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
转载请注明，文章出自
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] != '$') 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 '%s' 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;
…………
}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'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'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'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'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。
本文章为转载内容，我们尊重原作者对文章享有的著作权。如有内容错误或侵权问题，欢迎原作者联系我们进行内容更正或删除文章。
