MySQL Replication Troubleshooting

Understanding MySQL Replication Components

Before troubleshooting, it's important to understand the key components involved in MySQL replication.

Replication typically involves a master server that writes binary logs and one or more slave servers that read and apply these logs.

• Master server records changes in binary log files.
• Slave server reads the master's binary logs via the IO thread.
• Slave applies changes using the SQL thread.
• Replication status is monitored using SHOW SLAVE STATUS.

Common Replication Issues and Their Symptoms

Replication problems can manifest in various ways, including replication lag, stopped slave threads, or inconsistent data.

Recognizing symptoms early helps in faster diagnosis and resolution.

• Replication lag: Slave is behind master in applying changes.
• IO thread stopped: Slave cannot read master's binary log.
• SQL thread stopped: Slave cannot apply changes.
• Error messages in SHOW SLAVE STATUS indicating issues.
• Data inconsistency between master and slave.

Using SHOW SLAVE STATUS for Diagnosis

The SHOW SLAVE STATUS command provides detailed information about the slave's replication state.

Key fields to check include Slave_IO_Running, Slave_SQL_Running, Last_Error, and Seconds_Behind_Master.

• Slave_IO_Running and Slave_SQL_Running should both be 'Yes' for healthy replication.
• Last_Error shows the most recent replication error.
• Seconds_Behind_Master indicates replication lag in seconds.

Troubleshooting Steps for Common Errors

Follow these practical steps to resolve typical replication issues.

• Check network connectivity between master and slave.
• Verify replication user privileges and passwords.
• Inspect MySQL error logs for detailed error messages.
• Restart slave threads using START SLAVE or STOP SLAVE and START SLAVE.
• Skip problematic transactions carefully using SET GLOBAL SQL_SLAVE_SKIP_COUNTER.
• Resynchronize slave with master if data inconsistency occurs.

Preventive Measures and Best Practices

Maintaining healthy replication requires proactive monitoring and good practices.

• Regularly monitor replication status and lag.
• Keep replication user privileges minimal and secure.
• Use consistent schema changes across master and slaves.
• Perform regular backups and test recovery procedures.
• Document replication topology and configuration changes.