为什么 redis 提示 need enable AOF
有些程序在连接 Redis 的时候可能出于方便的原因会做一些管理性的操作, 比如 bgsave, bgrewriteaof, slaveof 等操作, 这些可能会产生一些意想不到的错误, 比如以下提示信息, 虽然不是致命的错误, 但看起来也很困惑:
[95138] 28 Jul 03:18:45.763 # Redis needs to enable the AOF but can't trigger a background AOF rewrite operation. Check the above logs for more info about the error.
[95138] 28 Jul 03:18:45.763 # Redis needs to enable the AOF but can't trigger a background AOF rewrite operation. Check the above logs for more info about the error.
可以看到这个提示是和 AOF 相关的, 默认没有开启 redis 的 AOF 设置, 不过程序在连接建立的时候会执行一次 config set appendonly yes 操作, 在 redis 中, config set appendonly yes 操作会自动触发一次 startAppendOnly() 操作, 如下所示:
# src/config.c
642 } else if (!strcasecmp(c->argv[2]->ptr,"appendonly")) {
643 int enable = yesnotoi(o->ptr);
644
645 if (enable == -1) goto badfmt;
646 if (enable == 0 && server.aof_state != REDIS_AOF_OFF) {
647 stopAppendOnly();
648 } else if (enable && server.aof_state == REDIS_AOF_OFF) { // 之前为 no, 修改为 yes 时
649 if (startAppendOnly() == REDIS_ERR) { // 执行 startAppendOnly
650 addReplyError(c,
651 "Unable to turn on AOF. Check server logs.");
652 return;
653 }
654 }
startAppendOnly 则主要执行了 BGREWRITEAOF 相关的操作, 如下所示:
# src/aof.c
190 /* Called when the user switches from "appendonly no" to "appendonly yes"
191 * at runtime using the CONFIG command. */
192 int startAppendOnly(void) {
193 server.aof_last_fsync = server.unixtime;
194 server.aof_fd = open(server.aof_filename,O_WRONLY|O_APPEND|O_CREAT,0644);
195 redisAssert(server.aof_state == REDIS_AOF_OFF);
196 if (server.aof_fd == -1) {
197 redisLog(REDIS_WARNING,"Redis needs to enable the AOF but can't open the append only file: %s",strerror(errno));
198 return REDIS_ERR;
199 }
200 if (rewriteAppendOnlyFileBackground() == REDIS_ERR) {
201 close(server.aof_fd);
202 redisLog(REDIS_WARNING,"Redis needs to enable the AOF but can't trigger a background AOF rewrite operation. Check the above logs for more info about the error.");
203 return REDIS_ERR;
204 }
205 /* We correctly switched on AOF, now wait for the rerwite to be complete
206 * in order to append data on disk. */
207 server.aof_state = REDIS_AOF_WAIT_REWRITE;
208 return REDIS_OK;
209 }
...
993 /* This is how rewriting of the append only file in background works:
994 *
995 * 1) The user calls BGREWRITEAOF
996 * 2) Redis calls this function, that forks():
997 * 2a) the child rewrite the append only file in a temp file.
998 * 2b) the parent accumulates differences in server.aof_rewrite_buf.
999 * 3) When the child finished '2a' exists.
1000 * 4) The parent will trap the exit code, if it's OK, will append the
1001 * data accumulated into server.aof_rewrite_buf into the temp file, and
1002 * finally will rename(2) the temp file in the actual file name.
1003 * The the new file is reopened as the new append only file. Profit!
1004 */
1005 int rewriteAppendOnlyFileBackground(void) {
1006 pid_t childpid;
1007 long long start;
1008
1009 if (server.aof_child_pid != -1) return REDIS_ERR;
1010 start = ustime();
1011 if ((childpid = fork()) == 0) {
1012 char tmpfile[256];
......
因为已经自动触发了 BGREWRITEAOF 操作(数据越多, bgrewriteaof 的耗时越长), 程序的连接再执行此操作的时候就会满足条件 if (server.aof_child_pid != -1) return REDIS_ERR;, 相应的也会满足下面的条件:
200 if (rewriteAppendOnlyFileBackground() == REDIS_ERR) {
201 close(server.aof_fd);
202 redisLog(REDIS_WARNING,"Redis needs to enable the AOF but can't trigger a background AOF rewrite operation. Check the above logs for more info about the error.");
203 return REDIS_ERR;
204 }
因此就会产生上述的错误提示 "Redis needs to enable the AOF but can't trigger a background AOF rewrite operation. Check the above logs for more info about the error.", 不过该提示不会影响程序正常的请求处理.
备注: 如果redis 初始开启了 aof 或者手动在运行过程中设置了
config set appendonly yes, 之后程序每次连接执行BGREWRITEAOF就不会触发startAppendOnly()操作, 相应的也就不会满足条件if (server.aof_child_pid != -1) return REDIS_ERR;, 所以不会再有上述的提示.
如何避免此类问题?
对应用程序而言, 我们使用 redis 是建议去掉所有 redis 管理类的命令使用, 并遵守以下原则:
1. 程序仅做常规的数据操作命令;
2. 管理类命令由中间层工具或管理员操作;