Redis源码剖析之AOF

appendonly no  # aof开关，默认关闭
appendfilename "appendonly.aof"  # 保存的文件名，默认appendonly.aof
# 有三种刷数据的策略
appendfsync always  # always是只要有数据改动，就把数据刷到磁盘里，最安全但性能也最差
appendfsync everysec  # 每隔一秒钟刷一次数据，数据安全性和性能折中，这也是redis默认和推荐的配置。 
appendfsync no # 不主动刷，什么时候数据刷到磁盘里取决于操作系统，在大多数Linux系统中每30秒提交一次，性能最好，但数据安全性最差。

dirty = server.dirty;
    prev_err_count = server.stat_total_error_replies;
    updateCachedTime(0);
    elapsedStart(&call_timer);
    c->cmd->proc(c); // 执行命令
    const long duration = elapsedUs(call_timer);
    c->duration = duration;
    dirty = server.dirty-dirty;
    if (dirty < 0) dirty = 0;

void propagate(struct redisCommand *cmd, int dbid, robj **argv, int argc,
               int flags)
{
    if (server.in_exec && !server.propagate_in_transaction)
        execCommandPropagateMulti(dbid);
    /* This needs to be unreachable since the dataset should be fixed during 
     * client pause, otherwise data may be lossed during a failover. */
    serverAssert(!(areClientsPaused() && !server.client_pause_in_transaction));
    if (server.aof_state != AOF_OFF && flags & PROPAGATE_AOF)
        feedAppendOnlyFile(cmd,dbid,argv,argc); // 如果aof开启了，就会向aof传播该命令。
    if (flags & PROPAGATE_REPL)
        replicationFeedSlaves(server.slaves,dbid,argv,argc);
}

void feedAppendOnlyFile(struct redisCommand *cmd, int dictid, robj **argv, int argc) {
    sds buf = sdsempty();
    /* The DB this command was targeting is not the same as the last command
     * we appended. To issue a SELECT command is needed. */
    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;
    }
    if (cmd->proc == expireCommand || cmd->proc == pexpireCommand ||
        cmd->proc == expireatCommand) {
        /* 把 EXPIRE/PEXPIRE/EXPIREAT 命令转化为 PEXPIREAT 命令*/
        buf = catAppendOnlyExpireAtCommand(buf,cmd,argv[1],argv[2]);
    } else if (cmd->proc == setCommand && argc > 3) {
        robj *pxarg = NULL;
        /* When SET is used with EX/PX argument setGenericCommand propagates them with PX millisecond argument.
         * So since the command arguments are re-written there, we can rely here on the index of PX being 3. */
        if (!strcasecmp(argv[3]->ptr, "px")) {
            pxarg = argv[4];
        }
        /* 把set命令的expired所带的相对时间转化为绝对时间(ms). */
        if (pxarg) {
            robj *millisecond = getDecodedObject(pxarg);
            long long when = strtoll(millisecond->ptr,NULL,10);
            when += mstime();
            decrRefCount(millisecond);
            robj *newargs[5];
            newargs[0] = argv[0];
            newargs[1] = argv[1];
            newargs[2] = argv[2];
            newargs[3] = shared.pxat;
            newargs[4] = createStringObjectFromLongLong(when);
            buf = catAppendOnlyGenericCommand(buf,5,newargs);
            decrRefCount(newargs[4]);
        } else {
            buf = catAppendOnlyGenericCommand(buf,argc,argv);
        }
    } else {
        /* 其他的命令都不需要转化 */
        buf = catAppendOnlyGenericCommand(buf,argc,argv);
    }
    /* 追加到AOF缓冲区。在重新进入事件循环之前，数据将被刷新到磁盘上，因此在客户端在执行前就会得到回复。*/
    if (server.aof_state == AOF_ON)
        server.aof_buf = sdscatlen(server.aof_buf,buf,sdslen(buf));
    /* 如果后台正在进行AOF重写，我们希望将子数据库和当前数据库之间的差异累积到缓冲区中，
     * 以便在子进程执行其工作时，我们可以将这些差异追加到新的只追加文件中。 */
    if (server.child_type == CHILD_TYPE_AOF)
        aofRewriteBufferAppend((unsigned char*)buf,sdslen(buf));
    sdsfree(buf);
}