ECT 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 |