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,选择强一致性还是若已执行,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。
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,这里我们把它成为积压空间。
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]); } } |
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" ); } } ...... } |
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); } } } } |
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 } |
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(); } |
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。
捣乱 2014-4-22
深入剖析 redis 主从复制,布布扣,bubuko.com
原文:http://www.cnblogs.com/daoluanxiaozi/p/3724299.html