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[转载] 深入剖析 redis 主从复制

發布時間：2025/3/20 编程问答 17 豆豆

生活随笔收集整理的這篇文章主要介紹了 [转载] 深入剖析 redis 主从复制小編覺得挺不錯的,現在分享給大家,幫大家做個參考.

轉載自http://www.cnblogs.com/daoluanxiaozi/p/3724299.html

主從概述

redis 支持 master-slave（主從）模式，redis server 可以設置為另一個 redis server 的主機（從機），從機定期從主機拿數據。特殊的，一個從機同樣可以設置為一個 redis server 的主機，這樣一來 master-slave 的分布看起來就是一個有向無環圖 DAG，如此形成 redis server 集群，無論是主機還是從機都是 redis server，都可以提供服務）。

在配置后，主機可負責讀寫服務，從機只負責讀。redis 提高這種配置方式，為的是讓其支持數據的弱一致性，即最終一致性。在業務中，選擇強一致性還是若已執行，應該取決于具體的業務需求，像微博，完全可以使用弱一致性模型；像淘寶，可以選用強一致性模型。

redis 主從復制的實現主要在 replication.c 中。

這篇文章涉及較多的代碼，但我已經盡量刪繁就簡，達到能說明問題本質。為了保留代碼的原生性并讓讀者能夠閱讀原生代碼的注釋，剖析 redis 的幾篇文章都沒有刪除代碼中的英文注釋，并已加注釋。

積壓空間

在《深入剖析 redis AOF 持久化策略》中，介紹了更新緩存的概念，舉一個例子：客戶端發來命令：set name Jhon，這一數據更新被記錄為：*3\r\n$3\r\nSET\r\n$4\r\nname\r\n$3\r\nJhon\r\n，并存儲在更新緩存中。

同樣，在主從連接中，也有更新緩存的概念。只是兩者的用途不一樣，前者被寫入本地，后者被寫入從機，這里我們把它成為積壓空間。

更新緩存存儲在 server.repl_backlog，redis 將其作為一個環形空間來處理，這樣做節省了空間，避免內存再分配的情況。

struct?redisServer { ????/* Replication (master) */ ????// 最近一次使用（訪問）的數據集 ????int?slaveseldb;???????????????? /* Last SELECTed DB in replication output */ ????// 全局的數據同步偏移量 ????long?long?master_repl_offset;?? /* Global replication offset */ ????// 主從連接心跳頻率 ????int?repl_ping_slave_period;???? /* Master pings the slave every N seconds */ ????// 積壓空間指針 ????char?*repl_backlog;???????????? /* Replication backlog for partial syncs */ ????// 積壓空間大小 ????long?long?repl_backlog_size;??? /* Backlog circular buffer size */ ????// 積壓空間中寫入的新數據的大小 ????long?long?repl_backlog_histlen; /* Backlog actual data length */ ????// 下一次向積壓空間寫入數據的起始位置 ????long?long?repl_backlog_idx;???? /* Backlog circular buffer current offset */ ????// 積壓數據的起始位置，是一個宏觀值 ????long?long?repl_backlog_off;???? /* Replication offset of first byte in the ???????????????????????????????????????backlog buffer. */ ????// 積壓空間有效時間 ????time_t?repl_backlog_time_limit; /* Time without slaves after the backlog ???????????????????????????????????????gets released. */ }

積壓空間中的數據變更記錄是什么時候被寫入的？在執行一個 redis 命令的時候，如果存在數據的修改（寫），那么就會把變更記錄傳播。redis 源碼中是這么實現的：call()->propagate()->replicationFeedSlaves()

注釋：命令真正執行的地方在 call() 中，call() 如果發現數據被修改（dirty），則傳播 propagrate()，replicationFeedSlaves() 將修改記錄寫入積壓空間和所有已連接的從機。

這里可能會有疑問：為什么把數據添加入積壓空間，又把數據分發給所有的從機？為什么不僅僅將數據分發給所有從機呢？

因為有一些從機會因特殊情況（？？？）與主機斷開連接，注意從機斷開前有暫存主機的狀態信息，因此這些斷開的從機就沒有及時收到更新的數據。redis 為了讓斷開的從機在下次連接后能夠獲取更新數據，將更新數據加入了積壓空間。從 replicationFeedSlaves() 實現來看，在線的 slave 能馬上收到數據更新記錄；因某些原因暫時斷開連接的 slave，需要從積壓空間中找回斷開期間的數據更新記錄。如果斷開的時間足夠長，master 會拒絕 slave 的部分同步請求，從而 slave 只能進行全同步。

下面是源碼注釋：

// call() 函數是執行命令的核心函數，真正執行命令的地方 /* 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); ????// 臟數據標記，數據是否被修改 ????dirty = server.dirty; ????// 執行命令對應的函數 ????c->cmd->proc(c); ????dirty = server.dirty-dirty; ????duration = ustime()-start; ????...... ????// 將客戶端請求的數據修改記錄傳播給 AOF 和從機 ????/* Propagate the command into the AOF and replication link */ ????if?(flags & REDIS_CALL_PROPAGATE) { ????????int?flags = REDIS_PROPAGATE_NONE; ????????// 強制主從復制 ????????if?(c->flags & REDIS_FORCE_REPL) flags |= REDIS_PROPAGATE_REPL; ????????// 強制 AOF 持久化 ????????if?(c->flags & REDIS_FORCE_AOF) flags |= REDIS_PROPAGATE_AOF; ????????// 數據被修改 ????????if?(dirty) ????????????flags |= (REDIS_PROPAGATE_REPL | REDIS_PROPAGATE_AOF); ????????// 傳播數據修改記錄 ????????if?(flags != REDIS_PROPAGATE_NONE) ????????????propagate(c->cmd,c->db->id,c->argv,c->argc,flags); ????} ????...... } // 向 AOF 和從機發布數據更新 /* 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 策略需要打開，且設置 AOF 傳播標記，將更新發布給本地文件 ????if?(server.aof_state != REDIS_AOF_OFF && flags & REDIS_PROPAGATE_AOF) ????????feedAppendOnlyFile(cmd,dbid,argv,argc); ????// 設置了從機傳播標記，將更新發布給從機 ????if?(flags & REDIS_PROPAGATE_REPL) ????????replicationFeedSlaves(server.slaves,dbid,argv,argc); } // 向積壓空間和從機發送數據 void?replicationFeedSlaves(list *slaves, int?dictid, robj **argv, int?argc) { ????listNode *ln; ????listIter li; ????int?j, len; ????char?llstr[REDIS_LONGSTR_SIZE]; ????// 沒有積壓數據且沒有從機，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 的可以用共享對象 ????????/* For a few DBs we have pre-computed SELECT command. */ ????????if?(dictid >= 0 && dictid < REDIS_SHARED_SELECT_CMDS) { ????????????selectcmd = shared.select[dictid]; ????????} else?{ ????????// 不能使用共享對象，生成 SELECT 命令對應的 redis 對象 ????????????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)); ????????} ????????// 這里可能會有疑問：為什么把數據添加入積壓空間，又把數據分發給所有的從機？ ????????// 為什么不僅僅將數據分發給所有從機呢？ ????????// 因為有一些從機會因特殊情況（？？？）與主機斷開連接，注意從機斷開前有暫存 ????????// 主機的狀態信息，因此這些斷開的從機就沒有及時收到更新的數據。redis 為了讓 ????????// 斷開的從機在下次連接后能夠獲取更新數據，將更新數據加入了積壓空間。 ????????// 將 SELECT 命令對應的 redis 對象數據添加到積壓空間 ????????/* Add the SELECT command into the backlog. */ ????????if?(server.repl_backlog) feedReplicationBacklogWithObject(selectcmd); ????????// 將數據分發所有的從機 ????????/* Send it to slaves. */ ????????listRewind(slaves,&li); ????????while((ln = listNext(&li))) { ????????????redisClient *slave = ln->value; ????????????addReply(slave,selectcmd); ????????} ????????// 銷毀對象 ????????if?(dictid < 0 || dictid >= REDIS_SHARED_SELECT_CMDS) ????????????decrRefCount(selectcmd); ????} ????// 更新最近一次使用（訪問）的數據集 ????server.slaveseldb = dictid; ????// 將命令寫入積壓空間 ????/* Write the command to the replication backlog if any. */ ????if?(server.repl_backlog) { ????????char?aux[REDIS_LONGSTR_SIZE+3]; ????????// 命令個數 ????????/* 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); ????????// 逐個命令寫入 ????????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'; ????????????/* 每個命令格式如下： ????????????$3 ????????????*3 ????????????SET ????????????*4 ????????????NAME ????????????*4 ????????????Jhon*/ ????????????// 命令長度 ????????????feedReplicationBacklog(aux,len+3); ????????????// 命令 ????????????feedReplicationBacklogWithObject(argv[j]); ????????????// 換行 ????????????feedReplicationBacklog(aux+len+1,2); ????????} ????} ????// 立即給每一個從機發送命令 ????/* Write the command to every slave. */ ????listRewind(slaves,&li); ????while((ln = listNext(&li))) { ????????redisClient *slave = ln->value; ????????// 如果從機要求全同步，則不對此從機發送數據 ????????/* 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. */ ????????// 向從機命令的長度 ????????/* Add the multi bulk length. */ ????????addReplyMultiBulkLen(slave,argc); ????????// 向從機發送命令 ????????/* 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]); ????} }

主從數據同步機制概述

redis 主從同步有兩種方式（或者所兩個階段）：全同步和部分同步。

主從剛剛連接的時候，進行全同步；全同步結束后，進行部分同步。當然，如果有需要，slave 在任何時候都可以發起全同步。redis 策略是，無論如何，首先會嘗試進行部分同步，如不成功，要求從機進行全同步，并啟動 BGSAVE……BGSAVE 結束后，傳輸 RDB 文件；如果成功，允許從機進行部分同步，并傳輸積壓空間中的數據。

下面這幅圖，總結了主從同步的機制：

如需設置 slave，master 需要向 slave 發送 SLAVEOF hostname port，從機接收到后會自動連接主機，注冊相應讀寫事件（syncWithMaster())。

// 修改主機 void?slaveofCommand(redisClient *c) { ????if?(!strcasecmp(c->argv[1]->ptr,"no") && ????????!strcasecmp(c->argv[2]->ptr,"one")) { ????????// slaveof no one 斷開主機連接 ????????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; ????????// 可能已經連接需要連接的主機 ????????/* 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; ????????} ????????// 斷開之前連接主機的連接，連接新的。 replicationSetMaster() 并不會真正連接主機，只是修改 struct server 中關于主機的設置。真正的主機連接在 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); } // 設置新主機 /* 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; ????// 斷開之前主機的連接 ????if?(server.master) freeClient(server.master); ????disconnectSlaves(); /* Force our slaves to resync with us as well. */ ????// 取消緩存主機 ????replicationDiscardCachedMaster(); /* Don't try a PSYNC. */ ????// 釋放積壓空間 ????freeReplicationBacklog(); /* Don't allow our chained slaves to PSYNC. */ ????// cancelReplicationHandshake() 嘗試斷開數據傳輸和主機連接 ????cancelReplicationHandshake(); ????server.repl_state = REDIS_REPL_CONNECT; ????server.master_repl_offset = 0; } // 管理主從連接的定時程序定時程序，每秒執行一次 // 在 serverCorn() 中調用 /* --------------------------- REPLICATION CRON? ----------------------------- */ /* Replication cron funciton, called 1 time per second. */ void?replicationCron(void) { ????...... ????// 如果需要（ REDIS_REPL_CONNECT），嘗試連接主機，真正連接主機的操作在這里 ????/* 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"); ????????} ????} ????...... }

全同步

接著自動發起 PSYNC 請求 master 進行全同步。無論如何，redis 首先會嘗試部分同步，如果失敗才嘗試全同步。而剛剛建立連接的 master-slave 需要全同步。

從機連接主機后，會主動發起 PSYNC 命令，從機會提供 master_runid 和 offset，主機驗證 master_runid 和 offset 是否有效？master_runid 相當于主機身份驗證碼，用來驗證從機上一次連接的主機，offset 是全局積壓空間數據的偏移量。
驗證未通過則，則進行全同步：主機返回 +FULLRESYNC master_runid offset（從機接收并記錄 master_runid 和 offset，并準備接收 RDB 文件）接著啟動 BGSAVE 生成 RDB 文件，BGSAVE 結束后，向從機傳輸，從而完成全同步。

全同步請求的數據是 RDB 數據文件和積壓空間中的數據。關于 RDB 數據文件，請參看《深入剖析 redis RDB 持久化策略》。如果沒有后臺持久化 BGSAVE 進程，那么 BGSVAE 會被觸發，否則所有請求全同步的 slave 都會被標記為等待 BGSAVE 結束。BGSAVE 結束后，master 會馬上向所有的從機發送 RDB 文件。

// 主機 SYNC 和 PSYNC 命令處理函數，會嘗試進行部分同步和全同步 /* SYNC ad PSYNC command implemenation. */ void?syncCommand(redisClient *c) { ????...... ????// 主機嘗試部分同步，失敗的話向從機發送 +FULLRESYNC master_runid offset，接著啟動 BGSAVE ????// 執行全同步： ????/* 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 后臺進程。 ????????1.如果 master 現有所連接的所有從機 slaves 當中有存在 REDIS_REPL_WAIT_BGSAVE_END 的從機，那么將從機 c 設置為 REDIS_REPL_WAIT_BGSAVE_END； ????????2.否則，設置為 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; ????????// 檢測是否已經有從機申請全同步 ????????listRewind(server.slaves,&li); ????????while((ln = listNext(&li))) { ????????????slave = ln->value; ????????????if?(slave->replstate == REDIS_REPL_WAIT_BGSAVE_END) break; ????????} ????????if?(ln) { ????????// 存在狀態為 REDIS_REPL_WAIT_BGSAVE_END 的從機 slave， ????????// 就將此從機 c 狀態設置為 REDIS_REPL_WAIT_BGSAVE_END， ????????// 從而在 BGSAVE 進程結束后，可以發送 RDB 文件， ????????// 同時將從機 slave 中的更新復制到此從機 c。 ????????????/* Perfect, the server is already registering differences for ?????????????* another slave. Set the right state, and copy the buffer. */ ????????????// 將其他從機上的待回復的緩存復制到從機 c ????????????copyClientOutputBuffer(c,slave); ????????????// 修改從機 c 狀態為「等待 BGSAVE 進程結束」 ????????????c->replstate = REDIS_REPL_WAIT_BGSAVE_END; ????????????redisLog(REDIS_NOTICE,"Waiting for end of BGSAVE for SYNC"); ????????} else?{ ????????// 不存在狀態為 REDIS_REPL_WAIT_BGSAVE_END 的從機，就將此從機 c 狀態設置為 REDIS_REPL_WAIT_BGSAVE_START，即等待新的 BGSAVE 進程的開啟。 ????????????// 修改狀態為「等待 BGSAVE 進程開始」 ????????????/* 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 后臺進程，啟動一個新的 BGSAVE 進程 ????????/* 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; ????????} ????????// 將此從機 c 狀態設置為 REDIS_REPL_WAIT_BGSAVE_END，從而在 BGSAVE 進程結束后，可以發送 RDB 文件，同時將從機 slave 中的更新復制到此從機 c。 ????????c->replstate = REDIS_REPL_WAIT_BGSAVE_END; ????????// 清理腳本緩存？？？ ????????/* 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 結束后，會調用 /* A background saving child (BGSAVE) terminated its work. Handle this. */ void?backgroundSaveDoneHandler(int?exitcode, int?bysignal) { ????// 其他操作 ????...... ????// 可能從機正在等待 BGSAVE 進程的終止 ????/* 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); } // 當 RDB 持久化(backgroundSaveDoneHandler())結束后，會調用此函數 // RDB 文件就緒，給所有的從機發送 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 開始。調整狀態為等待下一次 BGSAVE 進程的結束 ????????if?(slave->replstate == REDIS_REPL_WAIT_BGSAVE_START) { ????????????startbgsave = 1; ????????????slave->replstate = REDIS_REPL_WAIT_BGSAVE_END; ????????// 等待 BGSAVE 結束。準備向 slave 發送 RDB 文件 ????????} else?if?(slave->replstate == REDIS_REPL_WAIT_BGSAVE_END) { ????????????struct?redis_stat buf; ????????????// 如果 RDB 持久化失敗， bgsaveerr 會被設置為 REDIS_ERR ????????????if?(bgsaveerr != REDIS_OK) { ????????????????freeClient(slave); ????????????????redisLog(REDIS_WARNING,"SYNC failed. BGSAVE child returned an error"); ????????????????continue; ????????????} ????????????// 打開 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; ????????????// 如果之前有注冊寫事件，取消 ????????????aeDeleteFileEvent(server.el,slave->fd,AE_WRITABLE); ????????????// 注冊新的寫事件,sendBulkToSlave() 傳輸 RDB 文件 ????????????if?(aeCreateFileEvent(server.el, slave->fd, AE_WRITABLE, sendBulkToSlave, slave) == AE_ERR) { ????????????????freeClient(slave); ????????????????continue; ????????????} ????????} ????} ????// startbgsave == REDIS_ERR 表示 BGSAVE 失敗，再一次進行 BGSAVE 嘗試 ????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 失敗，所有等待 BGSAVE 的從機都將結束連接。這是 redis 自我保護的措施，fork 失敗很可能是內存緊張*/ ????????????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); ????????????} ????????} ????} }

部分同步

如上所說，無論如何，redis 首先會嘗試部分同步。部分同步即把積壓空間緩存的數據，即更新記錄發送給從機。

從機連接主機后，會主動發起 PSYNC 命令，從機會提供 master_runid 和 offset，主機驗證 master_runid 和 offset 是否有效？
驗證通過則，進行部分同步：主機返回 +CONTINUE（從機接收后會注冊積壓數據接收事件），接著發送積壓空間數據。

// 連接主機 connectWithMaster() 的時候，會被注冊為回調函數 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); ????...... ????// 嘗試部分同步，主機允許進行部分同步會返回 +CONTINUE，從機接收后注冊相應的事件 ????/* 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. */ ????// 函數返回三種狀態： ????// PSYNC_CONTINUE：表示會進行部分同步，在 slaveTryPartialResynchronization() ?????????????????????// 中已經設置回調函數 readQueryFromClient() ????// PSYNC_FULLRESYNC：全同步，會下載 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; ????} ????// 執行全同步 ????...... } // 函數返回三種狀態： // PSYNC_CONTINUE：表示會進行部分同步，已經設置回調函數 // PSYNC_FULLRESYNC：全同步，會下載 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) { ????// 緩存了上一次與主機連接的信息，可以嘗試進行部分同步，減少數據傳輸 ????????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?{ ????// 未緩存上一次與主機連接的信息，進行全同步 ????// psync ? -1 可以獲取主機的 master_runid ????????redisLog(REDIS_NOTICE,"Partial resynchronization not possible (no cached master)"); ????????psync_runid = "?"; ????????memcpy(psync_offset,"-1",3); ????} ????// 向主機發送命令，并接收回復 ????/* 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?{ ????????????// 拷貝 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); ????????// 緩存主機替代現有主機，且為 PSYNC（部分同步）做好準備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. */ ????// 接收到主機發出的錯誤信息 ????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; } // 主機 SYNC 和 PSYNC 命令處理函數，會嘗試進行部分同步和全同步 /* SYNC ad PSYNC command implemenation. */ void?syncCommand(redisClient *c) { ????...... ????// 主機嘗試部分同步，允許則進行部分同步，會返回 +CONTINUE，接著發送積壓空間 ????/* 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?{ ????????// 部分同步失敗，會進行全同步，這時會收到來自客戶端的 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; ????} ????// 執行全同步： ????...... } // 主機嘗試是否能進行部分同步 /* 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)) { ????// 當因為異常需要與主機斷開連接的時候，從機會暫存主機的狀態信息，以便 ????// 下一次的部分同步。 ????// 1）master_runid 是從機提供一個因緩存主機的 runid， ????// 2）server.runid 是本機（主機）的 runid。 ????// 匹配失敗，說明是本機（主機）不是從機緩存的主機，這時候不能進行部分同步， ????// 只能進行全同步 ????????// "?" 表示從機要求全同步 ????????// 什么時候從機會要求全同步？？？ ????????/* 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; ????} ????// 從參數中解析整數，整數是從機指定的偏移量 ????/* 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; ????// 部分同步失敗的情況 ????if?(!server.repl_backlog || /*不存在積壓空間*/ ????????psync_offset < server.repl_backlog_off ||? /*psync_offset 太過小， ????????????????????????????????????????????????????即從機錯過太多更新記錄， ????????????????????????????????????????????????????安全起見，實行全同步*/ ????????????????????????????????????????????????????/*psync_offset 越界*/ ????????psync_offset > (server.repl_backlog_off + server.repl_backlog_histlen)) ????// 經檢測，不滿足部分同步的條件，轉而進行全同步 ????{ ????????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; ????} ????// 執行部分同步： ????// 1）標記客戶端為從機 ????// 2）通知從機準備接收數據。從機收到 +CONTINUE 會做好準備 ????// 3）開發發送數據 ????/* 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. */ ????// 將連接的客戶端標記為從機 ????c->flags |= REDIS_SLAVE; ????// 表示進行部分同步 ????// #define REDIS_REPL_ONLINE 9 /* RDB file transmitted, sending just ????// updates. */ ????c->replstate = REDIS_REPL_ONLINE; ????// 更新 ack 的時間 ????c->repl_ack_time = server.unixtime; ????// 添加入從機鏈表 ????listAddNodeTail(server.slaves,c); ????// 告訴從機可以進行部分同步，從機收到后會做相關的準備（注冊回調函數） ????/* 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; ????} ????// 向從機寫積壓空間中的數據，積壓空間存儲有「更新緩存」 ????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: ????...... ????// 向從機發送 +FULLRESYNC runid repl_offset }

暫緩主機

從機因為某些原因，譬如網絡延遲（PING 超時，ACK 超時等），可能會斷開與主機的連接。這時候，從機會嘗試保存與主機連接的信息，譬如全局積壓空間數據偏移量等，以便下一次的部分同步，并且從機會再一次嘗試連接主機。注意一點，如果斷開的時間足夠長，部分同步肯定會失敗的。

void?freeClient(redisClient *c) { ????listNode *ln; ????/* If this is marked as current client unset it */ ????if?(server.current_client == c) server.current_client = NULL; ????// 如果此機為從機，已經連接主機，可能需要保存主機狀態信息，以便進行 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; ????????} ????} ????...... } // 為了實現部分同步，從機會保存主機的狀態信息后才會斷開主機的連接，主機狀態信息 // 保存在 server.cached_master // 會在 freeClient() 中調用，保存與主機連接的狀態信息，以便進行 PSYNC void?replicationCacheMaster(redisClient *c) { ????listNode *ln; ????redisAssert(server.master != NULL && server.cached_master == NULL); ????redisLog(REDIS_NOTICE,"Caching the disconnected master state."); ????// 從客戶端列表刪除主機的信息 ????/* 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); ????// 保存主機的狀態信息 ????/* Save the master. Server.master will be set to null later by ?????* replicationHandleMasterDisconnection(). */ ????server.cached_master = server.master; ????// 注銷事件，關閉連接 ????/* 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; ????// 修改連接的狀態，設置 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(); }

總結

簡單來說，主從同步就是 RDB 文件的上傳下載；主機有小部分的數據修改，就把修改記錄傳播給每個從機。這篇文章詳述了 redis 主從復制的內部協議和機制。接下來的幾篇關于 redis 的文章，主要是其內部數據結構。

搗亂 2014-4-22

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轉載于:https://www.cnblogs.com/scott19820130/p/4916038.html

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