Redis数据持久化机制AOF原理分析一

本文所引用的源码全部来自Redis2.8.2版本。 Redis AOF数据持久化机制的实现相关代码是redis.c, redis.h, aof.c, bio.c, rio.c, config.c 在阅读本文之前请先阅读Redis数据持久化机制AOF原理分析之配置详解文章，了解AOF相关参数的解析，文章链接 http://blog

本文所引用的源码全部来自Redis2.8.2版本。

Redis AOF数据持久化机制的实现相关代码是redis.c, redis.h, aof.c, bio.c, rio.c, config.c

在阅读本文之前请先阅读Redis数据持久化机制AOF原理分析之配置详解文章，了解AOF相关参数的解析，文章链接

http://blog.csdn.net/acceptedxukai/article/details/18135219

转载请注明，文章出自http://blog.csdn.net/acceptedxukai/article/details/18136903

下面将介绍AOF数据持久化机制的实现

Server启动加载AOF文件数据

Server启动加载AOF文件数据的执行步骤为:main() -> initServerConfig() -> loadServerConfig() -> initServer() -> loadDataFromDisk()。initServerConfig()主要为初始化默认的AOF参数配置；loadServerConfig()加载配置文件redis.conf中AOF的参数配置，覆盖Server的默认AOF参数配置，如果配置appendonly on，那么AOF数据持久化功能将被激活，server.aof_state参数被设置为REDIS_AOF_ON；loadDataFromDisk()判断server.aof_state == REDIS_AOF_ON，结果为True就调用loadAppendOnlyFile函数加载AOF文件中的数据，加载的方法就是读取AOF文件中数据，由于AOF文件中存储的数据与客户端发送的请求格式相同完全符合Redis的通信协议，因此Server创建伪客户端fakeClient，将解析后的AOF文件数据像客户端请求一样调用各种指令，cmd->proc(fakeClient)，将AOF文件中的数据重现到Redis Server数据库中。

/* Function called at startup to load RDB or AOF file in memory. */
void loadDataFromDisk(void) {
    long long start = ustime();
    if (server.aof_state == REDIS_AOF_ON) {
        if (loadAppendOnlyFile(server.aof_filename) == REDIS_OK)
            redisLog(REDIS_NOTICE,"DB loaded from append only file: %.3f seconds",(float)(ustime()-start)/1000000);
    } else {
        if (rdbLoad(server.rdb_filename) == REDIS_OK) {
            redisLog(REDIS_NOTICE,"DB loaded from disk: %.3f seconds",
                (float)(ustime()-start)/1000000);
        } else if (errno != ENOENT) {
            redisLog(REDIS_WARNING,"Fatal error loading the DB: %s. Exiting.",strerror(errno));
            exit(1);
        }
    }
}

Server首先判断加载AOF文件是因为AOF文件中的数据要比RDB文件中的数据要新。

int loadAppendOnlyFile(char *filename) {
    struct redisClient *fakeClient;
    FILE *fp = fopen(filename,"r");
    struct redis_stat sb;
    int old_aof_state = server.aof_state;
    long loops = 0;

    //redis_fstat就是fstat64函数，通过fileno(fp)得到文件描述符，获取文件的状态存储于sb中，
    //具体可以参考stat函数，st_size就是文件的字节数
    if (fp && redis_fstat(fileno(fp),&sb) != -1 && sb.st_size == 0) {
        server.aof_current_size = 0;
        fclose(fp);
        return REDIS_ERR;
    }

    if (fp == NULL) {//打开文件失败
        redisLog(REDIS_WARNING,"Fatal error: can't open the append log file for reading: %s",strerror(errno));
        exit(1);
    }

    /* Temporarily disable AOF, to prevent EXEC from feeding a MULTI
     * to the same file we're about to read. */
    server.aof_state = REDIS_AOF_OFF;

    fakeClient = createFakeClient(); //建立伪终端
    startLoading(fp); // 定义于 rdb.c ，更新服务器的载入状态

    while(1) {
        int argc, j;
        unsigned long len;
        robj **argv;
        char buf[128];
        sds argsds;
        struct redisCommand *cmd;

        /* Serve the clients from time to time */
        // 有间隔地处理外部请求，ftello()函数得到文件的当前位置，返回值为long
        if (!(loops++ % 1000)) {
            loadingProgress(ftello(fp));//保存aof文件读取的位置，ftellno(fp)获取文件当前位置
            aeProcessEvents(server.el, AE_FILE_EVENTS|AE_DONT_WAIT);//处理事件
        }
        //按行读取AOF数据
        if (fgets(buf,sizeof(buf),fp) == NULL) {
            if (feof(fp))//达到文件尾EOF
                break;
            else
                goto readerr;
        }
        //读取AOF文件中的命令，依照Redis的协议处理
        if (buf[0] != '*') goto fmterr;
        argc = atoi(buf+1);//参数个数
        if (argc < 1) goto fmterr;

        argv = zmalloc(sizeof(robj*)*argc);//参数值
        for (j = 0; j < argc; j++) {
            if (fgets(buf,sizeof(buf),fp) == NULL) goto readerr;
            if (buf[0] != &#39;$&#39;) goto fmterr;
            len = strtol(buf+1,NULL,10);//每个bulk的长度
            argsds = sdsnewlen(NULL,len);//新建一个空sds
            //按照bulk的长度读取
            if (len && fread(argsds,len,1,fp) == 0) goto fmterr;
            argv[j] = createObject(REDIS_STRING,argsds);
            if (fread(buf,2,1,fp) == 0) goto fmterr; /* discard CRLF 跳过\r\n*/
        }

        /* Command lookup */
        cmd = lookupCommand(argv[0]->ptr);
        if (!cmd) {
            redisLog(REDIS_WARNING,"Unknown command &#39;%s&#39; reading the append only file", (char*)argv[0]->ptr);
            exit(1);
        }
        /* Run the command in the context of a fake client */
        fakeClient->argc = argc;
        fakeClient->argv = argv;
        cmd->proc(fakeClient);//执行命令

        /* The fake client should not have a reply */
        redisAssert(fakeClient->bufpos == 0 && listLength(fakeClient->reply) == 0);
        /* The fake client should never get blocked */
        redisAssert((fakeClient->flags & REDIS_BLOCKED) == 0);

        /* Clean up. Command code may have changed argv/argc so we use the
         * argv/argc of the client instead of the local variables. */
        for (j = 0; j < fakeClient->argc; j++)
            decrRefCount(fakeClient->argv[j]);
        zfree(fakeClient->argv);
    }

    /* This point can only be reached when EOF is reached without errors.
     * If the client is in the middle of a MULTI/EXEC, log error and quit. */
    if (fakeClient->flags & REDIS_MULTI) goto readerr;

    fclose(fp);
    freeFakeClient(fakeClient);
    server.aof_state = old_aof_state;
    stopLoading();
    aofUpdateCurrentSize(); //更新server.aof_current_size，AOF文件大小
    server.aof_rewrite_base_size = server.aof_current_size;
    return REDIS_OK;
    &hellip;&hellip;&hellip;&hellip;
}

在前面一篇关于AOF参数配置的博客遗留了一个问题，server.aof_current_size参数的初始化，下面解决这个疑问。

void aofUpdateCurrentSize(void) {
    struct redis_stat sb;

    if (redis_fstat(server.aof_fd,&sb) == -1) {
        redisLog(REDIS_WARNING,"Unable to obtain the AOF file length. stat: %s",
            strerror(errno));
    } else {
        server.aof_current_size = sb.st_size;
    }
}

redis_fstat是作者对Linux中fstat64函数的重命名，该还是就是获取文件相关的参数信息，具体可以Google之，sb.st_size就是当前AOF文件的大小。这里需要知道server.aof_fd即AOF文件描述符，该参数的初始化在initServer()函数中

/* Open the AOF file if needed. */
    if (server.aof_state == REDIS_AOF_ON) {
        server.aof_fd = open(server.aof_filename,O_WRONLY|O_APPEND|O_CREAT,0644);
        if (server.aof_fd == -1) {
            redisLog(REDIS_WARNING, "Can't open the append-only file: %s",strerror(errno));
            exit(1);
        }
    }

至此，Redis Server启动加载硬盘中AOF文件数据的操作就成功结束了。

Server数据库产生新数据如何持久化到硬盘

当客户端执行Set等修改数据库中字段的指令时就会造成Server数据库中数据被修改，这些修改的数据应该被实时更新到AOF文件中，并且也要按照一定的fsync机制刷新到硬盘中，保证数据不会丢失。

在上一篇博客中，提到了三种fsync方式:appendfsync always, appendfsync everysec, appendfsync no. 具体体现在server.aof_fsync参数中。

首先看当客户端请求的指令造成数据被修改，Redis是如何将修改数据的指令添加到server.aof_buf中的。

call() -> propagate() -> feedAppendOnlyFile()，call()函数判断执行指令后是否造成数据被修改。

feedAppendOnlyFile函数首先会判断Server是否开启了AOF，如果开启AOF，那么根据Redis通讯协议将修改数据的指令重现成请求的字符串，注意在超时设置的处理方式，接着将字符串append到server.aof_buf中即可。该函数最后两行代码需要注意，这才是重点，如果server.aof_child_pid != -1那么表明此时Server正在重写rewrite AOF文件，需要将被修改的数据追加到server.aof_rewrite_buf_blocks链表中，等待rewrite结束后，追加到AOF文件中。具体见下面代码的注释。

/* 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)
{
    //将cmd指令变动的数据追加到AOF文件中
    if (server.aof_state != REDIS_AOF_OFF && flags & REDIS_PROPAGATE_AOF)
        feedAppendOnlyFile(cmd,dbid,argv,argc);
    if (flags & REDIS_PROPAGATE_REPL)
        replicationFeedSlaves(server.slaves,dbid,argv,argc);
}

//cmd指令修改了数据，先将更新的数据写到server.aof_buf中
void feedAppendOnlyFile(struct redisCommand *cmd, int dictid, robj **argv, int argc) {
    sds buf = sdsempty();
    robj *tmpargv[3];

    /* The DB this command was targeting is not the same as the last command
     * we appendend. To issue a SELECT command is needed. */
    // 当前 db 不是指定的 aof db，通过创建 SELECT 命令来切换数据库
    if (dictid != server.aof_selected_db) {
        char seldb[64];

        snprintf(seldb,sizeof(seldb),"%d",dictid);
        buf = sdscatprintf(buf,"*2\r\n$6\r\nSELECT\r\n$%lu\r\n%s\r\n",
            (unsigned long)strlen(seldb),seldb);
        server.aof_selected_db = dictid;
    }

    // 将 EXPIRE / PEXPIRE / EXPIREAT 命令翻译为 PEXPIREAT 命令
    if (cmd->proc == expireCommand || cmd->proc == pexpireCommand ||
        cmd->proc == expireatCommand) {
        /* Translate EXPIRE/PEXPIRE/EXPIREAT into PEXPIREAT */
        buf = catAppendOnlyExpireAtCommand(buf,cmd,argv[1],argv[2]);
    }// 将 SETEX / PSETEX 命令翻译为 SET 和 PEXPIREAT 组合命令
    else if (cmd->proc == setexCommand || cmd->proc == psetexCommand) {
        /* Translate SETEX/PSETEX to SET and PEXPIREAT */
        tmpargv[0] = createStringObject("SET",3);
        tmpargv[1] = argv[1];
        tmpargv[2] = argv[3];
        buf = catAppendOnlyGenericCommand(buf,3,tmpargv);
        decrRefCount(tmpargv[0]);
        buf = catAppendOnlyExpireAtCommand(buf,cmd,argv[1],argv[2]);
    } else {//其他的指令直接追加
        /* All the other commands don't need translation or need the
         * same translation already operated in the command vector
         * for the replication itself. */
        buf = catAppendOnlyGenericCommand(buf,argc,argv);
    }

    /* Append to the AOF buffer. This will be flushed on disk just before
     * of re-entering the event loop, so before the client will get a
     * positive reply about the operation performed. */
    // 将 buf 追加到服务器的 aof_buf 末尾，在beforeSleep中写到AOF文件中，并且根据情况fsync刷新到硬盘
    if (server.aof_state == REDIS_AOF_ON)
        server.aof_buf = sdscatlen(server.aof_buf,buf,sdslen(buf));

    /* If a background append only file rewriting is in progress we want to
     * accumulate the differences between the child DB and the current one
     * in a buffer, so that when the child process will do its work we
     * can append the differences to the new append only file. */
    //如果server.aof_child_pid不为1，那就说明有快照进程正在写数据到临时文件(已经开始rewrite)，
    //那么必须先将这段时间接收到的指令更新的数据先暂时存储起来，等到快照进程完成任务后，
    //将这部分数据写入到AOF文件末尾，保证数据不丢失
    //解释为什么需要aof_rewrite_buf_blocks，当server在进行rewrite时即读取所有数据库中的数据，
    //有些数据已经写到新的AOF文件，但是此时客户端执行指令又将该值修改了，因此造成了差异
    if (server.aof_child_pid != -1)
        aofRewriteBufferAppend((unsigned char*)buf,sdslen(buf));
    /*这里说一下server.aof_buf和server.aof_rewrite_buf_blocks的区别
      aof_buf是正常情况下aof文件打开的时候，会不断将这份数据写入到AOF文件中。
      aof_rewrite_buf_blocks 是如果用户主动触发了写AOF文件的命令时，比如 config set appendonly yes命令
      那么redis会fork创建一个后台进程,也就是当时的数据快照，然后将数据写入到一个临时文件中去。
      在此期间发送的命令，我们需要把它们记录起来，等后台进程完成AOF临时文件写后，serverCron定时任务
      感知到这个退出动作，然后就会调用backgroundRewriteDoneHandler进而调用aofRewriteBufferWrite函数，
      将aof_rewrite_buf_blocks上面的数据，也就是diff数据写入到临时AOF文件中，然后再unlink替换正常的AOF文件。
      因此可以知道，aof_buf一般情况下比aof_rewrite_buf_blocks要少，
      但开始的时候可能aof_buf包含一些后者不包含的前面部分数据。*/

    sdsfree(buf);
}

Server在每次事件循环之前会调用一次beforeSleep函数，下面看看这个函数做了什么工作？

/* This function gets called every time Redis is entering the
 * main loop of the event driven library, that is, before to sleep
 * for ready file descriptors. */
void beforeSleep(struct aeEventLoop *eventLoop) {
    REDIS_NOTUSED(eventLoop);
    listNode *ln;
    redisClient *c;

    /* Run a fast expire cycle (the called function will return
     * ASAP if a fast cycle is not needed). */
    if (server.active_expire_enabled && server.masterhost == NULL)
        activeExpireCycle(ACTIVE_EXPIRE_CYCLE_FAST);

    /* Try to process pending commands for clients that were just unblocked. */
    while (listLength(server.unblocked_clients)) {
        ln = listFirst(server.unblocked_clients);
        redisAssert(ln != NULL);
        c = ln->value;
        listDelNode(server.unblocked_clients,ln);
        c->flags &= ~REDIS_UNBLOCKED;

        /* Process remaining data in the input buffer. */
        //处理客户端在阻塞期间接收到的客户端发送的请求
        if (c->querybuf && sdslen(c->querybuf) > 0) {
            server.current_client = c;
            processInputBuffer(c);
            server.current_client = NULL;
        }
    }

    /* Write the AOF buffer on disk */
    //将server.aof_buf中的数据追加到AOF文件中并fsync到硬盘上
    flushAppendOnlyFile(0);
}

通过上面的代码及注释可以发现，beforeSleep函数做了三件事：1、处理过期键，2、处理阻塞期间的客户端请求，3、将server.aof_buf中的数据追加到AOF文件中并fsync刷新到硬盘上，flushAppendOnlyFile函数给定了一个参数force，表示是否强制写入AOF文件，0表示非强制即支持延迟写，1表示强制写入。

void flushAppendOnlyFile(int force) {
    ssize_t nwritten;
    int sync_in_progress = 0;
    if (sdslen(server.aof_buf) == 0) return;
    // 返回后台正在等待执行的 fsync 数量
    if (server.aof_fsync == AOF_FSYNC_EVERYSEC)
        sync_in_progress = bioPendingJobsOfType(REDIS_BIO_AOF_FSYNC) != 0;

    // AOF 模式为每秒 fsync ，并且 force 不为 1 如果可以的话，推延冲洗
    if (server.aof_fsync == AOF_FSYNC_EVERYSEC && !force) {
        /* With this append fsync policy we do background fsyncing.
         * If the fsync is still in progress we can try to delay
         * the write for a couple of seconds. */
        // 如果 aof_fsync 队列里已经有正在等待的任务
        if (sync_in_progress) {
            // 上一次没有推迟冲洗过，记录推延的当前时间，然后返回
            if (server.aof_flush_postponed_start == 0) {
                /* No previous write postponinig, remember that we are
                 * postponing the flush and return. */
                server.aof_flush_postponed_start = server.unixtime;
                return;
            } else if (server.unixtime - server.aof_flush_postponed_start < 2) {
                // 允许在两秒之内的推延冲洗
                /* We were already waiting for fsync to finish, but for less
                 * than two seconds this is still ok. Postpone again. */
                return;
            }
            /* Otherwise fall trough, and go write since we can&#39;t wait
             * over two seconds. */
            server.aof_delayed_fsync++;
            redisLog(REDIS_NOTICE,"Asynchronous AOF fsync is taking too long (disk is busy?). Writing the AOF buffer without waiting for fsync to complete, this may slow down Redis.");
        }
    }
    /* If you are following this code path, then we are going to write so
     * set reset the postponed flush sentinel to zero. */
    server.aof_flush_postponed_start = 0;

    /* We want to perform a single write. This should be guaranteed atomic
     * at least if the filesystem we are writing is a real physical one.
     * While this will save us against the server being killed I don&#39;t think
     * there is much to do about the whole server stopping for power problems
     * or alike */
    // 将 AOF 缓存写入到文件，如果一切幸运的话，写入会原子性地完成
    nwritten = write(server.aof_fd,server.aof_buf,sdslen(server.aof_buf));
    if (nwritten != (signed)sdslen(server.aof_buf)) {//出错
        /* Ooops, we are in troubles. The best thing to do for now is
         * aborting instead of giving the illusion that everything is
         * working as expected. */
        if (nwritten == -1) {
            redisLog(REDIS_WARNING,"Exiting on error writing to the append-only file: %s",strerror(errno));
        } else {
            redisLog(REDIS_WARNING,"Exiting on short write while writing to "
                                   "the append-only file: %s (nwritten=%ld, "
                                   "expected=%ld)",
                                   strerror(errno),
                                   (long)nwritten,
                                   (long)sdslen(server.aof_buf));

            if (ftruncate(server.aof_fd, server.aof_current_size) == -1) {
                redisLog(REDIS_WARNING, "Could not remove short write "
                         "from the append-only file.  Redis may refuse "
                         "to load the AOF the next time it starts.  "
                         "ftruncate: %s", strerror(errno));
            }
        }
        exit(1);
    }
    server.aof_current_size += nwritten;

    /* Re-use AOF buffer when it is small enough. The maximum comes from the
     * arena size of 4k minus some overhead (but is otherwise arbitrary). */
    // 如果 aof 缓存不是太大，那么重用它，否则，清空 aof 缓存
    if ((sdslen(server.aof_buf)+sdsavail(server.aof_buf)) < 4000) {
        sdsclear(server.aof_buf);
    } else {
        sdsfree(server.aof_buf);
        server.aof_buf = sdsempty();
    }

    /* Don&#39;t fsync if no-appendfsync-on-rewrite is set to yes and there are
     * children doing I/O in the background. */
    //aof rdb子进程运行中不支持fsync并且aof rdb子进程正在运行，那么直接返回，
    //但是数据已经写到aof文件中，只是没有刷新到硬盘
    if (server.aof_no_fsync_on_rewrite &&
        (server.aof_child_pid != -1 || server.rdb_child_pid != -1))
            return;

    /* Perform the fsync if needed. */
    if (server.aof_fsync == AOF_FSYNC_ALWAYS) {//总是fsync，那么直接进行fsync
        /* aof_fsync is defined as fdatasync() for Linux in order to avoid
         * flushing metadata. */
        aof_fsync(server.aof_fd); /* Let&#39;s try to get this data on the disk */
        server.aof_last_fsync = server.unixtime;
    } else if ((server.aof_fsync == AOF_FSYNC_EVERYSEC &&
                server.unixtime > server.aof_last_fsync)) {
        if (!sync_in_progress) aof_background_fsync(server.aof_fd);//放到后台线程进行fsync
        server.aof_last_fsync = server.unixtime;
    }
}

上述代码中请关注server.aof_fsync参数，即设置Redis fsync AOF文件到硬盘的策略，如果设置为AOF_FSYNC_ALWAYS，那么直接在主进程中fsync，如果设置为AOF_FSYNC_EVERYSEC，那么放入后台线程中fsync，后台线程的代码在bio.c中。

小结

文章写到这，已经解决的了Redis Server启动加载AOF文件和如何将客户端请求产生的新的数据追加到AOF文件中，对于追加数据到AOF文件中，根据fsync的配置策略如何将写入到AOF文件中的新数据刷新到硬盘中，直接在主进程中fsync或是在后台线程fsync。

至此，AOF数据持久化还剩下如何rewrite AOF，接受客户端发送的BGREWRITEAOF请求，此部分内容待下篇博客中解析。

感谢此篇博客给我在理解Redis AOF数据持久化方面的巨大帮助，http://chenzhenianqing.cn/articles/786.html

本人Redis-2.8.2的源码注释已经放到Github中，有需要的读者可以下载，我也会在后续的时间中更新，https://github.com/xkeyideal/annotated-redis-2.8.2

本人不怎么会使用Git，望有人能教我一下。