一个线上数据丢失故障案例，引出了在 GTID 模式下 AUTO POSITION MODE 的必要性。
作者：孙绪宗
新浪微博 DBA 团队工程师，主要负责 MySQL、PostgreSQL 等关系型数据库运维。
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阅读前需知
- Retrieved_Gtid_Set：从库已经接收到主库的事务编号（从库的 IO 线程已经接受到了）。
- Executed_Gtid_Set：已经执行的事务编号（从库的执行 SQL 线程已经执行了的 SQL）。
故障现象
主从数据不一致，但是看复制是正常状态（双 Yes）。此时主库执行，从库本该报错 1062 或者 1032 的 SQL，从库复制线程还是双 Yes，没有报错。
MySQL 版本：5.7.35
故障复现
查看主库状态
MySQL [xuzong]> show master status;
+------------------+----------+--------------+------------------+-----------------------------------------------+
| File | Position | Binlog_Do_DB | Binlog_Ignore_DB | Executed_Gtid_Set |
+------------------+----------+--------------+------------------+-----------------------------------------------+
| mysql-bin.000008 | 39349641 | | | c233aec0-58d3-11ec-a74a-a0a33ba8b3ff:1-104345 |
+------------------+----------+--------------+------------------+-----------------------------------------------+
从库上只需要设置 GTID 值的 gno 大于主库的值即可复现。
mysql> stop slave;
Query OK, 0 rows affected (0.00 sec)
mysql> reset slave all;
Query OK, 0 rows affected (0.01 sec)
mysql> reset master;
Query OK, 0 rows affected (0.01 sec)
mysql> set @@GLOBAL.GTID_PURGED='c233aec0-58d3-11ec-a74a-a0a33ba8b3ff:1-1000000'; #1000000>104345
Query OK, 0 rows affected (0.00 sec)
然后用 POSITION MODE 点位复制，启动从库，执行 show slave status \G 就会看到如下：
Slave_IO_Running: Yes
Slave_SQL_Running: Yes
......
Last_IO_Error_Timestamp:
Last_SQL_Error_Timestamp:
Master_SSL_Crl:
Master_SSL_Crlpath:
Retrieved_Gtid_Set: c233aec0-58d3-11ec-a74a-a0a33ba8b3ff:104221-104345
Executed_Gtid_Set: c233aec0-58d3-11ec-a74a-a0a33ba8b3ff:1-1000000
Auto_Position: 0
Replicate_Rewrite_DB:
双 Yes，看上去一点问题都没有，但是这时候在主库执行任何操作，都不会被复制，因为复制时校验 GTID 时从库表示已经执行过了。
主库
MySQL [xuzong]> insert into test(passtext) values('test');
Query OK, 1 row affected (0.00 sec)
从库
mysql> select * from xuzong.test;
+----+----------------------------------+
| id | passtext |
+----+----------------------------------+
| 1 | 7e5a44af63552be3f2f819cebbe0832a |
| 2 | 7e5a44af63552be3f2f819cebbe0832a |
| 3 | 7e5a44af63552be3f2f819cebbe0832a |
| 4 | d13baf7019431e0c75dee85bc923a91b |
| 5 | 11 |
+----+----------------------------------+
5 rows in set (0.00 sec)
# 从库能看出来 Retrieved_Gtid_Set 在变化
mysql> show slave status \G
Retrieved_Gtid_Set: c233aec0-58d3-11ec-a74a-a0a33ba8b3ff:104221-104367
Executed_Gtid_Set: c233aec0-58d3-11ec-a74a-a0a33ba8b3ff:1-1000000
问题在于，这种不应该报错嘛，从库的 GTID 大于主库的 GTID ？？？
猜测
根源在于没有设置 Auto_Position=1，那么接下来验证一下猜测。
Last_IO_Error: Got fatal error 1236 from master when reading data from binary log: 'Slave has more GTIDs than the master has, using the master's SERVER_UUID. This may indicate that the end of the binary log was truncated or that the last binary log file was lost, e.g., after a power or disk failure when sync_binlog != 1. The master may or may not have rolled back transactions that were already replicated to the slave. Suggest to replicate any transactions that master has rolled back from slave to master, and/or commit empty transactions on master to account for transactions that have been'
Last_SQL_Errno: 0
Last_SQL_Error:
Replicate_Ignore_Server_Ids:
Master_Server_Id: 1362229455
Master_UUID: c233aec0-58d3-11ec-a74a-a0a33ba8b3ff
Master_Info_File: mysql.slave_master_info
SQL_Delay: 0
SQL_Remaining_Delay: NULL
Slave_SQL_Running_State: Slave has read all relay log; waiting for more updates
Master_Retry_Count: 86400
Master_Bind:
Last_IO_Error_Timestamp: 230817 11:11:44
Last_SQL_Error_Timestamp:
Master_SSL_Crl:
Master_SSL_Crlpath:
Retrieved_Gtid_Set:
Executed_Gtid_Set: c233aec0-58d3-11ec-a74a-a0a33ba8b3ff:1-1000000
Auto_Position: 1
果不其然报错了，验证了猜测。
问题处理
这种情况目前看只能重做从库。
因为案例中复制进程为双 Yes，无法被监控捕获，同时无法得知数据不一致出现的时间点，所以无法通过分析 binlog 恢复。
本案例中的实例还进行过主从切换，只能联系业务方做一次全量的数据对比，得到一个完整数据的实例，然后重新构建从库。
总结
最后总结一下 Auto_Position 的作用。
- 会立即清理原来的 Relay Log。
- 根据从库的
Executed_Gtid_Set和Retrieved_Gtid_Set的并集定位 Binlog。MASTER_LOG_FILE和MASTER_LOG_POS不存实例的值。
生产上实例开了 GTID 的话，主从复制建议用 AUTO_POSITION MODE 的方式，即 Auto_Position=1。避免一些未知操作导致从库复制线程没有预期的报错，进而导致从库未正确复制，数据丢失。
拓展：源码解读
rel_slave.cc
--> request_dump
部分源码：
.....
enum_server_command command= mi->is_auto_position() ?
COM_BINLOG_DUMP_GTID : COM_BINLOG_DUMP;
......
if (command == COM_BINLOG_DUMP_GTID)
{
if (gtid_executed.add_gtid_set(mi->rli->get_gtid_set()) != RETURN_STATUS_OK || // 加Retrieved_Gtid_Set
gtid_executed.add_gtid_set(gtid_state->get_executed_gtids()) !=
RETURN_STATUS_OK) // 加Executed_Gtid_Set
......
int2store(ptr_buffer, binlog_flags);
ptr_buffer+= ::BINLOG_FLAGS_INFO_SIZE;
int4store(ptr_buffer, server_id);
ptr_buffer+= ::BINLOG_SERVER_ID_INFO_SIZE;
int4store(ptr_buffer, static_cast<uint32>(BINLOG_NAME_INFO_SIZE));
ptr_buffer+= ::BINLOG_NAME_SIZE_INFO_SIZE;
memset(ptr_buffer, 0, BINLOG_NAME_INFO_SIZE); // 设置MASTER_LOG_FILE为0
ptr_buffer+= BINLOG_NAME_INFO_SIZE;
int8store(ptr_buffer, 4LL); // 设置MASTER_LOG_POS为4
ptr_buffer+= ::BINLOG_POS_INFO_SIZE; // 存gtid_set
如果 relay_log_recovery 是打开的，则忽略 Retrieved_Gtid_Set 值。
/*
In the init_gtid_set below we pass the mi->transaction_parser.
This will be useful to ensure that we only add a GTID to
the Retrieved_Gtid_Set for fully retrieved transactions. Also, it will
be useful to ensure the Retrieved_Gtid_Set behavior when auto
positioning is disabled (we could have transactions spanning multiple
relay log files in this case).
We will skip this initialization if relay_log_recovery is set in order
to save time, as neither the GTIDs nor the transaction_parser state
would be useful when the relay log will be cleaned up later when calling
init_recovery.
*/ // 注释解释的很清楚，relay_log_recovery=1 会跳过初始化 gtid_retrieved_initialized
if (!is_relay_log_recovery &&
!gtid_retrieved_initialized &&
relay_log.init_gtid_sets(>id_set, NULL,
opt_slave_sql_verify_checksum,
true/*true=need lock*/,
&mi->transaction_parser, >id_partial_trx))
{
sql_print_error("Failed in init_gtid_sets() called from Relay_log_info::rli_init_info().");
DBUG_RETURN(1);
}
gtid_retrieved_initialized= true;
#ifndef NDEBUG
global_sid_lock->wrlock();
gtid_set.dbug_print("set of GTIDs in relay log after initialization");
global_sid_lock->unlock();
#endif