[Debian] How To Repair MySQL Replication – MyHosting Help Center

How To Repair MySQL Replication

If you have set up MySQL replication, you probably know this problem: sometimes there are invalid MySQL queries which cause the replication to not work anymore. In this short guide I explain how you can repair the replication on the MySQL slave without the need to set it up from scratch again.

Identify The Problem

To find out whether replication is/is not working and what has caused to stop it, you can take a look at the logs. On Debian, for example, MySQL logs to /var/log/syslog:

grep mysql /var/log/syslog

The output may look similar to this

Slave1:/home/admin# grep mysql /var/log/syslog

May 29 09:56:08 http2 mysqld[1380]: 080529 9:56:08 [ERROR] Slave: Error 'Table 'mydb.taggregate_temp_1212047760'

doesn't exist' on query. Default database: 'mydb'. Query: 'UPDATE thread AS thread,taggregate_temp_1212047760 AS aggregate

May 29 09:56:08 http2 mysqld[1380]: ^ISET thread.views = thread.views + aggregate.views

May 29 09:56:08 http2 mysqld[1380]: ^IWHERE thread.threadid = aggregate.threadid', Error_code: 1146

May 29 09:56:08 http2 mysqld[1380]: 080529 9:56:08 [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.001079' position 203015142

Slave1:/home/admin#

You can see what query caused the error, and at what log position the replication stopped.

To verify that the replication is really not working, log in to MySQL on the Slave:

mysql -u root -p

On the MySQL shell, run:

SHOW SLAVE STATUS \G

If one of Slave_IO_Running or Slave_SQL_Running is set to No, then the replication is broken:

mysql> SHOW SLAVE STATUS \G

*************************** 1. row ***************************

Slave_IO_State: Waiting for master to send event

Master_Host: 1.2.3.4

Master_User: slave_user

Master_Port: 3306

Connect_Retry: 60

Master_Log_File: mysql-bin.001079

Read_Master_Log_Pos: 269214454

Relay_Log_File: slave-relay.000130

Relay_Log_Pos: 100125935

Relay_Master_Log_File: mysql-bin.001079

Slave_IO_Running: Yes

Slave_SQL_Running: No

Replicate_Do_DB: mydb

Replicate_Ignore_DB:

Replicate_Do_Table:

Replicate_Ignore_Table:

Replicate_Wild_Do_Table:

Replicate_Wild_Ignore_Table:

Last_Errno: 1146

Last_Error: Error 'Table 'mydb.taggregate_temp_1212047760' doesn't exist' on query. Default database: 'mydb'.

Query: 'UPDATE thread AS thread,taggregate_temp_1212047760 AS aggregate

SET thread.views = thread.views + aggregate.views

WHERE thread.threadid = aggregate.threadid'

Skip_Counter: 0

Exec_Master_Log_Pos: 203015142

Relay_Log_Space: 166325247

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

1 row in set (0.00 sec)

Repairing The Replication

First, we stop the slave:

STOP SLAVE;

Fixing the problem is actually quite easy. We tell the slave to simply skip the invalid SQL query:

SET GLOBAL SQL_SLAVE_SKIP_COUNTER = 1;

This tells the slave to skip one query (which is the invalid one that caused the replication to stop). If you'd like to skip two queries, you'd use SET GLOBAL SQL_SLAVE_SKIP_COUNTER = 2; instead and so on.

Now we can start the slave again

START SLAVE;

Now check if replication is working again: