYNC, we are dealing with an old implementation          * of the replication protocol (like redis-cli --slave). Flag the client          * so that we don't expect to receive REPLCONF ACK feedbacks. */         c->flags |= REDIS_PRE_PSYNC_SLAVE;     }                                                      // 执行全同步：     ...... }                                                  // 主机尝试是否能进行部分同步 /* This function handles the PSYNC command from the point of view of a * master receiving a request for partial resynchronization. * * On success return REDIS_OK, otherwise REDIS_ERR is returned and we proceed * with the usual full resync. */ int masterTryPartialResynchronization(redisClient *c) {     long long psync_offset, psync_len;     char *master_runid = c->argv[1]->ptr;     char buf[128];     int buflen;                                                      /* Is the runid of this master the same advertised by the wannabe slave      * via PSYNC? If runid changed this master is a different instance and      * there is no way to continue. */     if (strcasecmp(master_runid, server.runid)) {     // 当因为异常需要与主机断开连接的时候，从机会暂存主机的状态信息，以便     // 下一次的部分同步。     // 1）master_runid 是从机提供一个因缓存主机的 runid，     // 2）server.runid 是本机（主机）的 runid。     // 匹配失败，说明是本机（主机）不是从机缓存的主机，这时候不能进行部分同步，     // 只能进行全同步                                                          // "?" 表示从机要求全同步         // 什么时候从机会要求全同步？？？         /* Run id "?" is used by slaves that want to force a full resync. */         if (master_runid[0] != '?') {             redisLog(REDIS_NOTICE,"Partial resynchronization not accepted: "                 "Runid mismatch (Client asked for '%s', I'm '%s')",                 master_runid, server.runid);         } else {             redisLog(REDIS_NOTICE, "Full resync requested by slave.");         }         goto need_full_resync;     }                                                      // 从参数中解析整数，整数是从机指定的偏移量     /* We still have the data our slave is asking for? */     if (getLongLongFromObjectOrReply(c, c->argv[2], &psync_offset, NULL) != REDIS_OK)         goto need_full_resync;                                                      // 部分同步失败的情况     if (!server.repl_backlog || /*不存在积压空间*/         psync_offset < server.repl_backlog_off ||  /*psync_offset 太过小，                                                     即从机错过太多更新记录，                                                     安全起见，实行全同步*/                                                     /*psync_offset 越界*/         psync_offset > (server.repl_backlog_off + server.repl_backlog_histlen))     // 经检测，不满足部分同步的条件，转而进行全同步     {         redisLog(REDIS_NOTICE,             "Unable to partial resync with the slave for lack of             backlog (Slave request was: %lld).", psync_offset);                                                  if (psync_offset > server.master_repl_offset) {             redisLog(REDIS_WARNING,                 "Warning: slave tried to PSYNC with an offset that is greater                 than the master replication offset.");         }         goto need_full_resync;     }                                                      // 执行部分同步：     // 1）标记客户端为从机     // 2）通知从机准备接收数据。从机收到 +CONTINUE 会做好准备     // 3）开发发送数据     /* If we reached this point, we are able to perform a partial resync:      * 1) Set client state to make it a slave.      * 2) Inform the client we can continue with +CONTINUE      * 3) Send the backlog data (from the offset to the end) to the slave. */                                                      // 将连接的客户端标记为从机     c->flags |= REDIS_SLAVE;                                                      // 表示进行部分同步     // #define REDIS_REPL_ONLINE 9 /* RDB file transmitted, sending just     // updates. */     c->replstate = REDIS_REPL_ONLINE;                                                      // 更新 ack 的时间     c->repl_ack_time = server.unixtime;                                                      // 添加入从机链表     listAddNodeTail(server.slaves, c);                                                      // 告诉从机可以进行部分同步，从机收到后会做相关的准备（注册回调函数）     /* We can't use the connection buffers since they are used to accumulate      * new commands at this stage. But we are sure the socket send buffer is      * emtpy so this write will never fail actually. */     buflen = snprintf(buf, sizeof(buf), "+CONTINUE\r\n");     if (write(c->fd, buf, buflen) != buflen) {         freeClientAsync(c);         return REDIS_OK;     }                                                      // 向从机写积压空间中的数据，积压空间存储有「更新缓存」     psync_len = addReplyReplicationBacklog(c, psync_offset);                                                      redisLog(REDIS_NOTICE,         "Partial resynchronization request accepted. Sending %lld bytes of backlog         starting from offset %lld.", psync_len, psync_offset);                                          /* Note that we don't need to set the selected DB at server.slaveseldb      * to -1 to force the master to emit SELECT, since the slave already      * has this state from the previous connection with the master. */     refreshGoodSlavesCount();     return REDIS_OK; /* The caller can return, no full resync needed. */                                                  need_full_resync:     ......     // 向从机发送 +FULLRESYNC runid repl_offset }

六、暂缓主机

从机因为某些原因，譬如网络延迟（PING 超时，ACK 超时等），可能会断开与主机的连接。这时候，从机会尝试保存与主机连接的信息，譬如全局积压空间数据偏移量等，以便下一次的部分同步，并且从机会再一次尝试连接主机。注意一点，如果断开的时间足够长，部分同步肯定会失败的。

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void freeClient(redisClient *c) {     listNode *ln;                                                                        /* If this is marked as current client unset it */     if (server.current_client == c) server.current_client = NULL;                                                                        // 如果此机为从机，已经连接主机，可能需要保存主机状态信息，以便进行 PSYNC     /* If it is our master that's beging disconnected we should make sure      * to cache the state to try a partial resynchronization later.      *      * Note that before doing this we make sure that the client is not in      * some unexpected state, by checking its flags. */     if (server.master && c->flags & REDIS_MASTER) {         redisLog(REDIS_WARNING,"Connection with master lost.");         if (!(c->flags & (REDIS_CLOSE_AFTER_REPLY|                           REDIS_CLOSE_ASAP|                           REDIS_BLOCKED|                           REDIS_UNBLOCKED)))         {             replicationCacheMaster(c);             return;         }     }     ...... }                                                                    // 为了实现部分同步，从机会保存主机的状态信息后才会断开主机的连接，主机状态信息 // 保存在 server.cached_master // 会在 freeClient() 中调用，保存与主机连接的状态信息，以便进行 PSYNC void replicationCacheMaster(redisClient *c) {     listNode *ln;                                                                        redisAssert(server.master != NULL && server.cached_master == NULL);     redisLog(REDIS_NOTICE,"Caching the disconnected master state.");                                                                        // 从客户端列表删除主机的信息     /* 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. */     ln = listSearchKey(server.clients,c);     redisAssert(ln != NULL);     listDelNode(server.clients,ln);                                                                        // 保存主机的状态信息     /* Save the master. Server.master will be set to null later by      * replicationHandleMasterDisconnection(). */     server.cached_master = server.master;                                                                        // 注销事件，关闭连接     /* Remove the event handlers and close the socket. We'll later reuse      * the socket of the new connection with the master during PSYNC. */     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;                                                                        // 修改连接的状态，设置 server.master = 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(); }

七、总结

简单来说，主从同步就是 RDB 文件的上传下载；主机有小部分的数据修改，就把修改记录传播给每个从机。这篇文章详述了 Redis 主从复制的内部协议和机制。接下来的几篇关于 Redis 的文章，主要是其内部数据结构。