redis支持2种持久化功能，分别是RDB持久化和AOF(Append Only File)持久化 。
今天总结下redis加载AOF文件过程，以及加载AOF过程中遇到的问题 。
由于AOF文件里面包含了重建redis数据库状态所需要的所有命令，因此在redis启动过程中需要加载一次AOF文件（前提是redis配置文件中使用的是aof文件），这样就可以还原之前的redis所有状态了 。
redis在启动时加载AOF过程如下
int main(int argc, char **argv) {...// 如果服务器不是运行在 SENTINEL 模式，那么执行以下代码if (!server.sentinel_mode) { //sentinel和集群只能二选1/* Things not needed when running in Sentinel mode. */redisLog(REDIS_WARNING,"Server started, Redis version " REDIS_VERSION);#ifdef __linux__linuxOvercommitMemoryWarning();#endif// 从 AOF 文件或者 RDB 文件中载入数据loadDataFromDisk();// 判断是否启动集群if (server.cluster_enabled) { //在该函数前会先载入cluster配置nodes.conf，见initServer->clusterInit;if (verifyClusterConfigWithData() == REDIS_ERR) {redisLog(REDIS_WARNING,"You can't have keys in a DB different than DB 0 when in ""Cluster mode. Exiting.");exit(1);}}// 打印 TCP 端口if (server.ipfd_count > 0)redisLog(REDIS_NOTICE,"The server is now ready to accept connections on port %d", server.port);// 打印本地套接字端口if (server.sofd > 0)redisLog(REDIS_NOTICE,"The server is now ready to accept connections at %s", server.unixsocket);} else { //sentinel和集群只能二选1sentinelIsRunning();}/* Warning the user about suspicious maxmemory setting. */// 检查不正常的 maxmemory 配置if (server.maxmemory > 0 && server.maxmemory < 1024*1024) {redisLog(REDIS_WARNING,"WARNING: You specified a maxmemory value that is less than 1MB (current value is %llu bytes). Are you sure this is what you really want?", server.maxmemory);}// 运行事件处理器，一直到服务器关闭为止aeSetBeforeSleepProc(server.el,beforeSleep);aeMain(server.el);// 服务器关闭，停止事件循环aeDeleteEventLoop(server.el);return 0;}从上面代码得知，redis在loadDataFromDisk()中加载AOF文件 。
void loadDataFromDisk(void) { //loadDataFromDisk和rdbSave对应加载写入// 记录开始时间long long start = ustime();// AOF 持久化是否已打开if (server.aof_state == REDIS_AOF_ON) {// 尝试载入 AOF 文件if (loadAppendOnlyFile(server.aof_filename) == REDIS_OK)// 打印载入信息，并计算载入耗时长度redisLog(REDIS_NOTICE,"DB loaded from append only file: %.3f seconds",(float)(ustime()-start)/1000000);// AOF 持久化未打开} else {// 尝试载入 RDB 文件if (rdbLoad(server.rdb_filename) == REDIS_OK) {// 打印载入信息，并计算载入耗时长度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);}}}若开启使用的是AOF持久化，则调用loadAppendOnlyFile进行加载AOF文件 。
int loadAppendOnlyFile(char *filename) {//伪客户端struct redisClient *fakeClient;// 打开 AOF 文件FILE *fp = fopen(filename,"r");、struct redis_stat sb;int old_aof_state = server.aof_state;long loops = 0;// 检查文件的正确性if (fp && redis_fstat(fileno(fp),&sb) != -1 && sb.st_size == 0) {server.aof_current_size = 0;fclose(fp);return REDIS_ERR;}// 检查文件是否正常打开if (fp == NULL) {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.* 暂时性地关闭 AOF ，防止在执行 MULTI 时，* EXEC 命令被传播到正在打开的 AOF 文件中 。*/server.aof_state = REDIS_AOF_OFF;//创建一个伪客户端fakeClient = createFakeClient();// 设置服务器的状态为：正在载入startLoading(fp); //startLoading 定义于 rdb.cwhile(1) {int argc, j;unsigned long len;robj **argv;char buf[128];sds argsds;struct redisCommand *cmd;/* Serve the clients from time to time* 间隔性地处理客户端发送来的请求* 因为服务器正处于载入状态，所以能正常执行的只有 PUBSUB 等模块*/if (!(loops++ % 1000)) {loadingProgress(ftello(fp));processEventsWhileBlocked();}// 读入文件内容到缓存if (fgets(buf,sizeof(buf),fp) == NULL) {if (feof(fp))break; // 文件已经读完，跳出elsegoto readerr;}// 确认协议格式，比如 *3rnif (buf[0] != '*') goto fmterr;// 取出命令参数，比如 *3rn 中的 3argc = atoi(buf+1);// 至少要有一个参数（被调用的命令）if (argc < 1) goto fmterr;// 从文本中创建字符串对象：包括命令，以及命令参数// 例如 $3rnSETrn$3rnKEYrn$5rnVALUErn// 将创建三个包含以下内容的字符串对象：// SET 、 KEY 、 VALUEargv = zmalloc(sizeof(robj*)*argc);for (j = 0; j < argc; j++) {if (fgets(buf,sizeof(buf),fp) == NULL) goto readerr;if (buf[0] != '$') goto fmterr;// 读取参数值的长度len = strtol(buf+1,NULL,10);// 读取参数值argsds = sdsnewlen(NULL,len);if (len && fread(argsds,len,1,fp) == 0)goto fmterr;// 为参数创建对象argv[j] = createObject(REDIS_STRING,argsds);if (fread(buf,2,1,fp) == 0) goto fmterr; /* discard CRLF */}/* Command lookup*查找命令*/cmd = lookupCommand(argv[0]->ptr);if (!cmd) {redisLog(REDIS_WARNING,"Unknown command '%s' reading the append only file", (char*)argv[0]->ptr);exit(1);}/* Run the command in the context of a fake client* 调用伪客户端，执行命令*/fakeClient->argc = argc;fakeClient->argv = argv;cmd->proc(fakeClient);/* 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.* 清理命令和命令参数对象*/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.* 如果能执行到这里，说明 AOF 文件的全部内容都可以正确地读取，* 但是，还要检查 AOF 是否包含未正确结束的事务*/if (fakeClient->flags & REDIS_MULTI) goto readerr;fclose(fp);// 释放伪客户端freeFakeClient(fakeClient);// 复原 AOF 状态server.aof_state = old_aof_state;// 停止载入stopLoading();// 更新服务器状态中， AOF 文件的当前大小aofUpdateCurrentSize();// 记录前一次重写时的大小server.aof_rewrite_base_size = server.aof_current_size;return REDIS_OK;// 读入错误readerr:// 非预期的末尾，可能是 AOF 文件在写入的中途遭遇了停机if (feof(fp)) {redisLog(REDIS_WARNING,"Unexpected end of file reading the append only file");// 文件内容出错} else {redisLog(REDIS_WARNING,"Unrecoverable error reading the append only file: %s", strerror(errno));}exit(1);// 内容格式错误fmterr:redisLog(REDIS_WARNING,"Bad file format reading the append only file: make a backup of your AOF file, then use ./redis-check-aof --fix <filename>");exit(1);}