redis aof命令缓冲区的写入源码

需要通过aof记录的命令主要为set命令和过期命令，这些命令在执行完毕后需要记录到aof缓冲区中以便写入到aof文件中。

sds aof_buf;      /* AOF buffer, written before entering the event loop */

在结构体redisServer中通过一个sds来作为aof的缓冲区。

Redis服务器在执行完相应的命令的时候，如果开启了aof功能，就会在feedAppendOnlyFile()函数中准备将命令添加至aof缓冲区当中。

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 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) {/* Translate EXPIRE/PEXPIRE/EXPIREAT into PEXPIREAT */buf = catAppendOnlyExpireAtCommand(buf,cmd,argv[1],argv[2]);} 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 if (cmd->proc == setCommand && argc > 3) {int i;robj *exarg = NULL, *pxarg = NULL;/* Translate SET [EX seconds][PX milliseconds] to SET and PEXPIREAT */buf = catAppendOnlyGenericCommand(buf,3,argv);for (i = 3; i < argc; i ++) {if (!strcasecmp(argv[i]->ptr, "ex")) exarg = argv[i+1];if (!strcasecmp(argv[i]->ptr, "px")) pxarg = argv[i+1];}serverAssert(!(exarg && pxarg));if (exarg)buf = catAppendOnlyExpireAtCommand(buf,server.expireCommand,argv[1],exarg);if (pxarg)buf = catAppendOnlyExpireAtCommand(buf,server.pexpireCommand,argv[1],pxarg);} else {/* 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. */if (server.aof_state == 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. */if (server.aof_child_pid != -1)aofRewriteBufferAppend((unsigned char*)buf,sdslen(buf));sdsfree(buf);
}

首先，判断该命令的执行dbid是否是和当前的dbid一致，如果不一致，需要额外写入select命令切换到命令所执行的dbid上。

而后，如果该命令属于expire，perpire, expireat这三个纯粹设置过期时间的命令，直接通过catAppendOnlyExpireAtCommand()方法转换成过期命令写到结果sds中。

对于setex和psetex两个存放和过期键同时设置的命令，首先根据这两条命令的格式：

SETEX key seconds value，取到第一个和第三个参数，通过catAppendOnlyGenericCommand()方法转换成set命令写到aof缓冲区中，再根据第一个参数和第二个参数组成pexpireat命令写到结果sds中，在这里被转换成了两条命令。redisrdb，

而如果是set [ex seconds] [px millisecond] 形式的命令，则根据参数数数量来确定。

如果是这样的命令，也是拆分成set部分和pexpireat部分拆分为多段命令。

如果只是简单的set命令，就简单通过catAppendOnlyGenericCommand()方法插入结果sds。

在完成了上述操作后，判断如果开启了aof，那么则将结果sds写入到server的aof缓冲区中。

如果此时redis正有子进程进行重写操作，那么直接将新的sds加入到重写的内容中。

最后清空结果sds以节约空间。redis mset。

sds catAppendOnlyGenericCommand(sds dst, int argc, robj **argv) {char buf[32];int len, j;robj *o;buf[0] = '*';len = 1+ll2string(buf+1,sizeof(buf)-1,argc);buf[len++] = '\r';buf[len++] = '\n';dst = sdscatlen(dst,buf,len);for (j = 0; j < argc; j++) {o = getDecodedObject(argv[j]);buf[0] = '$';len = 1+ll2string(buf+1,sizeof(buf)-1,sdslen(o->ptr));buf[len++] = '\r';buf[len++] = '\n';dst = sdscatlen(dst,buf,len);dst = sdscatlen(dst,o->ptr,sdslen(o->ptr));dst = sdscatlen(dst,"\r\n",2);decrRefCount(o);}return dst;
}

catAppendOnlyGenericCommand()方法用来将set命令转换为sds以便写入到aof缓冲区中，每次在写命令的开头，都会计算包括命令和参数的个数总和，加上*号写在命令上一行。同时每次在写具体值之前，都要计算写入的参数的具体长度，使用$加在前一行。作为解析对象的o每次在使用玩一次，都会通过decrRefCount()方法减少一次引用次数便于回收。