MySQL案例01：Last_SQL_Errno: 1755 Cannot execute the current event group in the parallel mode

周五同事监控报警，有个MySQL从库复制状态异常，让我帮忙排查下，经过排查发现是MySQL5.6并行复制的一个Bug所致，具体处理过程如下：

一、错误信息

登录mysql从库服务器，检查复制状态

mysql> show slave status G
*************************** 1. row ***************************
               Slave_IO_State: Waiting for master to send event
                  Master_Host: xx.xx.xx.xx
                  Master_User: repuser
                  Master_Port: 3306
                Connect_Retry: 60
              Master_Log_File: mysql-bin.001689
          Read_Master_Log_Pos: 145325388
               Relay_Log_File: mysql2-relay-bin.004390
                Relay_Log_Pos: 147850306
        Relay_Master_Log_File: mysql-bin.001673
             Slave_IO_Running: Yes
            Slave_SQL_Running: No
              Replicate_Do_DB: 
          Replicate_Ignore_DB: 
           Replicate_Do_Table: 
       Replicate_Ignore_Table: 
      Replicate_Wild_Do_Table: 
  Replicate_Wild_Ignore_Table: 
                   Last_Errno: 1755
                   Last_Error: Cannot execute the current event group in the parallel mode. Encountered event Format_desc, relay-log name ./mysql2-relay-bin.004392, position 238 which prevents execution of this event group in parallel mode. Reason: possible malformed group of events from an old master.
                 Skip_Counter: 0
          Exec_Master_Log_Pos: 147850096
              Relay_Log_Space: 4440639819
              Until_Condition: None
               Until_Log_File: 
                Until_Log_Pos: 0
           Master_SSL_Allowed: No
           Master_SSL_CA_File: 
           Master_SSL_CA_Path: 
              Master_SSL_Cert: 
            Master_SSL_Cipher: 
               Master_SSL_Key: 
        Seconds_Behind_Master: NULL
Master_SSL_Verify_Server_Cert: No
                Last_IO_Errno: 0
                Last_IO_Error: 
               Last_SQL_Errno: 1755
               Last_SQL_Error: Cannot execute the current event group in the parallel mode. Encountered event Format_desc, relay-log name ./mysql2-relay-bin.004392, position 238 which prevents execution of this event group in parallel mode. Reason: possible malformed group of events from an old master.
  Replicate_Ignore_Server_Ids: 
             Master_Server_Id: 46
                  Master_UUID: f6d575b8-6a5b-11e5-8313-0050568d23c9
             Master_Info_File: mysql.slave_master_info
                    SQL_Delay: 0
          SQL_Remaining_Delay: NULL
      Slave_SQL_Running_State: 
           Master_Retry_Count: 86400
                  Master_Bind: 
      Last_IO_Error_Timestamp: 
     Last_SQL_Error_Timestamp: 180314 01:01:17
               Master_SSL_Crl: 
           Master_SSL_Crlpath: 
           Retrieved_Gtid_Set: f6d575b8-6a5b-11e5-8313-0050568d23c9:9393159-70288025
            Executed_Gtid_Set: 33a835bb-6a5a-11e5-8308-0050568d685b:1-146,
f6d575b8-6a5b-11e5-8313-0050568d23c9:1-69584557
                Auto_Position: 1
1 row in set (0.01 sec)

mysql错误日志内容如下

018-03-14 01:01:16 26349 [Note] Event Scheduler: Loaded 0 events
2018-03-14 01:01:16 26349 [Note] /data/mysql/bin/mysqld: ready for connections.
Version: '5.6.16-log'  socket: '/tmp/mysql.sock'  port: 3306  Source distribution
2018-03-14 01:01:17 26349 [ERROR] Slave SQL: Cannot execute the current event group in the parallel mode. Encountered event Format_desc, relay-log name ./mysql2-relay-bin.004392, position 238 which prevents execution of this event group in parallel mode. Reason: possible malformed group of events from an old master. Error_code: 1755
2018-03-14 01:01:17 26349 [Warning] Slave: Cannot execute the current event group in the parallel mode. Encountered event Format_desc, relay-log name ./mysql2-relay-bin.004392, position 238 which prevents execution of this event group in parallel mode. Reason: possible malformed group of events from an old master. Error_code: 1755
2018-03-14 01:01:17 26349 [ERROR] Error running query, slave SQL thread aborted. Fix the problem, and restart the slave SQL thread with "SLAVE START". We stopped at log 'mysql-bin.001673' position 147850096

二、错误原因

从错误日志字面意思理解，出现这个错误是因为数据库开启了并行复制，一些event group在并行复制下，不支持从而导致复制失败，主从状态异常。

检查数据库版本：

mysql>  select version();
+------------+
| version()  |
+------------+
| 5.6.16-log |
+------------+
1 row in set (0.07 sec)

检查数据库并行复制信息：

mysql> show variables like '%parall%';
+------------------------+-------+
| Variable_name          | Value |
+------------------------+-------+
| slave_parallel_workers | 1     |
+------------------------+-------+
1 row in set (0.05 sec)

官方搜索错误信息，发现此错误是一bug，具体bug编号为Bug #71495、Bug #72537

https://bugs.mysql.com/bug.php?id=71495

https://bugs.mysql.com/bug.php?id=72537

三、解决方案

1、既然是bug，通过分析binlog内容，然后跳过的方式跳过事物

(非gtid模式，通过set sql_slave_skip_counter=1;跳过事务)

主库分析binlog：

mysqlbinlog -vv --base64-output='decode-rows'   --start-position=147850096  --stop-position=147850096   mysql-bin.001674  >a.sql

SHOW BINLOG EVENTS in 'mysql-bin.001673' from 147850096  limit 10;
从库跳过事物（gtid模式）

stop slave;

SET @@SESSION.GTID_NEXT= 'f6d575b8-6a5b-11e5-8313-0050568d23c9:9393159-70288025';

begin;

commit;

SET SESSION GTID_NEXT = AUTOMATIC;

start slave;

2、通过关闭并行复制来恢复（最佳方法）

stop slave;

set global slave_parallel_workers = 0 ;

start slave;

（  更改my.cnf中并行复制参数slave_parallel_workers = 0）

3、如果上述方法都不行，可以使用重新部署从库的方式来恢复，恢复后关闭并行复制（5.6并行复制存在bug，不推荐使用）

如果使用mysqldump备份，最好指定参数--set-gtid-purged=off，否则需要充值gtid

主库备份数据库：

innobackupexp方式：innobackupex  --socket=/tmp/mysql.sock  /20180316/

从库恢复数据库：

innobackupex  --redo-only --apply-log /soft/2018-03-16_17-45-30
innobackupex  --copy-back /soft/2018-03-16_17-45-30

查看事物应用binlog信息：cat xtrabackup_info

binlog_pos = filename 'mysql-bin.001690', position 46997344, GTID of the last change 'f6d575b8-6a5b-11e5-8313-0050568d23c9:1-70316241'

innodb_from_lsn = 0
innodb_to_lsn = 84446557453

设置从库gtid信息

set global gtid_purged='f6d575b8-6a5b-11e5-8313-0050568d23c9:1-70316241';  

重新部署从库：

change master to master_host='xx.xx.xx.xx',master_user='usernamer',master_password='password',MASTER_AUTO_POSITION=84446557453;

(上述使用了reset master,否则可以直接指定从MASTER_AUTO_POSITION=1开始)