REdis-4.0之前的AOF文件没有文件头，而从REdis-4.0开始AOF文件带有一个文件头，文件头格式和RDB文件头相同。
REdis-4.0版本，如果开启aof-use-rdb-preamble（值为yes，5.0默认为yes，4.0默认为no），则AOF文件内容由三部分组成：
1) 文件头（和RDB文件头格式相同）
2) RDB格式的二进制数据段
3) AOF格式的文本数据段
AOF格式的文本数据段和之前版本的保持相同，如下图所示：
从REdis-4.0开始，AOF有文件头，目的是支持同时加载RDB格式和AOF格式数据。AOF文件头和RDB文件头基本相同，但RDB文件头多了三个字段。
先看AOF和RDB通用部分的文件头内容：
1) 头5字节固定为REDIS
2) 第6~9共四字节为RDB版本号
3) 接下来为redis-ver和它的值，即redis版本
4) 接着redis-bits和它的值，即redis的位数，值为32或64
5) 接着为ctime和它的值，值为当前时间戳
6) 接着为used-mem和它的值
7) 最后是aof-preamble和它的值，值为0或1，1表示RDB有效。
RDB的文件头和AOF的文件基本相同，但RDB文件头在aof-preamble之前多了如下三项：
1) repl-stream-db
2) repl-id
3) repl-offset
如果配置项appendonly值为no，则不会加载AOF文件，而是直接加载RDB文件，因为RDB文件保存了复制信息，所以进程重启可增量复制。而如果置项appendonly值为yes，因为AOF文件头不包含复制信息，所以只能全量复制。
相关代码：
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/* Replay the append log file. * On success C_OK is returned. On non fatal * error (the append only file is zero-length) * C_ERR is returned. On * fatal error an error message is logged * and the program exists. */ int loadAppendOnlyFile(char *filename) // aof.c:673 { /* Check if this AOF file has an RDB preamble. In that case we need to * load the RDB file and later continue loading the AOF tail. */ char sig[5]; /* "REDIS" */ if (fread(sig,1,5,fp) != 5 || memcmp(sig,"REDIS",5) != 0) // aof.c:707 { /* No RDB preamble, seek back at 0 offset. */ if (fseek(fp,0,SEEK_SET) == -1) goto readerr; } else { /* RDB preamble. Pass loading the RDB functions. */ if (rdbLoadRio(&rdb,NULL,1) != C_OK) { // aof.c:717 。。。。。。 } } }
/* Load an RDB file from the rio stream 'rdb'. * On success C_OK is returned, * otherwise C_ERR is returned and 'errno' is set accordingly. */ int rdbLoadRio(rio *rdb, rdbSaveInfo *rsi, int loading_aof) // rdb.c:1850 { 。。。。。。 }
/* Save a few default AUX fields with information about the RDB generated. */ int rdbSaveInfoAuxFields(rio *rdb, int flags, rdbSaveInfo *rsi) // rdb.c:1071 { int redis_bits = (sizeof(void*) == 8) ? 64 : 32; int aof_preamble = (flags & RDB_SAVE_AOF_PREAMBLE) != 0;
/* Add a few fields about the state when the RDB was created. */ if (rdbSaveAuxFieldStrStr(rdb,"redis-ver",REDIS_VERSION) == -1) return -1; if (rdbSaveAuxFieldStrInt(rdb,"redis-bits",redis_bits) == -1) return -1; if (rdbSaveAuxFieldStrInt(rdb,"ctime",time(NULL)) == -1) return -1; if (rdbSaveAuxFieldStrInt(rdb,"used-mem",zmalloc_used_memory()) == -1) return -1;
/* Handle saving options that generate aux fields. */ // 如果是AOF，传入的rsi为NULL， // 因此不会记录以下三项辅助数据 // 可参见函数rewriteAppendOnlyFile（aof.c:1344）的实现 if (rsi) { // rdb.c:1082 if (rdbSaveAuxFieldStrInt(rdb,"repl-stream-db",rsi->repl_stream_db) == -1) return -1; if (rdbSaveAuxFieldStrStr(rdb,"repl-id",server.replid) == -1) return -1; if (rdbSaveAuxFieldStrInt(rdb,"repl-offset",server.master_repl_offset) == -1) return -1; } if (rdbSaveAuxFieldStrInt(rdb,"aof-preamble",aof_preamble) == -1) return -1; return 1; }
/* Produces a dump of the database in RDB format sending it to the specified * Redis I/O channel. On success C_OK is returned, otherwise C_ERR * is returned and part of the output, or all the output, can be * missing because of I/O errors. * * When the function returns C_ERR and if 'error' is not NULL, the * integer pointed by 'error' is set to the value of errno just after the I/O * error. */ int rdbSaveRio(rio *rdb, int *error, int flags, rdbSaveInfo *rsi) // rdb.c:1102 { if (rdbSaveInfoAuxFields(rdb,flags,rsi) == -1) goto werr; // rdb.c:1114 }
/* Write a sequence of commands able to fully rebuild the dataset into * "filename". Used both by REWRITEAOF and BGREWRITEAOF. * * In order to minimize the number of commands needed in the rewritten * log Redis uses variadic commands when possible, such as RPUSH, SADD * and ZADD. However at max AOF_REWRITE_ITEMS_PER_CMD items per time * are inserted using a single command. */ int rewriteAppendOnlyFile(char *filename) // aof.c:1344 { // 对于AOF，传入的第三个参数rsi值为NULL， // 因此不会记录复制信息，这样进程重启需要全量复制。 if (rdbSaveRio(&aof,&error,RDB_SAVE_AOF_PREAMBLE,NULL) == C_ERR) { // aof.c:1367 } |
启动时调用顺序：
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main(server.c:4003) -> loadDataFromDisk(server.c:3850) -> loadAppendOnlyFile(aof.c:673，当配置appendonly值为yes时) -> rdbLoadRio(rdb.c:1850) |